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A High-resolution, Multi-epoch Spectral Atlas of Peculiar Stars Including RAVE, GAIA , and HERMES Wavelength Ranges
We present an Echelle+CCD, high signal-to-noise ratio, high-resolution(R = 20,000) spectroscopic atlas of 108 well-known objectsrepresentative of the most common types of peculiar and variable stars.The wavelength interval extends from 4600 to 9400 Å and includesthe RAVE, Gaia, and HERMES wavelength ranges. Multi-epoch spectra areprovided for the majority of the observed stars. A total of 425 spectraof peculiar stars, which were collected during 56 observing nightsbetween 1998 November and 2002 August, are presented. The spectra aregiven in FITS format and heliocentric wavelengths, with accuratesubtraction of both the sky background and the scattered light.Auxiliary material useful for custom applications (telluric dividers,spectrophotometric stars, flat-field tracings) is also provided. Theatlas aims to provide a homogeneous database of the spectral appearanceof stellar peculiarities, a tool useful both for classification purposesand inter-comparison studies. It could also serve in the planning anddevelopment of automated classification algorithms designed for RAVE,Gaia, HERMES, and other large-scale spectral surveys. The spectrum of XXOph is discussed in some detail as an example of the content of thepresent atlas.
| H? Emission Variability in the ?-ray Binary LS I +61 303
LS I +61 303 is an exceptionally rare example of a high-mass X-raybinary that also exhibits MeV-TeV emission, making it one of only ahandful of "?-ray binaries." Here we present H? spectra thatshow strong variability during the 26.5 day orbital period and overdecadal timescales. We detect evidence of a spiral density wave in theBe circumstellar disk over part of the orbit. The H? line profilealso exhibits a dramatic emission burst shortly before apastron,observed as a redshifted shoulder in the line profile, as the compactsource moves almost directly away from the observer. We investigateseveral possible origins for this red shoulder, including an accretiondisk, mass-transfer stream, and a compact pulsar wind nebula that formsvia a shock between the Be star's wind and the relativistic pulsar wind.
| HD 259440: The Proposed Optical Counterpart of the ?-ray Binary HESS J0632+057
HD 259440 is a B0pe star that was proposed as the optical counterpart tothe ?-ray source HESS J0632+057. Here, we present optical spectraof HD 259440 acquired to investigate the stellar parameters, theproperties of the Be star disk, and evidence of binarity in this system.Emission from the H? line shows evidence of a spiral density wavein the nearly edge-on disk. We find a best-fit stellar effectivetemperature of 27,500-30,000 K and a log surface gravity of 3.75-4.0,although our fits are somewhat ambiguous due to scattered light from thecircumstellar disk. We derive a mass of 13.2-19.0 M sun and aradius of 6.0-9.6 R sun. By fitting the spectral energydistribution, we find a distance between 1.1 and 1.7 kpc. We do notdetect any significant radial velocity shifts in our data, ruling outorbital periods shorter than one month. If HD 259440 is a binary, it islikely a long-period (>100 d) system.
| The Be Star HD 215227: A Candidate Gamma-ray Binary
The emission-line Be star HD 215227 lies within the positional errorcircle of the newly identified gamma-ray source AGL J2241+4454. Wepresent new blue spectra of the star, and we point out the morphologicaland variability similarities to other Be binaries. An analysis of theavailable optical photometry indicates a variation with a period of60.37 ± 0.04 days, which may correspond to an orbital modulationof the flux from the disk surrounding the Be star. The distance to thestar of 2.6 kpc and its relatively large Galactic latitude suggest thatthe binary was ejected from the plane by a supernova explosion thatcreated the neutron star or black hole companion. The binary and runawayproperties of HD 215227 make it an attractive candidate as the opticalcounterpart of AGL J2241+4454 and as a new member of the small class ofgamma-ray emitting binaries.
| High-energy neutrino emission from low-mass microquasars
In this paper we study the high-energy neutrino emission fromlow-mass microquasars (LMMQs) by assuming that an acceleration regionfor particles is located in the inner jet, based on the framework of ahadronic jet model. Adopting the parametrized formulae for inelastic ppinteractions and the recent p? parametrizations, we calculate thespectral energy distributions (SEDs) of various photon and neutrinospectra. Furthermore, we also consider the electromagnetic cascadeprocesses due to ?? absorption. We then apply the jet modelto the microquasar (MQ) GX339-4, and calculate the expected neutrinoevent rate for three years of observations using the newest effectivearea of the KM3NeT detector in the Mediterranean Sea. The resultingresults indicate that (1) photon spectra can roughly reproduceobservations from radio to X-ray energies, and can also predict emissionfluxes at high and very high energies (VHE); (2) GX339-4 is a potentialneutrino source, neutrinos from which are likely to be identified withsome years of observations from the next-generation -scale neutrinotelescopes.
