The Italian gamma-ray satellite AGILE has recently reported the detection of
some variable high-energy sources likely of galactic origin. These sources do
not have any obvious counterpart at lower energies. We propose that these
sources are produced in proton-dominated jets of galactic microquasars. We
develop a model for microquasar jets that takes into account both primary
leptons and protons and all relevant radiative processes, including secondary
particle emission and gamma-ray attenuation due to pair creation in the jet. We
obtain spectral energy distributions that correspond to what is observed by
AGILE, with most of the power concentrated between 100 MeV and 10 GeV and
reaching luminosities of $10^{34-35}$ erg s$^{-1}$. We make detailed spectral
predictions that can be tested by the Fermi gamma-ray telescope in the
immediate future. We conclude that hadronic jets in galactic accreting sources
can be responsible for the variable unidentified gamma-ray sources detected by
AGILE.Comment: 4 pages, 2 figures. Accepted for publication in Astronomy &
  Astrophysics (Letters

Aharonian

Blumenthal

Bogovalov

Bosch-Ramon

G. E. Romero

G. S. Vila

Grenier

Kaufman Bernadó

Kelner

Khangulyan

Levinson

Mirabel

Nolan

Punsly

Reynoso

Romero

Vila

English

arXiv

Context. The Italian gamma-ray satellite AGILE has recently reported the detection of some variable high-energy sources likely of galactic origin. These sources do not have any obvious counterpart at lower energies.
        Aims. We propose that these sources are produced in proton-dominated jets of galactic microquasars.
        Methods. We develop a model for microquasar jets that takes into account both primary leptons and protons and all relevant radiative processes, including secondary particle emission and gamma-ray attenuation due to pair creation in the jet.
        Results. We obtain spectral energy distributions that correspond to what is observed by AGILE, with most of the power concentrated between 100 MeV and 10 GeV and reaching luminosities of 1034-35 erg s-1. We make detailed spectral predictions that can be tested by the Fermi gamma-ray telescope in the immediate future.
        Conclusions. We conclude that hadronic jets in galactic accreting sources can be responsible for the variable unidentified gamma-ray sources detected by AGILE

EDP Sciences OAI-PMH repository (1.2.0)

On the nature of the AGILE galactic transient sources

Context. The Italian gamma-ray satellite AGILE has recently reported the detection of some variable high-energy sources likely of galactic origin. These sources do not have any obvious counterpart at lower energies. Aims. We propose that these sources are produced in proton-dominated jets of galactic microquasars. Methods. We develop a model for microquasar jets that takes into account both primary leptons and protons and all relevant radiative processes, including secondary particle emission and gamma-ray attenuation due to pair creation in the jet. Results. We obtain spectral energy distributions that correspond to what is observed by AGILE, with most of the power concentrated between 100 MeV and 10 GeV and reaching luminosities of 1034-35 erg s-1. We make detailed spectral predictions that can be tested by the Fermi gamma-ray telescope in the immediate future. Conclusions. We conclude that hadronic jets in galactic accreting sources can be responsible for the variable unidentified gamma-ray sources detected by AGILE.Fil: Romero, Gustavo Esteban. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Vila, Gabriela Soledad. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Romero, Gustavo Esteban

