Unlike high-mass gamma-ray binaries, low-mass microquasars lack external
sources of radiation and matter that could produce high-energy emission through
interactions with relativistic particles. In this work we consider the
synchrotron emission of protons and leptons that populate the jet of a low-mass
microquasar. In our model photohadronic and inverse Compton (IC) interactions
with synchrotron photons produced by both protons and leptons result in a
high-energy tail of the spectrum. We also estimate the contribution from
secondary pairs injected through photopair production. The high-energy emission
is dominated by radiation of hadronic origin, so we can call these objects
proton microquasars.Comment: 4 pages, 2 figures, accepted for publication in the International
  Journal of Modern Physics D, proceedings of HEPRO meeting, held in Dublin, in
  September 200

Aharonian F. A.

Corbel S.

Falcke H.

Fender R. P.

G. E. ROMERO

G. S. VILA

Ghisellini G.

Markoff S.

Romero G. E.

English

arXiv

Crossref

MODELS FOR GAMMA-RAY PRODUCTION IN LOW-MASS MICROQUASARS

Unlike high-mass gamma-ray binaries, low-mass microquasars lack external  sources of radiation and matter that could produce high-energy emission  through interactions with relativistic particles. In this work we  consider the synchrotron emission of protons and leptons that populate  the jet of a low-mass microquasar. In our model photohadronic and  inverse Compton (IC) interactions with synchrotron photons produced by  both protons and leptons result in a high-energy tail of the spectrum.  We also estimate the contribution from secondary pairs injected through  photopair production. The high-energy emission is dominated by radiation  of hadronic origin, so we can call these objects proton microquasars.Fil: 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; ArgentinaFil: 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; ArgentinaHigh-Energy Phenomena in Relativistic Outflows I (HEPRO I)DublinIrlandaDublin Institute for Advanced Studie