| Magic Constraints on ?-ray Emission from Cygnus X-3
Cygnus X-3 is a microquasar consisting of an accreting compact objectorbiting around a Wolf-Rayet star. It has been detected at radiofrequencies and up to high-energy ? rays (above 100 MeV). However,many models also predict a very high energy (VHE) emission (abovehundreds of GeV) when the source displays relativistic persistent jetsor transient ejections. Therefore, detecting such emission would improvethe understanding of the jet physics. The imaging atmospheric Cherenkovtelescope MAGIC observed Cygnus X-3 for about 70 hr between 2006 Marchand 2009 August in different X-ray/radio spectral states and also duringa period of enhanced ?-ray emission. MAGIC found no evidence for aVHE signal from the direction of the microquasar. An upper limit to theintegral flux for energies higher than 250 GeV has been set to 2.2× 10-12 photons cm-2 s-1 (95%confidence level). This is the best limit so far to the VHE emissionfrom this source. The non-detection of a VHE signal during the period ofactivity in the high-energy band sheds light on the location of thepossible VHE radiation favoring the emission from the innermost regionof the jets, where absorption is significant. The current and futuregenerations of Cherenkov telescopes may detect a signal under precisespectral conditions.
| Variability in the Orbital Profiles of the X-ray Emission of the ?-ray Binary LS I +61° 303
We report on the analysis of Rossi X-Ray Timing Explorer ProportionalCounter Array (PCA) monitoring observations of the ?-ray binarysystem LS I +61° 303, covering 35 full cycles of its orbital motion.This constitutes the largest continuous X-ray-monitoring data setanalyzed to date for this source. Such an extended analysis allows us toreport (1) the discovery of variability in the orbital profiles of theX-ray emission, (2) the existence of a few (recent) short flares on topof the overall behavior typical of the source, which, given the PCAfield of view, may or may not be associated with LS I +61° 303, and(3) the determination of the orbital periodicity using soft X-ray dataalone.
| Masses of Neutron Stars in High-mass X-ray Binaries with Optical Astrometry
Determining the type of matter that is inside a neutron star (NS) hasbeen a long-standing goal of astrophysics. Despite this, most of the NSequations of state (EOS) that predict maximum masses in the range1.4-2.8 M_{\mathord \odot } are still viable. Most of the precise NSmass measurements that have been made to date show values close to 1.4M_{\mathord \odot }, but a reliable measurement of an overmassive NSwould constrain the EOS possibilities. Here, we investigate how opticalastrometry at the microarcsecond level can be used to map out the orbitsof High-mass X-ray Binaries (HMXBs), leading to tight constraints on NSmasses. While previous studies by Unwin and coworkers and Tomsick andcoworkers discuss the fact that the future Space Interferometry Missionshould be capable of making such measurements, the current workdescribes detailed simulations for six HMXB systems, including predictedconstraints on all orbital parameters. We find that the direct NS massescan be measured to an accuracy of ~2.5% (1?) in the best case (XPer), to ~6.5% for Vela X-1, and to ~10% for two other HMXBs.
| Discovery of a GeV Blazar Shining Through the Galactic Plane
The Fermi Large Area Telescope (LAT) discovered a new gamma-ray sourcenear the Galactic plane, Fermi J0109+6134, when it flared brightly in2010 February. The low Galactic latitude (b = -1fdg2) indicatedthat the source could be located within the Galaxy, which motivatedrapid multi-wavelength follow-up including radio, optical, and X-rayobservations. We report the results of analyzing all 19 months of LATdata for the source, and of X-ray observations with both Swift and theChandra X-ray Observatory. We determined the source redshift, z = 0.783,using a Keck Low-Resolution Imaging Spectrometer observation. Finally,we compiled a broadband spectral energy distribution (SED) from bothhistorical and new observations contemporaneous with the 2010 Februaryflare. The redshift, SED, optical line width, X-ray absorption, andmulti-band variability indicate that this new GeV source is a blazarseen through the Galactic plane. Because several of the optical emissionlines have equivalent width >5 Å, this blazar belongs in theflat-spectrum radio quasar category.