Vila, Gabriela Soledad

CONICET Digital

A&A 494, L33–L36 (2009)DOI: 10.1051/0004-6361:200811283c© ESO 2009Astronomy&AstrophysicsLetter to the EditorOn the nature of the AGILE galactic transient sourcesG. E. Romero1,2, and G. S. Vila1,1 Instituto Argentino de Radioastronomía (IAR), CCT La Plata (CONICET), C.C.5, (1894) Villa Elisa, Buenos Aires, Argentinae-mail: romero@iar-conicet.gov.ar2 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900, La Plata, ArgentinaReceived 3 November 2008 / Accepted 25 December 2008ABSTRACTContext. The Italian gamma-ray satellite AGILE has recently reported the detection of some variable high-energy sources likely ofgalactic origin. These sources do not have any obvious counterpart at lower energies.Aims. We propose that these sources are produced in proton-dominated jets of galactic microquasars.Methods. We develop a model for microquasar jets that takes into account both primary leptons and protons and all relevant radiativeprocesses, including secondary particle emission and gamma-ray attenuation due to pair creation in the jet.Results. We obtain spectral energy distributions that correspond to what is observed by AGILE, with most of the power concentratedbetween 100 MeV and 10 GeV and reaching luminosities of 1034−35 erg s−1. We make detailed spectral predictions that can be testedby the Fermi gamma-ray telescope in the immediate future.Conclusions. We conclude that hadronic jets in galactic accreting sources can be responsible for the variable unidentified gamma-raysources detected by AGILE.Key words. X-rays: binaries – gamma rays: theory – radiation mechanisms: non-thermal1. IntroductionThe Italian gamma-ray satellite AGILE (Astro-rivelatoreGamma a Immagini Leggero) has detected several non-identifiedvariable sources likely of galactic origin. These sources includethe strong source AGL J2021+4029 located in the Cygnus re-gion, with the center of gravity of the error box at l = 78.01 degand b = 2.19 deg (Longo et al. 2008), the variable source inthe Musca region (error box centered at l = 312.2 deg andb = −0.3 deg, Pittori et al. 2008), and the high galactic lati-tude transient AGL J0229+2054, with the error box centered atl = 151.7 deg and b = −36.4 deg (Bullgarelli et al. 2008). TheCygnus source showed some significant re-brightening after itsdiscovery (Giuliani et al. 2008; Chen et al. 2008). SimultaneousX-ray observations with Super-AGILE did not show any coun-terpart in the 20–60 keV band. A steady and weak source was de-tected within the large error box by Swift/BAT (15–55 keV, seeAjello et al. 2008), but there is no clear relationship. AdditionalX-ray observations have been performed with XMM-Newton,without adding new clues (Pandel et al. 2008). Radio observa-tions with the very large array (VLA) have shown no clear coun-terpart (Cheung 2008).Concerning the source in the Musca region, a potentialarchival X-ray counterpart has been claimed on the basis ofBeppoSAX observations dating from January 2001 (Orlandiniet al. 2008). The high-latitude transient, on the other hand, mightbe a halo galactic source or a blazar (the AGN 1ES 0229+200is at 61.3 arcmin from Gamma-Ray Imaging Detector error boxcentroid). Member of CONICET, Argentina. Fellow of CONICET, Argentina.The fact that these sources are highly variable implies thatthe high-energy radiation should be produced in a compact re-gion. The absence of detection with Super-AGILE means thatthe ratio of gamma-ray to X-ray luminosities Lγ/LX should be1. These characteristics recall those of the population of vari-able EGRET sources found in the Galactic Plane and in theHalo (Romero 2001; Grenier 2001, 2004; Nolan et al. 2003).Actually, the AGILE detections in the Cygnus region and theMusca region partially overlap with the location error box of thesources 3EG J2020+4017 and 3EG J1410-6147, respectively. Ithas been proposed that the unidentified variable gamma-raysources sources at MeV–GeV energies might be high-mass mi-croquasars with the emission dominated by inverse Comptonup-scattering of UV stellar photons from the hot donor star(Kaufman-Bernadó et al. 2002; Bosch-Ramon et al. 2005). Onthe Galactic Plane, the donor star could be strongly obscured,rendering diﬃcult its detection. These models, however, predicta significant production of X-rays, something that is at oddswith the new AGILE and Super-Agile observations. Grenieret al. (2005) proposed that the variable high-latitude unidentifiedsources might be old, low-mass microquasars expelled long agofrom the Galactic Plane or from globular clusters (see Mirabelet al. 2001). They also showed that external Compton mod-els cannot account for the energetics required by the sources.Romero & Vila (2008) and Vila & Romero (2008) showed that“proton” microquasars with low-mass donor stars might explainthe halo EGRET sources through proton synchrotron radiationand photo-meson production.In this Letter we propose that microquasars with proton dom-inated jets can produce spectral energy distributions that satisfyall constraints imposed by AGILE observations and we makeArticle published by EDP SciencesL34 G. E. Romero and G. S. Vila: AGILE sourcesFig. 1. Acceleration and cooling rates at the base of the jet for primary protons and electrons, and secondary pions and muons, calculated forrepresentative values of the model parameters (proton-to-lepton energy ratio a = 1000, and primary injection spectral index α = 1.5). The valueof the magnetic field is B ∼ 107 G, and the acceleration eﬃciency parameter η is indicated.some predictions that can be used to test our proposal with theGLAST-Fermi satellite, and ground-based Cherenkov telescopearrays.2. The modelThe model we are going to present is based on the galactic jetmodel described by Romero & Vila (2008), but it incorporatesseveral refinements. The jet is assumed to carry most of the ac-cretion power, in accordance with the dissipationless disk modelof Bogovalov & Kelner (2005, 2008).A small fraction of the jet power is transformed into relativis-tic particles in a “one-zone” acceleration region located close tothe compact object (Khangulyan et al. 2008). In this region, themagnetic field (obtained by equipartition) is extremely high (ofthe order of 107 G). This produces the immediate cooling of pri-mary electrons and significant cooling of protons. In Fig. 1 weshow the cooling rates for both primary electrons and protons inthe acceleration region, as well as the cooling and decaying rateof secondary muons and pions. The maximum energies of theprimary particles are obtained equating the cooling rates to theacceleration rate, which is assumed to be produced by mildly rel-ativistic shocks at the base of the jet through a first-order Fermimechanism. In this work we adopt an acceleration eﬃciency ofthe order of 10%, which rougly corresponds to a shock velocityof 0.3 c and a mean free path similar to the Larmor radius. We seethat electrons, eVen for such a high acceleration eﬃciency, reachonly GeV energies, whereas protons can attain much higher en-ergies, well into the PeV band.Our model takes into account: 1) synchrotron emission fromboth types (electrons and protons) of primary particles, as wellas emission from secondary leptons (electrons, positrons, andmuons) and hadrons (charged pions); 2) inverse Compton (IC)emission from all leptons in the strong total radiation field inthe emitting region; 3) photo-pair and photo-meson productionby both protons and pions; 4) inelastic collisions between rela-tivistic protons in the jet and the cold material that forms mostof the same outflow (see Bosch-Ramon et al. 2006, for detailsof the physics of a cold matter dominated jet); 5) relativis-tic Bremsstrahlung from electrons and muons; and 6) internalphoton absorption in the local radiation fields (calculated as inAharonian et al. 2008); 7) re-injection of secondary pairs, whichis negligible due to the low opacity of the proton-dominated jet(see Romero & Vila 2008, for a discussion). The eﬀect of lossesof muons and pions in a strong magnetic field on the resultinggamma-ray and secondary lepton spectra has been recently dis-cussed by Reynoso & Romero (2009) and we adopt here theirtreatment. For the other processes we follow the classical for-mulae (Blumenthal & Gould 1970), the photo-meson productiontreatment already used by Romero & Vila (2008), and the ex-pressions given by Kelner et al. (2006) for pp interactions (thisformalism is only valid for proton energies above 0.1 TeV; atlower energies we use the standard treatment).G. E. Romero and G. S. Vila: AGILE sources L35Fig. 2. Spectral energy distributions of a proton-dominated microquasar (a = 1000). Each panel corresponds to a diﬀerent acceleration eﬃciency(η = 0.1 on the left, η = 0.01 on the right). Fermi point source sensitivity is indicated by a thick blue line.The ratio of relativistic protons to electrons a in the jet isunknown and is used as the basic free parameter in our model.We have calculated a number of models for diﬀerent acceler-ation eﬃciencies, and