Vila, Gabriela Soledad

Romero, Gustavo Esteban

CONICET Digital

arXiv:0805.2085v1  [astro-ph]  14 May 2008May 14, 2008 15:19 WSPC - Proceedings Trim Size: 11in x 8.5in vila˙HEPRO1Models for gamma-ray production in low-mass microquasarsG. S. Vila‡Instituto Argentino de Radioastronomı́a (IAR-CONICET),C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina‡E-mail: gvila@iar-conicet.gov.arG. E. Romero∗,∗∗Instituto Argentino de Radioastronomı́a (IAR-CONICET),C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina∗E-mail: romero@iar-conicet.gov.arFacultad de Ciencias Astronómicas y Geofsicas, UNLP,Paseo del Bosque s/n, 1900 La Plata, Buenos Aires, Argentina∗∗E-mail: romero@fcaglp.unlp.edu.arUnlike high-mass gamma-ray binaries, low-mass microquasars lack external sources of radiation and matter that couldproduce high-energy emission through interactions with relativistic particles. In this work we consider the synchrotronemission of protons and leptons that populate the jet of a low-mass microquasar. In our model photohadronic andinverse Compton (IC) interactions with synchrotron photons produced by both protons and leptons result in a high-energy tail of the spectrum. We also estimate the contribution from secondary pairs injected through photopairproduction. The high-energy emission is dominated by radiation of hadronic origin, so we can call these objects‘proton microquasars’.Keywords: X-rays:binaries - gamma rays: theory - radiation mechanisms: non-thermal1. IntroductionMicroquasars are accreting binary systems thataccelerate particles up to relativistic energies, asdemonstrated by the detection of synchrotron radi-ation from radio up to X-ray energies.1,2 In micro-quasars with a high-mass donor star, the relativis-tic jets ejected from the surroundings of the com-pact object must traverse photon and matter fieldsproduced by the star. This results in interactionsthat can produce high-energy gamma rays, as dis-cussed by several authors.3,4 In fact, three high-mass microquasars have already been detected asgamma ray emitters.5–7 Low-mass microquasars, onthe contrary, have not been detected so far. High-energy emission from these objects has been mod-eled in Refs. 4, 8, 9 and 10. These authors have fo-cused only on leptonic processes. Here we presenta new family of models for the non-thermal emis-sion of low-mass microquasars which include severalimprovements. Among other things, we consider anhadronic content in the jets, proton synchrotron ra-diation, photomeson and photopair production, lep-tonic processes for primary and secondary particles,etc. Relativistic particle distributions are calculatedin self-consistent way, solving the transport equationunder reasonable assumptions. Our models predictthat low-mass microquasars might be detectable byinstruments like GLAST and Cherenkov telescopeslike VERITAS, HESS and MAGIC, and other futureextensions of these arrays.2. ModelWe consider an inhomogeneous jet of conical geome-try perpendicular to the orbital plane of the binary.The jet is launched at a distance z0 from the com-pact object and has an initial radius r0 = 0.1 z0. Theoutflow carries a significant fraction of the accretionpower,11,12 Ljet = qjet Laccr. We further assume thata fraction Lrel = ξLjet is in the form of relativisticprotons and leptons, Lrel = Lp + Le. We relate theenergy budget of both species as Lp = aLe. Parti-cles are accelerated by diffusive shock accelerationleading to a power-law injectionQ (E, z) =Q0zE−α [Q] = erg−1s−1cm−3. (1)We consider a canonical value α = 2.2 for the spec-tral index, and we assume that the injection ratedecreases with the distance to the compact object.Acceleration of particles takes place in a compact re-May 14, 2008 15:19 WSPC - Proceedings Trim Size: 11in x 8.5in vila˙HEPRO2gion centered at zacc, with a thickness ∆z = 5r (zacc).The efficiency of the acceleration mechanism is char-acterized by the parameter η, in such a way that theacceleration rate ist−1acc = ηceBE−1. (2)We assume an efficient accelerator with η = 0.1. Themagnetic field in the jet decreases as the jet expandsas B = B0(z0/z). Its value at z0 is determined byequipartition between magnetic and kinetic energydensities, UB(z0) = Ukinjet (z0). This yields B0 ∼ 107 Gand makes synchrotron losses the dominant channelof radiative particle cooling. For a particle of mass mand energy E, the synchrotron cooling rate is givenbyt−1synchr =43(mem)3 cσT UBmec2Emc2. (3)Particle distributions in the steady stateNp,e(Ep,e, z) (erg−1cm−3) are