After the second year of Fermi orbiting, the number of galactic sources associated with Pulsar Wind Nebulae (PWNe) and SuperNova Remnants (SNRs) has largely increased. For all these sources multi-wavelenghts spectral energy

distributions have been investigated and information about acceleration mechanisms and interaction sites have been collected and studied. The GeV-TeV connection of some recently detected sources will be presented and different interpretation of the observed spectra will be discussed

Giordano, F.

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DOI 10.1393/ncc/i2011-10887-8Colloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011High-energy Pulsar Wind Nebulae and SuperNova RemnantsF. Giordano for the Fermi-LAT CollaborationDipartimento di Fisica “M. Merlin” dell’Universita` e del Politecnico di BariINFN, Sezione di Bari - Bari, Italy(ricevuto il 25 Febbraio 2011; pubblicato online l’1 Giugno 2011)Summary. — After the second year of Fermi orbiting, the number of galacticsources associated with Pulsar Wind Nebulae (PWNe) and SuperNova Remnants(SNRs) has largely increased. For all these sources multi-wavelenghts spectral energydistributions have been investigated and information about acceleration mechanismsand interaction sites have been collected and studied. The GeV-TeV connection ofsome recently detected sources will be presented and different interpretation of theobserved spectra will be discussed.PACS 95.85.Pw – γ-ray.PACS 98.35.Ac – Origin, formation, evolution, age, and star formation.PACS 98.38.Mz – Supernova remnants.PACS 98.70.Sa – Cosmic rays (including sources, origin, acceleration, and interac-tions).1. – SNRs and PWNe as galactic cosmic-ray sourcesPAMELA observations of the positron fraction e+/(e− + e+) [1] showing an excessrelative to the prediction of a cosmic-ray (CR) propagation model [2] in the energyrange between 10 and 100GeV has opened new windows of possible interpretations inexotic scenario and astrophysical field. According to the model, the e+ are producedas secondaries by the Galactic CRs interactions with the interstellar medium along thepropagation path, assuming a power-law spectrum with an index of 2.75. Other indica-tions that more effort needs to be done to completely understand the cosmic-ray theorycomes from the electron and positron spectrum observed by the Fermi-LAT [3]. In orderto try to solve the mystery, different sources of e+ have been proposed, including pulsars(PSRs), Pulsar Wind Nebulae (PWNe) [4-8], or SuperNova Remnants (SNRs) [9-11],together with propagation effects [12,13] and dark matter annihilation or decay interpre-tations [7,14]. For this reason, it has become more crucial than before to be able to detectthese sources as high-energy γ-ray sources and try to investigate in details the origin ofthe observed spectra, disentangling all contributions coming from leptonic populationand/or hadronic accelerated particles.c© Societa` Italiana di Fisica 183184 F. GIORDANO for the FERMI-LAT COLLABORATIONFig. 1. – High-energy PWNe detected in two years by Fermi-LAT.2. – High-energy Pulsar Wind NebulaeSince the launch of the Fermi Gamma-Ray Space Telescope the number of detectedpulsars in the γ-ray domain has dramatically increased [15]. The list of Fermi-LATpulsars now contains 56 bright sources and certainly many more will be detected inthe upcoming months. Yet most of the pulsar spin-down luminosity is not observed aspulsed photon emission and is instead carried away as a magnetized particle wind [16].The deceleration of the pulsar-driven wind as it sweeps up ejecta from the supernovaexplosion generates a termination shock at which the particles are scattered and acceler-ated to ultra-relativistic energies. The PWN emission, including synchrotron and inverseCompton components, extends across the electromagnetic spectrum from radio to TeVenergies. PWNe studies can supply information on particle acceleration mechanisms atrelativistic shocks, on the evolution of the pulsar spin-down and, at later phases, on theambient interstellar gas. Figure 1 shows the PWNe Fermi-LAT detections after 2 yearsof sky survey [17].2.1. The Fermi PSRs off-pulse detections. – During the first year of Fermi data taking,56 bright PSRs have been detected and studied in details to try to highlight possibleemission in the off-pulse windows.The Crab Nebula. The Crab Nebula