a set of standard parameters: black holemass ∼10 M, accretion rate ∼1037 erg s−1 (i.e., ∼10−2 of theEddington luminosity), injection ∝ E−1.5, viewing angle 30◦, jetbulk Lorentz factor Γ = 1.5, and location of the accelerationzone at z0 ≈ 108 cm (see Romero & Vila 2008).The transport equation is solved for all types of particles. Atsteady state it reads:∂∂E[dEdt∣∣∣∣∣lossN(E, z)]+N(E, z)tesc= Q(E). (1)where tesc is the particle escape time from the acceleration re-gion (tesc ≈ Δz/vjet) and Q is the injection function that can benormalized in accordance with the energy budget of relativisticparticles through:Le,p =∫Vd3r∫ Emaxe,p (z)Emine,pdEe,p Ee,p Qe,p(Ee,p). (2)The minimum kinetic energy is taken to be of the order of therest mass energy of the corresponding particle. The equation forpions and muons has an additional term N(E, z)/tdec, that takesinto account the decay of the particles on a timescale tdec thatdepends on the energy in the lab system.In general, variability can be obtained introducing a variableinjection Q(E, t). Once the particle distributions are known, theradiative output can be calculated as mentioned above. The inter-nal attenuation results from photon annihilation: γ+γ→ e−+e+.The opacity for a gamma ray of energy Eγ is:τ(Eγ) = 12∫l∫ maxth∫ umax−1(1 − u) σ(Eγ, , u)× n(, z) du d dl, (3)where n(, z) is the photon number density at energy  and lo-cation z, u = cosϑ, ϑ is the angle between the momenta of thecolliding photons, l is the γ-ray photon path, and the cross sec-tion for the interaction is given by (e.g. Levinson 2006):σγγ(Eγ, , ϑ) = 316σT(1 − β2)×[(3 − β4)ln(1 + β1 − β)− 2β(2 − β2)]. (4)Here, β is the speed of the electron/positron in the center of mo-mentum frame, i.e.,(1 − β2) = 2m2ec4(1 − u)Eγ ; 0 ≤ β < 1. (5)The threshold energy th is defined by β = 1 with ϑ = 0.3. ResultsIn Fig. 2 we show the spectral energy distribution (SED) cal-culated for a proton dominated (a = 1000) jet, with a totalpower of 1037 erg s−1. We show the diﬀerent contributions fromall significant cooling processes for all primary and secondaryparticles, in the case of two diﬀerent acceleration eﬃciencies η(0.1 and 0.01, from left to right, respectively). Bremsstrahlungand IC losses are negligible, since synchrotron radiation is thedominat cooling channel for leptons. The diﬀerent accelerationeﬃciencies not only aﬀect the maximum energy of the radia-tion but modify the whole shape of the SED, since the dom-inant synchrotron losses follow the square of the energy anddepend significantly on the mass of the particles, resulting ina global photon redistribution. In both cases, however, the peakof the SED is determined by proton synchrotron radiation, fol-lowed by pion-synchrotron emission. For the higher eﬃciencythe synchrotron peak is sharper, reaching almost 1035 erg s−1.In the case of a lower eﬃciency, the peak is slightly above1034 erg s−1. In the first case most of the emission is concen-trated in the range 108–1012 eV, whereas in the second it is be-tween 108 and 1010 eV, with a soft slope beyond 109 eV. In bothcases there is a high ratio Lγ/LX , in accordance with what isinferred from AGILE observations. Soft X-rays, due mainly toelectron synchrotron radiation, are at the level of 1032 erg s−1.The hard X-ray component is dominated by muon synchrotronemission. Contrary to models with equipartition (a = 1) in rel-ativistic particles, photo-meson production is not significant inproton dominated jets, since the synchrotron field is relativelyweak (see Romero & Vila 2008).The quoted luminosities correspond to the flux reported byAGILE and the upper X-ray limits, at a distance of the order of∼2 kpc. And accurate determination of the distance can be usedto constrain the energy budget of the sources. At TeV energiesthe luminosities (due mainly to pp interactions) are at the levelof 1032–1033 erg s−1, and hence the sources are not detectable byL36 G. E. Romero and G. S. Vila: AGILE sourcescurrent Cherenkov atmospheric telescopes, although they couldfall within the sensitivity of upgraded instruments like HESS IIand MAGIC II.4. Discussion and summaryA crucial feature of a synchrotron-proton dominated jet is that avery strong magnetic field is necessary to produce detectable ra-diation in γ-rays. This means that the acceleration region shouldbe located very close to the compact object (at ∼108 cm in ourmodels). Intrinsic absorption is then not very important sincethe low energy fields, which are responsible for the opacity togamma-ray propagation, are weak. In models with a high con-tent of primary electrons these eﬀects, at the base of the jet, arevery significant leading to a complete suppression of all emis-sion above ∼100 GeV (Romero & Vila 2008). Such models pro-duce a huge amount of X-rays, something that is not observed inthe unidentified MeV-GeV sources.In our model we introduce a hard injection spectrum in or-der to achieve a strong contrast between leptonic and hadronicpeaks. A softer injection would reduce the Lγ/LX ratio. Wenotice that the losses in the high magnetic field strongly af-fect the overall leptonic particle spectrum. Observations withthe LAT instrument of the Fermi satellite will allow us to de-termine the photon spectrum of these sources in the range100 MeV–100 GeV, allowing a better determination of the phys-ical parameters.We remark that IC models are possible for these type ofsources only if the radiation is produced in a region with a sig-nificantly smaller (∼10–100 G) magnetic field, see for examplePunsly et al. (2000). Such a field would occur at distances of1012–1013 cm from the compact object. How the injector of rel-ativistic particles could be located there will be discussed else-where (see, nonetheless, Bosch-Ramon 2007).In summary, we propose a complete lepto/hadronic jet modelto explain the unidentified variable AGILE sources. This modelassumes a strong component of relativistic primary protons andtakes into account all radiative processes that might occur at thebase of the jets. The predicted SEDs are in accordance with whatwe know about of these sources. The jet model is independentof the nature of the donor star, so it could explain both low- andhigh-latitude galactic sources. Fermi observations will allowus to determine better constraints on the spectral features, thenmaking possible inferences about the actual conditions in thesources. Development of models for γ-ray production far fromthe compact object and models that take into account externaleﬀects is in progress.Acknowledgements. We thank Matías Reynoso for many fruitful discussions.GER acknowledges support by the Ministerio de Educación y Ciencia (Spain)under grant AYA 2007-68034-C03-01, FEDER funds. G.S.V. thanks NicolásCasco for his great help with numerical computations.ReferencesAharonian, F., Akhperjanian, A. G., Aye, K.-M., et al. (HESS Coll.) 2005,Science, 309, 746Aharonian, F. A., Khangulyan, D., & Costamante, L. 2008, MNRAS, 387, 1206Ajello, M., Greiner, J., Kabach, G., & Gehrels, N. 2008, ATel, 1497Blumenthal, G. R., & Gould, R. J. 1970, Rev. Mod. Phys., 42, 237Bogovalov, S. V., & Kelner, S. R. 2005, Astron. Rep., 49, 57Bogovalov, S. V., & Kelner, S. R. 2008, MNRAS, submitted[arXiv:0809.0429v1]Bosch-Ramon, V. 2007, Ap&SS, 309, 321Bosch-Ramon, V., Romero, G. E., & Paredes, J. M. 2005, A&A, 429, 267Bosch-Ramon, V., Romero, G. E., & Paredes, J. M. 2006, A&A, 447, 263Bulgarelli, A. Tavani, M., Pucella, G., et al. 2008, ATel, 1641Chen, A., Caraveo, P., Giuliani, A., et al. 2008, ATel, 1585Cheung, C. C. 2008, ATel, 1589Giuliani, A., Caraveo, P., Chen, A., et al. 2008, ATel, 1547Grenier, I. A. 2001, in The Nature of Unidentified High-Energy Gamma-RaySources, ed. A. Carramiñana et al. (Dordrecht: Kluwer Academic Publishers),51Grenier, I. A. 2004, in Cosmic Gamma-Ray Sources, ed. K. S. Cheng, & G. E.Romero (Dordrecht: Kluwer Academic Publishers), 47Grenier, I. A., Kaufman Bernadó, M. M., & Romero, G. E. 2005, Ap&SS, 297,109Kaufman Bernadó, M. M., Romero, G. E., & Mirabel, I. F. 2002, A&A, 385, L10Kelner, S. R., Aharonian, F. A., & Bugayov, V. V. 2006, Phys. Rev. D, 74, 034018Khangulyan, D., Aharonian, F. A., & Bosch-Ramon, V. 2008, MNRAS, 383, 467Levinson, A. 2006, Int. J. Mod. Phys. A, 21, 6115Longo, F., Chen, A., Vercellone, S., et al. 2008, ATel, 1429Mirabel, I. F., Dhawan, V., Mignani, R. P., et al. 2001, Nature, 413, 139Nolan, P. L., Tompkins, W. F., Grenier, I. A., & Michelson, P. F. 2003, ApJ, 597,615Orlandini, M., Frontera, F., Bassani, L., Landi, R., & Sguera, V. 2008, ATel, 1419Pandel, D., Kaaret, P., & Lang, C. C. 2008, ATel, 1595Pittori, C., Verrecchia, F., Santolamazza, P., et al. 2008, ATel, 1394Punsly, B., Romero, G. E., Torres, D. F., & Combi, J. A. 2000, A&A, 364, 552Reynoso, M. M., & Romero, G. E. 2009, A&A, 493, 1Romero, G. E. 2001, in The Nature of Unidentified High-Energy Gamma-RaySources, ed. A. Carramiñana et al. (Dordrecht: Kluwer Academic Publishers),65Romero, G. E., & Vila, G. S. 2008, A&A, 485, 623Vila, G. S., & Romero, G. E. 2008, Int. J. Mod. Phys. D, 17, 1903