calculated solving atransport equation that takes into account parti-cle injection, cooling and particle escape from theacceleration region,∂∂E[dEdt∣∣∣∣lossN (E, z)]+N (E, z)tesc= Q (E, z) , (4)where dE/dt|loss is the sum of all radiative and me-chanical losses for each type of particle.We estimate the escape time as tesc = zacc/vjet,where vjet ∼ 0.7 c corresponds to a bulk Lorentz fac-tor Γjet = 1.5, typical of a jet in the low-hard stateof the microquasars.13 The proton spectrum goes asNp ∝ E−2.2p for all Ep. In the case of electrons, in-stead, synchrotron losses introduce a steepening thatleads to Ne ∝ E−3.2e at all energies before the cutoff.The acceleration and cooling rates for both protonsand electrons at the acceleration region are shown inFigure 1. The maximum energy of all particles is de-termined by the synchrotron cooling rate. In Table1 we show some representative values of the relevantparameters.Table 1. Values of the various parameters characterizing the jet andthe distributions of relativistic particles that were used in the calcula-tions.Parameter ValueLaccr: accretion power 1.7× 1039 erg s−1(1)qjet: coupling constant accretion-jet power 0.1z0: jet’s launching point 2× 108 cm(2)zacc: location of the acceleration region 109 cm∆z: thickness of the acceleration region 5r(zacc)Γjet: jet’s bulk Lorentz factor 1.5(1)θ: viewing angle 30◦ξ: ratio Lrel/Ljet 0.1, 10−3a: ratio Lp/Le 1, 100, 103η: acceleration efficiency 0.1B0: magnetic field at z0 2× 107 GEminp,e: minimum particle energy 100 mp,ec2Emaxp,e: maximum particle energy p, e− 1010, 7× 1016 eV(3)(1) Typical value for the low-mass microquasar XTE J1118+48014(2) 50RSchw for a 8 M⊙ black hole as XTE J1118+48015(3) These are the maximum values obtained along the jet.3. ResultsIn addition to synchrotron radiation, we considerother two photon production mechanisms: IC andproton-photon colissions (pγ) on the synchrotron ra-diation fields of both protons and leptons. The laterprocess has two main branches: photomeson produc-tionp + γ → p + aπ0 + b(π+ + π−)and photopair productionp + γ → p + e+ + e−The spectrum from the decay of π0 → 2γ is esti-mated following Ref. 16. For the injection of elec-tron/positron pairs we use the formulae given byRefs. 17 and 18. The IC spectra are calculated inthe local approximation of Ref. 19, whereas for syn-chrotron radiation we use the classical expressions,as given, for example, by Ref. 20. All the calcula-tions are performed in the jet co-moving referenceframe and then transformed to the observer frameusing the appropriate Doppler factor. We fix θ = 30◦as the viewing angle for numerical estimates. Sincethe jet is only mildly relativistic, none of our resultsdepends strongly on this value as long as it is notvery close to zero.May 14, 2008 15:19 WSPC - Proceedings Trim Size: 11in x 8.5in vila˙HEPRO38 9 10 11 12 13123456789101112  Log t-1 (s-1 )Log Ee (eV) IC synchrotron acceleration adiabatic11 12 13 14 15 16 17-4-3-2-1012345678Log t-1 (s-1 )Log Ep (eV) synchrotron adiabatic acceleration pFig. 1. Acceleration and cooling rates at zacc for electrons (left panel) and protons (right panel), for a = 1. In the case of protons,the pγ cooling rate is the sum of the photopair and photomeson production contributions. The dominant channel of energy lossfor electrons is synchrotron radiation. For protons, adiabatic (low energies) and synchrotron (high energies) losses are the maincooling mechanisms.Figure 2 (b), (c) and (d) correspond to a proton-dominated jet. The peak of the spectral energy dis-tribution in the three cases is due to the proton syn-chrotron radiation. At energies beyond 10 TeV, thedominant contribution comes from the decay of neu-tral pions produced through photomeson mechanism.In the energy range between 1 TeV and 100 TeV, thesynchrotron radiation from photopairs is systemati-cally more significant than that generated by sec-ondary pairs from charged pion decays.Case (a) corresponds to equipartition betweenprotons and leptons (a = 1). As expected, the leptonsynchrotron luminosity rises. A denser photon fieldresults in an enhanced (although relatively small)leptonic inverse Compton (IC) emission, but an ap-preciable pγ luminosity at very high energies, charac-terized by a hard spectrum. In case (d) the relativis-tic particle content is smaller, ξ = Lrel/Ljet = 10−3.This reduces synchrotron, IC and pγ luminosities byone and two orders of magnitude, respectively.Concerning the radio emission, our models,which have a high