was the first detection as off-pulse emissionfrom PSRJ0534+2200 Fermi-LAT. The Crab Nebula belongs to the class of filled-centersupernova remnants [18] and is well studied in almost all wavelength bands of the electro-magnetic spectrum from the radio (10−5 eV) to very high-energy γ-rays (nearly 1014 eV).Several models [19-21] describe the photon production processes taking place in thisnebula as a double component spectrum: a synchrotron radiation from high-energy elec-trons in the nebular magnetic field, responsible for the observed spectrum from radioto MeV, and an Inverse Compton (IC) scattering of the primary accelerated electronsoff the synchrotron photons, far infrared and Cosmic Microwave Background (CMB),which produces high-energy γ-rays [22]. Other possible deviations from this simplifiedpicture [23] propose that significant production of high-energy γ-rays may come frombremsstrahlung radiation of relativistic electrons taking place in the Crab filaments.The VelaX PWN. The Vela pulsar (PSR B0833-45) at a distance of 290 pc is one ofthe closest pulsars to Earth and among the brightest and has always generated a lot ofHIGH-ENERGY PULSAR WIND NEBULAE AND SUPERNOVA REMNANTS 185Fig. 2. – The SEDs of the MSH 15-52 (left) and of the HESS J1640-465 (right). For the MSH15-52, one single electron population is enough to interpret correctly the observed data [29]. Forthe HESS J1640-665, like the VelaX PWN, a scenario with a low-energy Maxwellian electroncomponent replacing the low-energy portion of the electron power-law spectrum is needed toexplain the observed GeV excess [34].interest in studying the mechanisms at the base of such huge energy emission. Aboutthe 99% of the pulsar spin-down luminosity is not observed as pulsed photon emissionand is apparently carried away as a magnetized particle wind. Radio and X-ray observa-tions established the presence of large-scale diffuse emission surrounding PSR B0833-45possibly related to the Vela supernova remnant [24-26]. These radio observations showthat in roughly 8′ diameter there are three distinct central regions (Vela-X, Vela-Y andVela-Z). The most intense of these, Vela-X, is an extremely bright (∼ 1000 Jy) diffuseradio structure of size 2′–3′ located close to PSR B0833-45. The observations of a flatradio spectral index and the large degree of radio polarization in Vela-X were interpretedas indications of a diffuse radio emission from a PWN powered by the spin-down ofPSR B0833-45. The PWN emission extends across the electromagnetic spectrum in syn-chrotron and inverse Compton components from radio to TeV energies [16, 27]. Froma detailed study of the overall Spectral Energy Distribution (SED) [28], there are indi-cations of a cocoon emission by significantly cooled electrons, dominated by relativelyrecent injection of high-energy electrons from the pulsar and its termination shock, anda halo component consisting of older electrons.Ths MSH 15-52. Two possible interpretations for the origins of γ-ray photons fromPWNe, i.e. hadronic (from proton-proton interactions) or leptonic (via the inverse Comp-ton process) have been proposed recently [29] for the MSH 15-52. The multi-wavelengthpicture of MSH 15-52 is shown in fig. 2 (left). With the Fermi-LAT data added in themulti-wavelength spectrum [30], new constraints on the model parameters have been de-fined. A simple one-zone model has been assumed, which utilizes the interstellar radiationfield described in [31] as target photons for inverse Compton scattering, neglecting γ-raysproduction via bremsstrahlung due to the low density of the region (≤ 0.4 cm−3). Whatarises from the fit of the SED is that a single power-law electron spectrum does not repro-duce correctly the data; a more accurate description that takes into account an electronpopulation following a broken power law with an exponential cut-off surely interpretsmore appropriately the spectrum. The fitted average magnetic field strength is 17μG,which is in an excellent agreement with the value suggested by TeV observations [32] andconsistent with the lower limit of < 8μG obtained from X-ray observations [33].186 F. GIORDANO for the FERMI-LAT COLLABORATION2.2. The GeV counterpart of the H.E.S.S. PWNe. – The last two detections as PWNesources by the Fermi-LAT have been the HESS J1640-465 [34] and the HESS J1825-137 [35]. As in the case of evolved VelaX PWN, to