Astronomy and Astrophysics

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas

A&A 494, L33–L36 (2009)DOI: 10.1051/0004-6361:200811283c© ESO 2009Astronomy&AstrophysicsLetter to the EditorOn the nature of the AGILE galactic transient sourcesG. E. Romero1,2, and G. S. Vila1,1 Instituto Argentino de Radioastronomía (IAR), CCT La Plata (CONICET), C.C.5, (1894) Villa Elisa, Buenos Aires, Argentinae-mail: romero@iar-conicet.gov.ar2 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900, La Plata, ArgentinaReceived 3 November 2008 / Accepted 25 December 2008ABSTRACTContext. The Italian gamma-ray satellite AGILE has recently reported the detection of some variable high-energy sources likely ofgalactic origin. These sources do not have any obvious counterpart at lower energies.Aims. We propose that these sources are produced in proton-dominated jets of galactic microquasars.Methods. We develop a model for microquasar jets that takes into account both primary leptons and protons and all relevant radiativeprocesses, including secondary particle emission and gamma-ray attenuation due to pair creation in the jet.Results. We obtain spectral energy distributions that correspond to what is observed by AGILE, with most of the power concentratedbetween 100 MeV and 10 GeV and reaching luminosities of 1034−35 erg s−1. We make detailed spectral predictions that can be testedby the Fermi gamma-ray telescope in the immediate future.Conclusions. We conclude that hadronic jets in galactic accreting sources can be responsible for the variable unidentified gamma-raysources detected by AGILE.Key words. X-rays: binaries – gamma rays: theory – radiation mechanisms: non-thermal1. IntroductionThe Italian gamma-ray satellite AGILE (Astro-rivelatoreGamma a Immagini Leggero) has detected several non-identifiedvariable sources likely of galactic origin. These sources includethe strong source AGL J2021+4029 located in the Cygnus re-gion, with the center of gravity of the error box at l = 78.01 degand b = 2.19 deg (Longo et al. 2008), the variable source inthe Musca region (error box centered at l = 312.2 deg andb = −0.3 deg, Pittori et al. 2008), and the high galactic lati-tude transient AGL J0229+2054, with the error box centered atl = 151.7 deg and b = −36.4 deg (Bullgarelli et al. 2008). TheCygnus source showed some significant re-brightening after itsdiscovery (Giuliani et al. 2008; Chen et al. 2008). SimultaneousX-ray observations with Super-AGILE did not show any coun-terpart in the 20–60 keV band. A steady and weak source was de-tected within the large error box by Swift/BAT (15–55 keV, seeAjello et al. 2008), but there is no clear relationship. AdditionalX-ray observations have been performed with XMM-Newton,without adding new clues (Pandel et al. 2008). Radio observa-tions with the very large array (VLA) have shown no clear coun-terpart (Cheung 2008).Concerning the source in the Musca region, a potentialarchival X-ray counterpart has been claimed on the basis ofBeppoSAX observations dating from January 2001 (Orlandiniet al. 2008). The high-latitude transient, on the other hand, mightbe a halo galactic source or a blazar (the AGN 1ES 0229+200is at 61.3 arcmin from Gamma-Ray Imaging Detector error boxcentroid). Member of CONICET, Argentina. Fellow of CONICET, Argentina.The fact that these sources are highly variable implies thatthe high-energy radiation should be produced in a compact re-gion. The absence of detection with Super-AGILE means thatthe ratio of gamma-ray to X-ray luminosities Lγ/LX should be1. These characteristics recall those of the population of vari-able EGRET sources found in the Galactic Plane and in theHalo (Romero 2001; Grenier 2001, 2004; Nolan et al. 2003).Actually, the AGILE detections in the Cygnus region and theMusca region partially overlap with the location error box of thesources 3EG J2020+4017 and 3EG J1410-6147, respectively. Ithas been proposed that the unidentified variable gamma-raysources sources at MeV–GeV energies might be high-mass mi-croquasars with the emission dominated by inverse Comptonup-scattering of UV stellar photons from the hot donor star(Kaufman-Bernadó et al. 2002; Bosch-Ramon et al. 2005). Onthe Galactic Plane, the donor star could be strongly obscured,rendering difficult its detection. These models, however, predicta significant production of X-rays, something that is at oddswith the new AGILE and Super-Agile observations. Grenieret al. (2005) proposed that the variable high-latitude unidentifiedsources might be old, low-mass microquasars expelled long agofrom the Galactic Plane or from globular clusters (see Mirabelet al. 2001). They also showed that external Compton mod-els cannot account for the energetics required by the sources.Romero & Vila (2008) and Vila & Romero (2008) showed that“proton” microquasars with low-mass donor stars might explainthe halo EGRET sources through proton synchrotron radiationand photo-meson production.In this Letter we propose that microquasars with proton dom-inated jets can produce spectral energy distributions that satisfyall constraints imposed by AGILE observations and we makeArticle published by EDP SciencesL34 G. E. Romero and G. S. Vila: AGILE sourcesFig. 1. Acceleration and cooling rates at the base of the jet for primary protons and electrons, and secondary pions and muons, calculated forrepresentative values of the model parameters (proton-to-lepton energy ratio a = 1000, and primary injection spectral index α = 1.5). The valueof the magnetic field is B ∼ 107 G, and the acceleration efficiency parameter η is indicated.some predictions that can be used to test our proposal with theGLAST-Fermi satellite, and ground-based Cherenkov telescopearrays.2. The modelThe model we are going to present is based on the galactic jetmodel described by Romero & Vila (2008), but it incorporatesseveral refinements. The jet is assumed to carry most of the ac-cretion power, in accordance with the dissipationless disk modelof Bogovalov & Kelner (2005, 2008).A small fraction of the jet power is transformed into relativis-tic particles in a “one-zone” acceleration region located close tothe compact object (Khangulyan et al. 2008). In this region, themagnetic field (obtained by equipartition) is extremely high (ofthe order of 107 G). This produces the immediate cooling of pri-mary electrons and significant cooling of protons. In Fig. 1 weshow the cooling rates for both primary electrons and protons inthe acceleration region, as well as the cooling and decaying rateof secondary muons and pions. The maximum energies of theprimary particles are obtained equating the cooling rates to theacceleration rate, which is assumed to be produced by mildly rel-ativistic shocks at the base of the jet through a first-order Fermimechanism. In this work we adopt an acceleration efficiency ofthe order of 10%, which rougly corresponds to a shock velocityof 0.3 c and a mean free path similar to the Larmor radius. We seethat electrons, eVen for such a high acceleration efficiency, reachonly GeV energies, whereas protons can attain much higher en-ergies, well into the PeV band.Our model takes into account: 1) synchrotron emission fromboth types (electrons and protons) of primary particles, as wellas emission from secondary leptons (electrons, positrons, andmuons) and hadrons (charged pions); 2) inverse Compton (IC)emission from all leptons in the strong total radiation field inthe emitting region; 3) photo-pair and photo-meson productionby both protons and pions; 4) inelastic collisions between rela-tivistic protons in the jet and the cold material that forms mostof the same outflow (see Bosch-Ramon et al. 2006, for detailsof the physics of a cold matter dominated jet); 5) relativis-tic Bremsstrahlung from electrons and muons; and 6) internalphoton absorption in the local radiation fields (calculated as inAharonian et al. 2008); 7) re-injection of secondary pairs, whichis negligible due to the low opacity of the proton-dominated jet(see Romero & Vila 2008, for a discussion). The effect of lossesof muons and pions in a strong magnetic field on the resultinggamma-ray and secondary lepton spectra has been recently dis-cussed by Reynoso & Romero (2009) and we adopt here theirtreatment. For the other processes we follow the classical for-mulae (Blumenthal & Gould 1970), the photo-meson productiontreatment already used by Romero & Vila (2008), and the ex-pressions given by Kelner et al. (2006) for pp interactions (thisformalism is only valid for proton energies above 0.1 TeV; atlower energies we use the standard treatment).G. E. Romero and G. S. Vila: AGILE sources L35Fig. 2. Spectral energy distributions of a proton-dominated microquasar (a = 1000). Each panel corresponds to a different acceleration efficiency(η = 0.1 on the left, η = 0.01 on the right). Fermi point source sensitivity is indicated by a thick blue line.The ratio of relativistic protons to electrons a in the jet isunknown and is used as the basic free parameter in our model.We have calculated a number of models for different acceler-ation