minimum injection energy forelectrons, are not intended to reproduce the observ-able radio spectrum of systems like XTE J1118+480(Lradio ∼ 1028−29 erg s−121). However, models withEmin ∼ 2mec2 can yield the expected luminosity andspectrum in this band.224. ConclusionsLow-mass microquasars with steady jets can be sig-nificant sources of high-energy radiation if they havean important hadronic content. These objects can befound out of the galactic plane, because they havelarge proper motions.23 They can also be sources ofneutrinos through charged pion decays from pγ inter-actions, as already noticed by Ref. 24. These ‘protonmicroquasars’ might be detected in the near futureby GLAST and ground-based Cherenkov telescopearrays, since gamma-ray luminosities in the range1033 − 1036 erg s−1 between 1 GeV and 10 TeV canbe expected under a variety of conditions. Moreover,some of the variable, unidentified sources detectedby EGRET around the galactic center at Eγ > 100MeV could be ‘proton microquasars’ ejected from thehalo or the plane of the Galaxy.AcknowledgmentsWe thank an anonymous referee for construc-tive comments. This work has been supported bygrants PIP 5375 (CONICET) and PICT 03-13291BID 1728/OC-AR (ANPCyT). GER acknowledgessupport by the Ministerio de Educación y Cien-cia (Spain) under grant AYA 2007-68034-C03-01,FEDER funds.May 14, 2008 15:19 WSPC - Proceedings Trim Size: 11in x 8.5in vila˙HEPRO4-2 0 2 4 6 8 10 12 14 16 18323334353637383940a=1   =0.1 synchr photopion synchr photopairLog L (erg s-1 )Log E  (eV) synchr e- synchr p leptonic IC p(a)-2 0 2 4 6 8 10 12 14 16 1832333435363738(b) synchr photopion synchr photopair  Log L (erg s-1 )Log E  (eV) synchr e- synchr p leptonic IC pa=100  =0.1-2 0 2 4 6 8 10 12 14 16 1832333435363738(c)a=1000 =0.1 synchr photopion synchr photopairLog L (erg s-1 )Log E  (eV) synchr e- synchr p leptonic IC p-2 0 2 4 6 8 10 12 14 16 1830313233343536(d)a=100  =10-3 synchr photopion synchr photopairLog L (erg s-1 )Log E  (eV) synchr e- synchr p leptonic IC pFig. 2. Production spectra obtained for different values of jet content of relativistic particles ξ, and proton-to-lepton luminosityratio a.References1. Mirabel I.F. et al., Nature 358, 215 (1992)2. Corbel S. et al., Science 289, 196 (2002)3. Romero G.E. et al., A&A 410, L1 (2003)4. Bosch-Ramon et al., A&A 447, 263 (2006)5. Aharonian F.A. et al., Science 304, 746 (2005)6. Albert J. et al., Science 312, 1771 (2006)7. Albert J. et al., ApJ 665, L51 (2007)8. Atoyan A.M. & Aharonian F.A., MNRAS 302, 253(1999)9. Markoff S. et al., A&A 372, L25 (2001)10. Grenier I.A. et al., Ap&SS 297, 109 (2005)11. Falcke H. & Biermann P., A&A 293, 665 (1995)12. Körding E.G., Fender R.P., & Migliari S., MNRAS369, 1451 (2006)13. Fender R.P., Belloni T.M., Gallo E., MNRAS 35,1105 (2004)14. Markoff S., Falcke H., & Fender R., BAAS 32, 1546(2000)15. Gelino D.M. et al., ApJ 642, 438 (2006)16. Atoyan A.M. & Dermer C. D., ApJ 586, 79 (2003)17. Chodorowski M.J. et al., ApJ 400, 181 (1992)18. Mastichiadis A. et al., A&A 433, 765 (2005)19. Ghisellini G., Maraschi L., & Treves A., A&A 146,204 (1985)20. Blumenthal G.R. & Gould R.J., Rev. Mod. Phys. 42,237 (1970)21. Fender R.P., Hjellming R.M., Tilanus R.P.J. et al.,MNRAS 322, L23 (2001)22. Romero G.E. & Vila G.S., A&A, submitted (2008)23. Mirabel I.F. et al., Nature 413, 139 (2001)24. Levinson A. & Waxman E., Phys. Rev. Lett. 87,171101 (2001)

International Journal of Modern Physics D

Models for gamma-ray production in low-mass microquasars

Unlike high-mass gamma-ray binaries, low-mass microquasars lack external sources of radiation and matter that could produce high-energy emission through interactions with relativistic particles. In this work we consider the synchrotron emission of protons and leptons that populate the jet of a low-mass microquasar. In our model photohadronic and inverse Compton (IC) interactions with synchrotron photons produced by both protons and leptons result in a high-energy tail of the spectrum. We also estimate the contribution from secondary pairs injected through photopair production. The high-energy emission is dominated by radiation of hadronic origin, so we can call these objects proton microquasars.Fil: 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; ArgentinaFil: 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; Argentin

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Models for gamma-ray production in low-mass microquasars

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