describe the SED of the HESS J1640-465, simple power-law models for the radiating particles fail to reproduce the observedbroadband spectrum. The presence of an excess population of low-energy electrons isinferred, and models for the IC scattering of photons from this population predict anexcess of γ-rays in the GeV range (fig. 2 (right)).3. – High-energy SuperNova RemnantsIt is almost uniformly accepted that SNRs are possible candidates sites for particleacceleration. There are some aspects still not perfectly understood, like the exact mecha-nisms which bring particles up to the knee of the galactic cosmic-ray spectrum, as well asthe evolution and the subsequent release of particles in the outer space of the remnants.To maintain the energy density of the Galactic CRs, the kinetic energy released in su-pernova explosions has to be efficiently transferred to CRs with a conversion efficiency of10%. For these reasons the discovery of the emission from many SNRs at GeV and TeVγ-ray energies has added new ingredients to the galactic cosmic-ray puzzle. Moreover,after two years of data taking, the number of SNRs has increased up to about 10, asample that is expected to double in the next future, collecting data from young andmiddle-aged SNRs, interacting with dense environment like molecular clouds or expand-ing in pretty clear ambient, which is guiding the comprehension of the properties of theradiating particle population and disentangling the main mechanisms which produce theobserved γ-ray spectra.3.1. Young SNRs: CasA and Tycho. – Regardless of the origin of the observed gammarays, the total content of CRs accelerated in Cas A can be obtained as the sum ofleptonic and hadronic contributions We+Wp and can be estimated to be about (1–4)×1049 erg. Moreover, as a complementary effect of the conversion of the shock energy,there is evidence of turbulent magnetic fields with amplification factors much beyond thevalue predicted by the compression of the interstellar field without CR perturbations,estimating for B a value of ∼ 0.12mG. Even though Cas A is considered to have enteredthe Sedov phase, the total amount of CRs accelerated in the remnant constitutes only aminor fraction (< 2%) of the total kinetic energy of the supernova. The bremsstrahlungspectrum and π0-decay spectrum have rather different predictions below 1GeV. Thehard spectrum below 1GeV (fig. 3) would favor the π0-decay origin, though the currentFermi-LAT data [36] does not rule out the bremsstrahlung model. In fig. 3 (left) thebremsstrahlung components (dashed lines) as well as the IC component (dotted lines)are shown for the leptonic model, for two different values of the magnetic field (0.12mGand 0.3mG). The π0-decay spectra are shown in the right panel for two possible protonspectra, one harder with an index of 2.1 which stops the acceleration at 10TeV (blueline) and the other with a softer population with an index of 2.3 without any cutoff (redline). The data do not strongly discriminate among any of the proposed interpretations.Recently the detection of the Tycho SNR has been announced by the VERITAScollaboration [37]. It is the remnants of the star explosion noted by the astronomerTycho Brahe in AD 1572. The GeV analysis with Fermi is in progress; very preliminaryresults seem to suggest also a GeV emission compatible with the position of the SNRfrom X-ray data [38].HIGH-ENERGY PULSAR WIND NEBULAE AND SUPERNOVA REMNANTS 187Fig. 3. – Energy spectrum of Cas A in a leptonic emission model (left) and in a hadronic emissionmodel (right). See the text for the different models shown.3.2. Middle-aged SNRs interacting with molecular clouds. – Bright and extended GeVγ-ray emission associated with middle-aged supernova remnants has recently been de-tected by the Fermi-LAT, and some also by AGILE. Specifically, SNRs W51C [39],W44 [40], IC443 [41, 42], and W28 [43, 44] are spatially resolved, all interacting withmolecular clouds. Some other SNRs [45] have still been detected by the Fermi-LAT inthe class of objects with an age of about few tens of thousands years, expanding in densemolecular environments. The list of detected SNRs is in table I. Some authors [46]started collecting similarities and differences in this class of SNRs studying the emissionin radio and in the γ-ray band. The synchrotron radio emission has a large flux of fewhundreds of Jy at 1GHz with