efficiencies, and a set of standard parameters: black holemass ∼10 M, accretion rate ∼1037 erg s−1 (i.e., ∼10−2 of theEddington luminosity), injection ∝ E−1.5, viewing angle 30◦, jetbulk Lorentz factor Γ = 1.5, and location of the accelerationzone at z0 ≈ 108 cm (see Romero & Vila 2008).The transport equation is solved for all types of particles. Atsteady state it reads:∂∂E[dEdt∣∣∣∣∣lossN(E, z)]+N(E, z)tesc= Q(E). (1)where tesc is the particle escape time from the acceleration re-gion (tesc ≈ Δz/vjet) and Q is the injection function that can benormalized in accordance with the energy budget of relativisticparticles through:Le,p =∫Vd3r∫ Emaxe,p (z)Emine,pdEe,p Ee,p Qe,p(Ee,p). (2)The minimum kinetic energy is taken to be of the order of therest mass energy of the corresponding particle. The equation forpions and muons has an additional term N(E, z)/tdec, that takesinto account the decay of the particles on a timescale tdec thatdepends on the energy in the lab system.In general, variability can be obtained introducing a variableinjection Q(E, t). Once the particle distributions are known, theradiative output can be calculated as mentioned above. The inter-nal attenuation results from photon annihilation: γ+γ→ e−+e+.The opacity for a gamma ray of energy Eγ is:τ(Eγ) =12∫l∫ εmaxεth∫ umax−1(1 − u) σ(Eγ, ε, u)× n(ε, z) du dε dl, (3)where n(ε, z) is the photon number density at energy ε and lo-cation z, u = cosϑ, ϑ is the angle between the momenta of thecolliding photons, l is the γ-ray photon path, and the cross sec-tion for the interaction is given by (e.g. Levinson 2006):σγγ(Eγ, ε, ϑ) =316σT(1 − β2)×[(3 − β4)ln(1 + β1 − β)− 2β(2 − β2)]. (4)Here, β is the speed of the electron/positron in the center of mo-mentum frame, i.e.,(1 − β2) = 2m2ec4(1 − u)Eγε ; 0 ≤ β < 1. (5)The threshold energy εth is defined by β = 1 with ϑ = 0.3. ResultsIn Fig. 2 we show the spectral energy distribution (SED) cal-culated for a proton dominated (a = 1000) jet, with a totalpower of 1037 erg s−1. We show the different contributions fromall significant cooling processes for all primary and secondaryparticles, in the case of two different acceleration efficiencies η(0.1 and 0.01, from left to right, respectively). Bremsstrahlungand IC losses are negligible, since synchrotron radiation is thedominat cooling channel for leptons. The different accelerationefficiencies not only affect the maximum energy of the radia-tion but modify the whole shape of the SED, since the dom-inant synchrotron losses follow the square of the energy anddepend significantly on the mass of the particles, resulting ina global photon redistribution. In both cases, however, the peakof the SED is determined by proton synchrotron radiation, fol-lowed by pion-synchrotron emission. For the higher efficiencythe synchrotron peak is sharper, reaching almost 1035 erg s−1.In the case of a lower efficiency, the peak is slightly above1034 erg s−1. In the first case most of the emission is concen-trated in the range 108–1012 eV, whereas in the second it is be-tween 108 and 1010 eV, with a soft slope beyond 109 eV. In bothcases there is a high ratio Lγ/LX , in accordance with what isinferred from AGILE observations. Soft X-rays, due mainly toelectron synchrotron radiation, are at the level of 1032 erg s−1.The hard X-ray component is dominated by muon synchrotronemission. Contrary to models with equipartition (a = 1) in rel-ativistic particles, photo-meson production is not significant inproton dominated jets, since the synchrotron field is relativelyweak (see Romero & Vila 2008).The quoted luminosities correspond to the flux reported byAGILE and the upper X-ray limits, at a distance of the order of∼2 kpc. And accurate determination of the distance can be usedto constrain the energy budget of the sources. At TeV energiesthe luminosities (due mainly to pp interactions) are at the levelof 1032–1033 erg s−1, and hence the sources are not detectable byL36 G. E. Romero and G. S. Vila: AGILE sourcescurrent Cherenkov atmospheric telescopes, although they couldfall within the sensitivity of upgraded instruments like HESS IIand MAGIC II.4. Discussion and summaryA crucial feature of a synchrotron-proton dominated jet is that avery strong magnetic field is necessary to produce detectable ra-diation in γ-rays. This means that the acceleration region shouldbe located very close to the compact object (at ∼108 cm in ourmodels). Intrinsic absorption is then not very important sincethe low energy fields, which are responsible for the opacity togamma-ray propagation, are weak. In models with a high con-tent of primary electrons these effects, at the base of the jet, arevery significant leading to a complete suppression of all emis-sion above ∼100 GeV (Romero & Vila 2008). Such models pro-duce a huge amount of X-rays, something that is not observed inthe unidentified MeV-GeV sources.In our model we introduce a hard injection spectrum in or-der to achieve a strong contrast between leptonic and hadronicpeaks. A softer injection would reduce the Lγ/LX ratio. Wenotice that the losses in the high magnetic field strongly af-fect the overall leptonic particle spectrum. Observations withthe LAT instrument of the Fermi satellite will allow us to de-termine the photon spectrum of these sources in the range100 MeV–100 GeV, allowing a better determination of the phys-ical parameters.We remark that IC models are possible for these type ofsources only if the radiation is produced in a region with a sig-nificantly smaller (∼10–100 G) magnetic field, see for examplePunsly et al. (2000). Such a field would occur at distances of1012–1013 cm from the compact object. How the injector of rel-ativistic particles could be located there will be discussed else-where (see, nonetheless, Bosch-Ramon 2007).In summary, we propose a complete lepto/hadronic jet modelto explain the unidentified variable AGILE sources. This modelassumes a strong component of relativistic primary protons andtakes into account all radiative processes that might occur at thebase of the jets. The predicted SEDs are in accordance with whatwe know about of these sources. The jet model is independentof the nature of the donor star, so it could explain both low- andhigh-latitude galactic sources. Fermi observations will allowus to determine better constraints on the spectral features, thenmaking possible inferences about the actual conditions in thesources. Development of models for γ-ray production far fromthe compact object and models that take into account externaleffects is in progress.Acknowledgements. We thank Matías Reynoso for many fruitful discussions.GER acknowledges support by the Ministerio de Educación y Ciencia (Spain)under grant AYA 2007-68034-C03-01, FEDER funds. G.S.V. thanks NicolásCasco for his great help with numerical computations.ReferencesAharonian, F., Akhperjanian, A. G., Aye, K.-M., et al. (HESS Coll.) 2005,Science, 309, 746Aharonian, F. A., Khangulyan, D., & Costamante, L. 2008, MNRAS, 387, 1206Ajello, M., Greiner, J., Kabach, G., & Gehrels, N. 2008, ATel, 1497Blumenthal, G. R., & Gould, R. J. 1970, Rev. Mod. Phys., 42, 237Bogovalov, S. V., & Kelner, S. R. 2005, Astron. Rep., 49, 57Bogovalov, S. V., & Kelner, S. R. 2008, MNRAS, submitted[arXiv:0809.0429v1]Bosch-Ramon, V. 2007, Ap&SS, 309, 321Bosch-Ramon, V., Romero, G. E., & Paredes, J. M. 2005, A&A, 429, 267Bosch-Ramon, V., Romero, G. E., & Paredes, J. M. 2006, A&A, 447, 263Bulgarelli, A. Tavani, M., Pucella, G., et al. 2008, ATel, 1641Chen, A., Caraveo, P., Giuliani, A., et al. 2008, ATel, 1585Cheung, C. C. 2008, ATel, 1589Giuliani, A., Caraveo, P., Chen, A., et al. 2008, ATel, 1547Grenier, I. A. 2001, in The Nature of Unidentified High-Energy Gamma-RaySources, ed. A. Carramiñana et al. (Dordrecht: Kluwer Academic Publishers),51Grenier, I. A. 2004, in Cosmic Gamma-Ray Sources, ed. K. S. Cheng, & G. E.Romero (Dordrecht: Kluwer Academic Publishers), 47Grenier, I. A., Kaufman Bernadó, M. M., & Romero, G. E. 2005, Ap&SS, 297,109Kaufman Bernadó, M. M., Romero, G. E., & Mirabel, I. F. 2002, A&A, 385, L10Kelner, S. R., Aharonian, F. A., & Bugayov, V. V. 2006, Phys. Rev. D, 74, 034018Khangulyan, D., Aharonian, F. A., & Bosch-Ramon, V. 2008, MNRAS, 383, 467Levinson, A. 2006, Int. J. Mod. Phys. A, 21, 6115Longo, F., Chen, A., Vercellone, S., et al. 2008, ATel, 1429Mirabel, I. F., Dhawan, V., Mignani, R. P., et al. 2001, Nature, 413, 139Nolan, P. L., Tompkins, W. F., Grenier, I. A., & Michelson, P. F. 2003, ApJ, 597,615Orlandini, M., Frontera, F., Bassani, L., Landi, R., & Sguera, V. 2008, ATel, 1419Pandel, D., Kaaret, P., & Lang, C. C. 2008, ATel, 1595Pittori, C., Verrecchia, F., Santolamazza, P., et al. 2008, ATel, 1394Punsly, B., Romero, G. E., Torres, D. F., & Combi, J. A. 2000, A&A, 364, 552Reynoso, M. M., & Romero, G. E. 2009, A&A, 493, 1Romero, G. E. 2001, in The Nature of Unidentified High-Energy Gamma-RaySources, ed. A. Carramiñana et al. (Dordrecht: Kluwer Academic Publishers),65Romero, G. E., & Vila, G. S. 2008, A&A, 485, 623Vila, G. S., & Romero, G. E. 2008, Int. J. Mod. Phys. D, 17, 1903

Context. The Italian gamma-ray satellite AGILE has recently reported, the detection of some variable high-energy sources likely of galactic origin. These sources do not have any obvious counterpart at lower energies. Aims. We propose that these sources are produced in proton-dominated jets of galactic microquasars. Methods. We develop a model for microquasar jets that takes into account both primary leptons and protons and all relevant radiative processes, including secondary particle emission and gamma-ray attenuation due to pair creation in the jet. Results. We obtain spectral energy distributions that correspond to what is observed by AGILE, with most of the power concentrated between 100 MeV and 10 GeV and reaching luminosities of 1034-35 erg s-1. We make detailed spectral predictions that can be tested by the Fermi, gamma-ray telescope in the immediate future. Conclusions. We conclude that hadronic jets in galactic accreting sources can be responsible for the variable unidentified gamma-ray sources detected by AGILE.Facultad de Ciencias Astronómicas y Geofísica

Vila, G. S.

El Servicio de Difusión de la Creación Intelectual

A&A 494, L33-L36 (2009)
DOI: 10.1051/0004-6361:200811283
© ESO 2009
Astronomy 
Astrophysics
Letter to the Editor
On the nature of the AGILE galactic transient sources
G. E. Romero1,2’* and G. S. Vila1’**
* Member of CONICET. Argentina.
★★ Fellow of CONICET. Argentina.
1 Instituto Argentino de Radioastronomía (IAR), OCT La Plata (CONICET), C.C.5, (1894) Villa Elisa. Buenos Aires. Argentina 
e-mail: romero@iar-conicet. gov. ar
2 Facultad de Ciencias Astronómicas y Geofísicas. Universidad Nacional de La Plata. Paseo del Bosque s/n. 1900, La Plata. Argentina
Received 3 November 2008 / Accepted 25 December 2008
ABSTRACT
Context. The Italian gamma-ray satellite AGILE has recently reported the detection of some variable high-energy sources likely of 
galactic origin. These sources do not have any obvious counterpart at lower energies.
Aims. We propose that these sources are produced in proton-dominated jets of galactic microquasars.
Methods. We develop a model for microquasar jets that takes into account both primary leptons and protons and all relevant radiative 
processes, including secondary particle emission and gamma-ray attenuation due to pair creation in the jet.
Results. We obtain spectral energy distributions that correspond to what is observed by AGILE, with most of the power concentrated 
between 100 MeV and 10 GeV and reaching luminosities of IO i4 i5 erg s'. We make detailed spectral predictions that can be tested 
by the Fermi gamma-ray telescope in the immediate future.
Conclusions. We conclude that hadronic jets in galactic accreting sources can be responsible for the variable unidentified gamma-ray 
sources detected by AGILE.
Key words. X-rays: binaries - gamma rays: theory - radiation mechanisms: non-thermal
1. Introduction
The Italian gamma-ray satellite AGILE (Astro-rivelatore 
Gamma a Immagini Leggero) has detected several non-identified 
variable sources likely of galactic origin. These sources include 
the strong source AGL J2021+4029 located in the Cygnus re­
gion, with the center of gravity of the error box at I = 78.01 deg 
and b = 2.19 deg (Longo et al. 2008), the variable source in 
the Musca region (error box centered at / = 312.2 deg and 
b = -0.3 deg, Pittori et al. 2008), and the high galactic lati­
tude transient AGL J0229+2054, with the error box centered at 
/ = 151.7 deg and b = -36.4 deg (Bullgarelli et al. 2008). The 
Cygnus source showed some significant re-brightening after its 
discovery (Giuliani et al. 2008; Chen et al. 2008). Simultaneous 
X-ray observations with Super-AGILE did not show any coun­
terpart in the 20-60 keV band. A steady and weak source was de­
tected within the large error box by Swift/BAT (15-55 keV, see 
Ajello et al. 2008), but there is no clear relationship. Additional 
X-ray observations have been performed with XMM-Newton, 
without adding new clues (Pandel et al. 2008). Radio observa­
tions with the very large array (VLA) have shown no clear coun­
terpart (Cheung 2008).
Concerning the source in the Musca region, a potential 
archival X-ray counterpart has been claimed on the basis of 
BeppoSAX observations dating from January 2001 (Orlandini 
et al. 2008). The high-latitude transient, on the other hand, might 
be a halo galactic source or a blazar (the AGN 1ES 0229+200 
is at 61.3 arcmin from Gamma-Ray Imaging Detector error box 
centroid).
The fact that these sources are highly variable implies that 
the high-energy radiation should be produced in a compact re­
gion. The absence of detection with Super-AGILE means that 
the ratio of gamma-ray to X-ray luminosities Ly/Lx should be 
»1. These characteristics recall those of the population of vari­
able EGRET sources found in the Galactic Plane and in the 
Halo (Romero 2001; Grenier 2001, 2004; Nolan et al. 2003). 
Actually, the AGILE detections in the Cygnus region and the 
Musca region partially overlap with the location error box of the 
sources 3EG J2020+4017 and 3EG J1410-6147, respectively. It 
has been proposed that the unidentified variable gamma-ray 
sources sources at MeV-GeV energies might be high-mass mi­
croquasars with the emission dominated by inverse Compton 
up-scattering of UV stellar photons from the hot donor star 
(Kaufman-Bernado et al. 2002; Bosch-Ramon et al. 2005). On 
the Galactic Plane, the donor star could be strongly obscured, 
rendering difficult its detection. These models, however, predict 
a significant production of X-rays, something that is at odds 
with the new AGILE and Super-Agile observations. Grenier 
et al. (2005) proposed that the variable high-latitude unidentified 
sources might be old, low-mass microquasars expelled long ago 
from the Galactic Plane or from globular clusters (see Mirabel 
et al. 2001). They also showed that external Compton mod­
els cannot account for the energetics required by the sources. 
Romero & Vila (2008) and Vila & Romero (2008) showed that 
"proton" microquasars with low-mass donor stars might explain 
the halo EGRET sources through proton synchrotron radiation 
and photo-meson production.
In this Letter we propose that microquasars with proton dom­
inated jets can produce spectral energy distributions that satisfy 
all constraints imposed by AGILE observations and we make
Article published by EDP Sciences
L34 G. E. Romero and G. S. Vila: AGILE sources
Fig-1- Acceleration and cooling rates at the base of the jet for primary protons and electrons, and secondary pions and muons, calculated for 
representative values of the model parameters (proton-to-lepton energy ratio a = 1000, and primary injection spectral index a = 1.5). The value 
of the magnetic Held is B ~ 10' G. and the acceleration efficiency parameter is indicated.
some predictions that can be used to test our proposal with the 
GLAST-Fermi satellite, and ground-based Cherenkov telescope 
arrays.
only GeV energies, whereas protons can attain much higher en­
ergies, well into the PeV band.
Our model takes into account: 1) synchrotron emission from 
both types (electrons and protons) of primary particles, as well 
as emission from secondary leptons (electrons, positrons, and 
muons) and hadrons (charged pions); 2) inverse Compton (IC) 
emission from all leptons in the strong total radiation field in 
the emitting region; 3) photo-pair and photo-meson production 
by both protons and pions; 4) inelastic collisions between rela­
tivistic protons in the jet and the cold material that forms most 
of the same outflow (see Bosch-Ramon et al. 2006, for details 
of the physics of a cold matter dominated jet); 5) relativis­
tic Bremsstrahlung from electrons and muons; and 6) internal 
photon absorption in the local radiation fields (calculated as in 
Aharonian et al. 2008); 7) re-injection of secondary pairs, which 
is negligible due to the low opacity of the proton-dominated jet 
(see Romero & Vila 2008, for a discussion). The effect of losses 
of muons and pions in a strong magnetic field on the resulting 
gamma-ray and secondary lepton spectra has been recently dis­
cussed by Reynoso & Romero (2009) and we adopt here their 
treatment. For the other processes we follow the classical for­
mulae (Blumenthal & Gould 1970), the photo-meson production 
treatment already used by Romero & Vila (2008), and the ex­
pressions given by Kelner et al. (2006) for pp interactions (this 
formalism is only valid for proton energies above 0.1 TeV; at 
lower energies we use the standard treatment).
2. The model
The model we are going to present is based on the galactic jet
model described by Romero & Vila (2008), but it incorporates 
several refinements. The jet is assumed to carry most of the ac­
cretion power, in accordance with the dissipationless disk model 
of Bogovalov & Kelner (2005, 2008).
A small fraction of the jet power is transformed into relativis­
tic particles in a "one-zone" acceleration region located close to 
the compact object (Khangulyan et al. 2008). In this region, the 
magnetic field ( obtained by equipartition) is extremely high ( of 
the order of 102 * * * * 7 G). This produces the immediate cooling of pri­
mary electrons and significant cooling of protons. In Fig. 1 we 
show the cooling rates for both primary electrons and protons in 
the acceleration region, as well as the cooling and decaying rate 
of secondary muons and pions. The maximum energies of the 
primary particles are obtained equating the cooling rates to the 
acceleration rate, which is assumed to be produced by mildly rel­
ativistic shocks at the base of the jet through a first-order Fermi 
mechanism. In this work we adopt an acceleration efficiency of 
the order of 10%, which rougly corresponds to a shock velocity 
of 0.3 c and a mean free path similar to the Larmor radius. We see 
that electrons, eVen for such a high acceleration efficiency, reach
G. E. Romero and G. S. Vila: AGILE sources L35
Fig.2. Spectral energy distributions of a proton-dominated microquasar (a = 1000). Each panel corresponds to a different acceleration efficiency 
(z/ = 0.1 on the left, q = 0.01 on the right). Fermi point source sensitivity is indicated by a thick blue line.
The ratio of relativistic protons to electrons a in the jet is 
unknown and is used as the basic free parameter in our model. 
We have calculated a number of models for different acceler­
ation efficiencies, and a set of standard parameters: black hole 
mass ~10 Mq, accretion rate ~1037 ergs-1 (i.e., ~10-2 of the 
Eddington luminosity), injection oc E-1-5, viewing angle 30°, jet 
bulk Lorentz factor T = 1.5, and location of the acceleration 
zone at a ~ 108 cm ( see Romero & Vila 2008).
The transport equation is solved for all types of particles. At 
steady state it reads:
Here,/? is the speed of the electron/positron in the center of mo­
mentum frame, i.e.,
(1 -/?•) =
2m2 c4
(1 - u)Eye' 0 </?< 1. (5)
The threshold energy eth is defined by/? = 1 with if = 0.
d_
dE
dE
N(E,z)di loss
N(E.z)
-i----------------
Iesc
= 2(E). (1)
where ieSc is the particle escape time from the acceleration re­
gion (iesc ~ Az/njet) and Q is the injection function that can be 
normalized in accordance with the energy budget of relativistic 
particles through:
Le,p = I d3r I dEe.pEe.p Ge.pfEe.p). (2)
JV Je™
The minimum kinetic energy is taken to be of the order of the 
rest mass energy of the corresponding particle. The equation for 
pions and muons has an additional term N(E,z)lti<x, that takes 
into account the decay of the particles on a timescale idee that 
depends on the energy in the lab system.
In general, variability can be obtained introducing a variable 
injection Q(E, t). Once the particle distributions are known, the 
radiative output can be calculated as mentioned above. The inter­
nal attenuation results from photon annihilation: y + y -» e- + e+. 
The opacity for a gamma ray of energy Ey is:
T(Ey) - ( (1 - H)
J-l
cr(Ey, e, u)
x n(e,z) du de d/, (3)
where n(e, z) is the photon number density at energy e and lo­
cation z, u = cos is the angle between the momenta of the 
colliding photons, I is the y-ray photon path, and the cross sec­
tion for the interaction is given by (e.g. Levinson 2006):
3. Results
In Fig. 2 we show the spectral energy distribution (SED) cal­
culated for a proton dominated (a = 1000) jet, with a total 
power of 1037 ergs-1. We show the different contributions from 
all significant cooling processes for all primary and secondary 
particles, in the case of two different acceleration efficiencies 7/ 
(0.1 and 0.01, from left to right, respectively). Bremsstrahlung 
and IC losses are negligible, since synchrotron radiation is the 
dominat cooling channel for leptons. The different acceleration 
efficiencies not only affect the maximum energy of the radia­
tion but modify the whole shape of the SED, since the dom­
inant synchrotron losses follow the square of the energy and 
depend significantly on the mass of the particles, resulting in 
a global photon redistribution. In both cases, however, the peak 
of the SED is determined by proton synchrotron radiation, fol­
lowed by pion-synchrotron emission. For the higher efficiency 
the synchrotron peak is sharper, reaching almost 1035 ergs-1. 
In the case of a lower efficiency, the peak is slightly above 
1034 ergs-1. In the first case most of the emission is concen­
trated in the range 108-1012 eV, whereas in the second it is be­
tween 108 and 101" eV, with a soft slope beyond 1 ()y eV. In both 
cases there is a high ratio LyILx, in accordance with what is 
inferred from AGILE observations. Soft X-rays, due mainly to 
electron synchrotron radiation, are at the level of 1032 ergs-1. 
The hard X-ray component is dominated by muon synchrotron 
emission. Contrary to models with equipartition (a = 1) in rel­
ativistic particles, photo-meson production is not significant in 
proton dominated jets, since the synchrotron field is relatively 
weak (see Romero & Vila 2008).
The quoted luminosities correspond to the flux reported by 
AGILE and the upper X-ray limits, at a distance of the order of 
~2 kpc. And accurate determination of the distance can be used 
to constrain the energy budget of the sources. At TeV energies 
the luminosities (due mainly to pp interactions) are at the level 
of 1032—1033 erg s-1, and hence the sources are not detectable by
L36 G. E. Romero and G. S. Vila: AGILE sources
current Cherenkov atmospheric telescopes, although they could 
fall within the sensitivity of upgraded instruments like HESS II 
and MAGIC II.
4. Discussion and summary
A crucial feature of a synchrotron-proton dominated jet is that a 
very strong magnetic field is necessary to produce detectable ra­
diation in y-rays. This means that the acceleration region should 
be located very close to the compact object (at ~108 cm in our 
models). Intrinsic absorption is then not very important since 
the low energy fields, which are responsible for the opacity to 
gamma-ray propagation, are weak. In models with a high con­
tent of primary electrons these effects, at the base of the jet, are 
very significant leading to a complete suppression of all emis­
sion above ~ 100 GeV (Romero & Vila 2008). Such models pro­
duce a huge amount of X-rays, something that is not observed in 
the unidentified MeV-GeV sources.
In our model we introduce a hard injection spectrum in or­
der to achieve a strong contrast between leptonic and hadronic 
peaks. A softer injection would reduce the Ly/Lx ratio. We 
notice that the losses in the high magnetic field strongly af­
fect the overall leptonic particle spectrum. Observations with 
the LAT instrument of the Fermi satellite will allow us to de­
termine the photon spectrum of these sources in the range 
100 MeV-100 GeV, allowing a better determination of the phys­
ical parameters.
We remark that IC models are possible for these type of 
sources only if the radiation is produced in a region with a sig­
nificantly smaller (—10—100 G) magnetic field, see for example 
Punsly et al. (2000). Such a field would occur at distances of 
1012—1013 cm from the compact object. How the injector of rel­
ativistic particles could be located there will be discussed else­
where (see, nonetheless, Bosch-Ramon 2007).
In summary, we propose a complete lepto/hadronic jet model 
to explain the unidentified variable AGILE sources. This model 
assumes a strong component of relativistic primary protons and 
takes into account all radiative processes that might occur at the 
base of the jets. The predicted SEDs are in accordance with what 
we know about of these sources. The jet model is independent 
of the nature of the donor star, so it could explain both low- and 
high-latitude galactic sources. Fermi observations will allow 
us to determine better constraints on the spectral features, then 
making possible inferences about the actual conditions in the 
sources. Development of models for y-ray production far from 
the compact object and models that take into account external 
effects is in progress.
Acknowledgements. We thank Matias Reynoso for many fruitful discussions. 
GER acknowledges support by the Ministerio de Educación y Ciencia (Spain) 
under grant AYA 2007-68034-C03-01, EEDER funds. G.S.V. thanks Nicolás 
Casco for his great help with numerical computations.
References
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Science. 309, 746
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Vila. G. S„ & Romero. G. E. 2008, Int. J. Mod. Phys. D. 17. 1903