flat spectral index (∼ α = 0.26–0.40). The GeV γ-rayspectrum commonly exhibits a spectral break at few GeV, and an integrated luminosityin the range 1–100GeV in the interval Lγ = (0.8–0.9)× 1035 erg s−1.The number of detected SNRs illuminating molecular clouds is enough to test differenttheories which may take into account the gamma luminosity at both GeV and TeV bandsand the radio luminosity. In [46] for instance is shown that the radio and γ-ray emissionobserved can be explained by a model in which a shocked cloud and shock-acceleratedcosmic rays (CRs) are simultaneously compressed by the supernova blast wave as a resultof formation of a radiative cloud shock. A re-acceleration process of pre-existing CRscan explain the observed γ-ray emission through the decays of π0-mesons produced inTable I. – List of Fermi detected SNRs in two years.Object Diameter (pc) Age (y) Cloud interaction Lγ (erg s−1) 1–100GeVCas A 5 330 No 4× 1034W49B 10 ∼ 3000 Yes 9× 10353C 391 15 ∼ 6000 Yes 6× 1034G349.7+0.2 17 ∼ 6000 Yes 9× 1034IC 443 20 ∼ 10000 Yes 8× 1034W44 25 ∼ 10000 Yes 3× 1035W28 28 ∼ 10000 Yes 9× 1034CTB 37A 50 ∼ 20000 Yes 9× 1034G8.7-0.1 63 ∼ 30000 Yes 8× 1034W51C 76 ∼ 30000 Yes 8× 1035188 F. GIORDANO for the FERMI-LAT COLLABORATIONhadronic interactions between high-energy protons and the gas in the compressed-cloudlayer. In [47] the GeV-TeV spectrum has been interpreted as a delayed emission of cosmicrays diffusing from IC 443 and interacting with a cloud in the foreground of the remnant,and in [48] the non thermal emission from a molecular cloud located in proximity of aSNR is the result of the interactions of CRs that penetrate the cloud.4. – ConclusionsAccumulating more data and fitting with more details and precision the galactic γ-raydiffuse emission, more information about PWNe and SNRs as high-energy γ-ray emittersare being collected. Hints about the possibility to have hadronic interpretation of thegamma emission from SNRs seem to confirm the idea to identify the SNRs as possibleCRs sources and accelerators. On the other sides, recent detections of PWNe and ofcourse a numerous class of high-energy pulsars are giving new ingredients to a possibleinterpretation of astrophysics explanation of the cosmic-rays spectra and compositiondetected so far [3]. It is well accepted that pulsars also dissipate their energy via mag-netized particle winds, which consist of electron and positron pairs confined in the windoutflow. A PWN is created at the collision of the particle wind and the surroundingsmedium. With the data in hand many new constraints have been obtained about thebulk injection from the pulsar and its termination shock, and it may be hoped thatstarting from what has been just discovered, together with new data and results on otherPWNe, will help in understanding the physics of these relativistic outflows. Concerningthe detections of SNRs as γ-ray sources, new indications about possible hadronic inter-pretation of the observed spectra are being acquired. Though, there are no clear andstringent signatures about p-p collisions as well as diffusion or emission from the crusheddense material surrounding the remnants. In the next future more data will be collectedand the sample of SNRs will account for different progenitors, environments and age,making possible differences and similarities crucial for the understanding of the processbehind the observed γ-ray emission.∗ ∗ ∗The Fermi-LAT Collaboration acknowledges support from a number of agencies andinstitutes for both development and the operation of the LAT as well as scientificdata analysis. These include NASA and DOE in the United States, CEA/Irfu andIN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan,and the K. A. Wallenberg Foundation, the Swedish Research Council and the NationalSpace Board in Sweden. Additional support from INAF in Italy and CNES in Francefor science analysis during the operations phase is also gratefully acknowledged.REFERENCES[1] Adriani O. et al., Nature, 458 (2009) 607.[2] Strong A. W. and Moskalenko I. V., Astrophys. J., 509 (1998) 212.[3] Abdo A. A. et al., Phys. Rev. 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High-energy Pulsar Wind Nebulae and SuperNova Remnants

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High-energy Pulsar Wind Nebulae and SuperNova Remnants

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