Crossref

A&A 494, L33-L36 (2009)DOI: 10.1051/0004-6361:200811283© ESO 2009Astronomy AstrophysicsLetter to the EditorOn the nature of the AGILE galactic transient sourcesG. E. Romero1,2’* and G. S. Vila1’*** Member of CONICET. Argentina.★★ Fellow of CONICET. Argentina.1 Instituto Argentino de Radioastronomía (IAR), OCT La Plata (CONICET), C.C.5, (1894) Villa Elisa. Buenos Aires. Argentina e-mail: romero@iar-conicet. gov. ar2 Facultad de Ciencias Astronómicas y Geofísicas. Universidad Nacional de La Plata. Paseo del Bosque s/n. 1900, La Plata. ArgentinaReceived 3 November 2008 / Accepted 25 December 2008ABSTRACTContext. The Italian gamma-ray satellite AGILE has recently reported the detection of some variable high-energy sources likely of galactic origin. These sources do not have any obvious counterpart at lower energies.Aims. We propose that these sources are produced in proton-dominated jets of galactic microquasars.Methods. We develop a model for microquasar jets that takes into account both primary leptons and protons and all relevant radiative processes, including secondary particle emission and gamma-ray attenuation due to pair creation in the jet.Results. We obtain spectral energy distributions that correspond to what is observed by AGILE, with most of the power concentrated between 100 MeV and 10 GeV and reaching luminosities of IO i4 i5 erg s'. We make detailed spectral predictions that can be tested by the Fermi gamma-ray telescope in the immediate future.Conclusions. We conclude that hadronic jets in galactic accreting sources can be responsible for the variable unidentified gamma-ray sources detected by AGILE.Key words. X-rays: binaries - gamma rays: theory - radiation mechanisms: non-thermal1. IntroductionThe Italian gamma-ray satellite AGILE (Astro-rivelatore Gamma a Immagini Leggero) has detected several non-identified variable sources likely of galactic origin. These sources include the strong source AGL J2021+4029 located in the Cygnus re­gion, with the center of gravity of the error box at I = 78.01 deg and b = 2.19 deg (Longo et al. 2008), the variable source in the Musca region (error box centered at / = 312.2 deg and b = -0.3 deg, Pittori et al. 2008), and the high galactic lati­tude transient AGL J0229+2054, with the error box centered at / = 151.7 deg and b = -36.4 deg (Bullgarelli et al. 2008). The Cygnus source showed some significant re-brightening after its discovery (Giuliani et al. 2008; Chen et al. 2008). Simultaneous X-ray observations with Super-AGILE did not show any coun­terpart in the 20-60 keV band. A steady and weak source was de­tected within the large error box by Swift/BAT (15-55 keV, see Ajello et al. 2008), but there is no clear relationship. Additional X-ray observations have been performed with XMM-Newton, without adding new clues (Pandel et al. 2008). Radio observa­tions with the very large array (VLA) have shown no clear coun­terpart (Cheung 2008).Concerning the source in the Musca region, a potential archival X-ray counterpart has been claimed on the basis of BeppoSAX observations dating from January 2001 (Orlandini et al. 2008). The high-latitude transient, on the other hand, might be a halo galactic source or a blazar (the AGN 1ES 0229+200 is at 61.3 arcmin from Gamma-Ray Imaging Detector error box centroid).The fact that these sources are highly variable implies that the high-energy radiation should be produced in a compact re­gion. The absence of detection with Super-AGILE means that the ratio of gamma-ray to X-ray luminosities Ly/Lx should be »1. These characteristics recall those of the population of vari­able EGRET sources found in the Galactic Plane and in the Halo (Romero 2001; Grenier 2001, 2004; Nolan et al. 2003). Actually, the AGILE detections in the Cygnus region and the Musca region partially overlap with the location error box of the sources 3EG J2020+4017 and 3EG J1410-6147, respectively. It has been proposed that the unidentified variable gamma-ray sources sources at MeV-GeV energies might be high-mass mi­croquasars with the emission dominated by inverse Compton up-scattering of UV stellar photons from the hot donor star (Kaufman-Bernado et al. 2002; Bosch-Ramon et al. 2005). On the Galactic Plane, the donor star could be strongly obscured, rendering difficult its detection. These models, however, predict a significant production of X-rays, something that is at odds with the new AGILE and Super-Agile observations. Grenier et al. (2005) proposed that the variable high-latitude unidentified sources might be old, low-mass microquasars expelled long ago from the Galactic Plane or from globular clusters (see Mirabel et al. 2001). They also showed that external Compton mod­els cannot account for the energetics required by the sources. Romero & Vila (2008) and Vila & Romero (2008) showed that "proton" microquasars with low-mass donor stars might explain the halo EGRET sources through proton synchrotron radiation and photo-meson production.In this Letter we propose that microquasars with proton dom­inated jets can produce spectral energy distributions that satisfy all constraints imposed by AGILE observations and we makeArticle published by EDP SciencesL34 G. E. Romero and G. S. Vila: AGILE sourcesFig-1- Acceleration and cooling rates at the base of the jet for primary protons and electrons, and secondary pions and muons, calculated for representative values of the model parameters (proton-to-lepton energy ratio a = 1000, and primary injection spectral index a = 1.5). The value of the magnetic Held is B ~ 10' G. and the acceleration efficiency parameter is indicated.some predictions that can be used to test our proposal with the GLAST-Fermi satellite, and ground-based Cherenkov telescope arrays.only GeV energies, whereas protons can attain much higher en­ergies, well into the PeV band.Our model takes into account: 1) synchrotron emission from both types (electrons and protons) of primary particles, as well as emission from secondary leptons (electrons, positrons, and muons) and hadrons (charged pions); 2) inverse Compton (IC) emission from all leptons in the strong total radiation field in the emitting region; 3) photo-pair and photo-meson production by both protons and pions; 4) inelastic collisions between rela­tivistic protons in the jet and the cold material that forms most of the same outflow (see Bosch-Ramon et al. 2006, for details of the physics of a cold matter dominated jet); 5) relativis­tic Bremsstrahlung from electrons and muons; and 6) internal photon absorption in the local radiation fields (calculated as in Aharonian et al. 2008); 7) re-injection of secondary pairs, which is negligible due to the low opacity of the proton-dominated jet (see Romero & Vila 2008, for a discussion). The effect of losses of muons and pions in a strong magnetic field on the resulting gamma-ray and secondary lepton spectra has been recently dis­cussed by Reynoso & Romero (2009) and we adopt here their treatment. For the other processes we follow the classical for­mulae (Blumenthal & Gould 1970), the photo-meson production treatment already used by Romero & Vila (2008), and the ex­pressions given by Kelner et al. (2006) for pp interactions (this formalism is only valid for proton energies above 0.1 TeV; at lower energies we use the standard treatment).2. The modelThe model we are going to present is based on the galactic jetmodel described by Romero & Vila (2008), but it incorporates several refinements. The jet is assumed to carry most of the ac­cretion power, in accordance with the dissipationless disk model of Bogovalov & Kelner (2005, 2008).A small fraction of the jet power is transformed into relativis­tic particles in a "one-zone" acceleration region located close to the compact object (Khangulyan et al. 2008). In this region, the magnetic field ( obtained by equipartition) is extremely high ( of the order of 102 * * * * 7 G). This produces the immediate cooling of pri­mary electrons and significant cooling of protons. In Fig. 1 we show the cooling rates for both primary electrons and protons in the acceleration region, as well as the cooling and decaying rate of secondary muons and pions. The maximum energies of the primary particles are obtained equating the cooling rates to the acceleration rate, which is assumed to be produced by mildly rel­ativistic shocks at the base of the jet through a first-order Fermi mechanism. In this work we adopt an acceleration efficiency of the order of 10%, which rougly corresponds to a shock velocity of 0.3 c and a mean free path similar to the Larmor radius. We see that electrons, eVen for such a high acceleration efficiency, reachG. E. Romero and G. S. Vila: AGILE sources L35Fig.2. Spectral energy distributions of a proton-dominated microquasar (a = 1000). Each panel corresponds to a different acceleration efficiency (z/ = 0.1 on the left, q = 0.01 on the right). Fermi point source sensitivity is indicated by a thick blue line.The ratio of relativistic protons to electrons a in the jet is unknown and is used as the basic free parameter in our model. We have calculated a number of models for different acceler­ation efficiencies, and a set of standard parameters: black hole mass ~10 Mq, accretion rate ~1037 ergs-1 (i.e., ~10-2 of the Eddington luminosity), injection oc E-1-5, viewing angle 30°, jet bulk Lorentz factor T = 1.5, and location of the acceleration zone at a ~ 108 cm ( see Romero & Vila 2008).The transport equation is solved for all types of particles. At steady state it reads:Here,/? is the speed of the electron/positron in the center of mo­mentum frame, i.e.,(1 -/?•) =2m2 c4(1 - u)Eye'0 </?< 1. (5)The threshold energy eth is defined by/? = 1 with if = 0.d_dEdEN(E,z)di lossN(E.z)-i----------------Iesc= 2(E). (1)where ieSc is the particle escape time from the acceleration re­gion (iesc ~ Az/njet) and Q is the injection function that can be normalized in accordance with the energy budget of relativistic particles through:Le,p = I d3r I dEe.pEe.p Ge.pfEe.p). (2)JV Je™The minimum kinetic energy is taken to be of the order of the rest mass energy of the corresponding particle. The equation for pions and muons has an additional term N(E,z)lti<x, that takes into account the decay of the particles on a timescale idee that depends on the energy in the lab system.In general, variability can be obtained introducing a variable injection Q(E, t). Once the particle distributions are known, the radiative output can be calculated as mentioned above. The inter­nal attenuation results from photon annihilation: y + y -» e- + e+. The opacity for a gamma ray of energy Ey is:T(Ey) - ( (1 - H)J-lcr(Ey, e, u)x n(e,z) du de d/, (3)where n(e, z) is the photon number density at energy e and lo­cation z, u = cos is the angle between the momenta of the colliding photons, I is the y-ray photon path, and the cross sec­tion for the interaction is given by (e.g. Levinson 2006):3. ResultsIn Fig. 2 we show the spectral energy distribution (SED) cal­culated for a proton dominated (a = 1000) jet, with a total power of 1037 ergs-1. We show the different contributions from all significant cooling processes for all primary and secondary particles, in the case of two different acceleration efficiencies 7/ (0.1 and 0.01, from left to right, respectively). Bremsstrahlung and IC losses are negligible, since synchrotron radiation is the dominat cooling channel for leptons. The different acceleration efficiencies not only affect the maximum energy of the radia­tion but modify the whole shape of the SED, since the dom­inant synchrotron losses follow the square of the energy and depend significantly on the mass of the particles, resulting in a global photon redistribution. In both cases, however, the peak of the SED is determined by proton synchrotron radiation, fol­lowed by pion-synchrotron emission. For the higher efficiency the synchrotron peak is sharper, reaching almost 1035 ergs-1. In the case of a lower efficiency, the peak is slightly above 1034 ergs-1. In the first case most of the emission is concen­trated in the range 108-1012 eV, whereas in the second it is be­tween 108 and 101" eV, with a soft slope beyond 1 ()y eV. In both cases there is a high ratio LyILx, in accordance with what is inferred from AGILE observations. Soft X-rays, due mainly to electron synchrotron radiation, are at the level of 1032 ergs-1. The hard X-ray component is dominated by muon synchrotron emission. Contrary to models with equipartition (a = 1) in rel­ativistic particles, photo-meson production is not significant in proton dominated jets, since the synchrotron field is relatively weak (see Romero & Vila 2008).The quoted luminosities correspond to the flux reported by AGILE and the upper X-ray limits, at a distance of the order of ~2 kpc. And accurate determination of the distance can be used to constrain the energy budget of the sources. At TeV energies the luminosities (due mainly to pp interactions) are at the level of 1032—1033 erg s-1, and hence the sources are not detectable byL36 G. E. Romero and G. S. Vila: AGILE sourcescurrent Cherenkov atmospheric telescopes, although they could fall within the sensitivity of upgraded instruments like HESS II and MAGIC II.4. Discussion and summaryA crucial feature of a synchrotron-proton dominated jet is that a very strong magnetic field is necessary to produce detectable ra­diation in y-rays. This means that the acceleration region should be located very close to the compact object (at ~108 cm in our models). Intrinsic absorption is then not very important since the low energy fields, which are responsible for the opacity to gamma-ray propagation, are weak. In models with a high con­tent of primary electrons these effects, at the base of the jet, are very significant leading to a complete suppression of all emis­sion above ~ 100 GeV (Romero & Vila 2008). Such models pro­duce a huge amount of X-rays, something that is not observed in the unidentified MeV-GeV sources.In our model we introduce a hard injection spectrum in or­der to achieve a strong contrast between leptonic and hadronic peaks. A softer injection would reduce the Ly/Lx ratio. We notice that the losses in the high magnetic field strongly af­fect the overall leptonic particle spectrum. Observations with the LAT instrument of the Fermi satellite will allow us to de­termine the photon spectrum of these sources in the range 100 MeV-100 GeV, allowing a better determination of the phys­ical parameters.We remark that IC models are possible for these type of sources only if the radiation is produced in a region with a sig­nificantly smaller (—10—100 G) magnetic field, see for example Punsly et al. (2000). Such a field would occur at distances of 1012—1013 cm from the compact object. How the injector of rel­ativistic particles could be located there will be discussed else­where (see, nonetheless, Bosch-Ramon 2007).In summary, we propose a complete lepto/hadronic jet model to explain the unidentified variable AGILE sources. This model assumes a strong component of relativistic primary protons and takes into account all radiative processes that might occur at the base of the jets. The predicted SEDs are in accordance with what we know about of these sources. The jet model is independent of the nature of the donor star, so it could explain both low- and high-latitude galactic sources. Fermi observations will allow us to determine better constraints on the spectral features, then making possible inferences about the actual conditions in the sources. Development of models for y-ray production far from the compact object and models that take into account external effects is in progress.Acknowledgements. We thank Matias Reynoso for many fruitful discussions. GER acknowledges support by the Ministerio de Educación y Ciencia (Spain) under grant AYA 2007-68034-C03-01, EEDER funds. G.S.V. thanks Nicolás Casco for his great help with numerical computations.ReferencesAharonian, E. Akhperjanian, A. G.. Aye. K.-M., et al. (HESS Coll.) 2005,Science. 309, 746Aharonian, F. A.. Khangulyan, D.. & Costamante, L. 2008, MNRAS, 387, 1206 Ajello. M„ Greiner. J.. Kabach, G„ & Gehrels, N. 2008, ATel. 1497 Blumenthal. G. R„ & Gould. R. J. 1970, Rev. Mod. Phys., 42. 237 Bogovalov, S. V.. & Keiner, S. R. 2005, Astron. Rep., 49. 57Bogovalov, S. V.. & Keiner, S. R. 2008, MNRAS. submittediarXiv : ®8®9. ®429vl]Bosch-Ramon, V. 2007, Ap&SS, 309, 321Bosch-Ramon, V.. Romero. G. E.. & Paredes. J. M. 2005, A&A. 429, 267 Bosch-Ramon, V.. Romero. G. E.. & Paredes. J. M. 2006, A&A. 447, 263 Bulgarelli, A. Tavani, M.. Pucella, G.. et al. 2008, ATel. 1641Chen. A.. Caraveo, R. Giuliani. A., et al. 2008, ATel. 1585 Cheung. C. C. 2008, ATel. 1589Giuliani. A.. Caraveo. R. Chen. A., et al. 2008, ATel. 1547Grenier. I. A. 2001, in The Nature of Unidentified High-Energy Gamma-RaySources, ed. A. Carraminana et al. (Dordrecht: Kluwer Academic Publishers). 51Grenier. I. A. 2004, in Cosmic Gamma-Ray Sources, ed. K. S. Cheng. & G. E. Romero (Dordrecht: Kluwer Academic Publishers). 47Grenier. I. A.. Kaufman Bernadó, M. M.. & Romero. G. E. 2005, Ap&SS. 297, 109Kaufman Bernadó. M. M.. Romero. G. E.. & Mirabel. I. F. 2002, A&A. 385, L10 Keiner, S. R.. Aharonian. E A.. & Bugayov, V. V. 2006, Phys. Rev. D. 74. 034018 Khangulyan. D.. Aharonian. F. A.. & Bosch-Ramon, V. 2008, MNRAS. 383, 467 Levinson. A. 2006, Int. J. Mod. Phys. A. 21. 6115Longo. E. Chen. A.. Vercellone, S.. et al. 2008, ATel. 1429Mirabel. I. E. Dhawan, V.. Mignani, R. R. et al. 2001, Nature. 413, 139Nolan. P. L.. Tompkins. W. E. Grenier. I. A.. & Michelson. P. F. 2003, ApJ, 597, 615Orlandini, M.. Frontera. F. Bassani. L.. Landi, R.. & Sguera, V. 2008, ATel. 1419 Pandel. D.. Kaaret, R. & Lang, C. C. 2008, ATel. 1595Pittori. C.. Verrecchia. E. Santolamazza, R. et al. 2008, ATel. 1394Punsly. B.. Romero. G. E.. Torres. D. E. & Combi. J. A. 2000, A&A. 364, 552 Reynoso. M. M.. & Romero. G. E. 2009, A&A. 493, 1Romero. G. E. 2001, in The Nature of Unidentified High-Energy Gamma-RaySources, ed. A. Carraminana et al. (Dordrecht: Kluwer Academic Publishers). 65Romero. G. E„ & Vila. G. S. 2008, A&A. 485, 623Vila. G. S„ & Romero. G. E. 2008, Int. J. Mod. Phys. D. 17. 1903

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