| On the possibility of sub-TeV -ray emission from CygX-3
The compact X-ray binary system CygX-3 has been recently discovered as asource of GeV ?-rays by the AGILE and the Fermi satellites. Itshows emission features in the GeV ?-rays similar to other?-ray binaries which were also observed in the TeV ?-rays(LS5039 and LSI+61303). The question appears whether CygX-3 can be alsodetected in the TeV ?-rays by the Cherenkov telescopes.Here we discuss this problem in detail based on the anisotropic inverseCompton (IC) pair cascade model successfully applied to TeV ?-raybinaries. We calculate the ?-ray light curves and ?-rayspectra expected from the cascade process occurring inside the CygX-3binary system. It is found that the ?-ray light curves at GeVenergies can be consistent with the ?-ray light curve observed bythe Fermi for reasonable parameters of the orbit of the injection sourceof relativistic electrons. Moreover, we show that in such a model thesub-TeV ?-ray emission (above 100GeV) is expected to be belowsensitivities of the present Cherenkov telescopes assuming thatelectrons are accelerated in CygX-3 to TeV energies. The next stageCherenkov telescopes (MAGIC II, HESS II) should have the energythreshold in the range 20-30GeV, in order to have a chance to detect thesignal from CygX-3. Otherwise, the positive detection of ?-rays atenergies above a few tens of GeV requires a telescope with thesensitivity of of Crab units. We conclude that detection of sub-TeV?-rays from CygX-3 by on-ground telescopes has to probably waitfor the construction of the Cherenkov Telescope Array.
| Deep Chandra observations of TeV binaries - I. LSI+61°303
We report on a 95ks Chandra observation of the TeV emitting high-massX-ray binary LSI+61°303, using the ACIS-S camera in continuousclocking mode to search for a possible X-ray pulsar in this system. Theobservation was performed while the compact object was passing fromphase 0.94 to 0.98 in its orbit around the Be companion star (henceclose to the apastron passage). We did not find any periodic orquasi-periodic signal (at this orbital phase) in a frequency range of0.005-175Hz. We derived an average pulsed fraction (PF) 3? upperlimit for the presence of a periodic signal of <~10 per cent(although this limit is strongly dependent on the frequency and theenergy band), the deepest limit ever reached for this object.Furthermore, the source appears highly variable in flux and spectrumeven in this very small orbital phase range, in particular we detect twoflares, lasting thousands of seconds, with a very hard X-ray spectrumwith respect to the average source spectral distribution. The X-ray PFlimits we derived are lower than the PF of any isolatedrotational-powered pulsar, in particular having a TeV counterpart. Inthis scenario, most of the X-ray emission of LSI+61°303 shouldnecessarily come from the interwind or innerpulsar wind zone shockrather than from the magnetosphere of the putative pulsar. On the otherhand, very low X-ray PFs are not unseen in accreting neutron starsystems, although we cannot at all exclude the black hole nature of thehosted compact object, a pulsar with a beam pointing away from our lineof sight or spinning faster than ~5.6ms, nor that pulsations might havea transient appearance in only a small fraction of the orbit.Furthermore, we did not find evidence for the previously suggestedextended X-ray emission.
| New Evidence for a Black Hole in the Compact Binary Cygnus X-3
The bright and highly variable X-ray and radio source known as CygnusX-3 was among the first X-ray sources discovered, yet it remains in manyways an enigma. It is known to consist of a massive, Wolf-Rayet primaryin an extremely tight orbit with a compact object. However, one of themost basic of parameters—the mass of the compact object—isnot known, nor is it even clear whether it is a neutron star or a blackhole (BH). In this paper, we present our analysis of the broadbandhigh-energy continua covering a substantial range in luminosity andspectral morphology. We apply these results to a recently identifiedscaling relationship that has been demonstrated to provide reliableestimates of the compact object mass in a number of accretion poweredbinaries. This analysis leads us to conclude that the compact object inCygnus X-3 has a mass greater than 4.2 M sun, thus clearlyindicative of a BH and as such, resolves a long-standing issue. The fullrange of uncertainty in our analysis and from using a range of recentlypublished distance estimates constrain the compact object mass to liebetween 4.2 M sun and 14.4 M sun. Our favoredestimate, based on a 9.0 kpc distance estimate, is ~10 M sun,with an error margin of 3.2 solar masses. This result may thus posechallenges to shared-envelope evolutionary models of compact binaries,as well as establishing Cygnus X-3 as the first confirmedaccretion-powered galactic gamma-ray source.
| Modulation Mechanism of Tev, Gev, and X-ray Emission in LS5039
The emission mechanism of the gamma-ray binary LS5039 in TeV, GeV, andX-ray energy bands is investigated. Observed light curves in LS5039 showthat TeV and GeV fluxes anticorrelate, whereas TeV and X-ray fluxescorrelate. However, such correlated variations have not yet beenreasonably explained at this stage. Assuming that relativistic electronsare injected constantly at the location of the compact object as a pointsource and that they lose energy only by the inverse Compton (IC)process, we calculate gamma-ray spectra and light curves by the MonteCarlo method, including the full electromagnetic cascade process.Moreover, we calculated X-ray spectra and light curves by using theresultant electron distribution. As a result, we are able toqualitatively reproduce spectra and light curves observed by H.E.S.S.,Fermi, and Suzaku for the inclination angle i = 30° and the index ofinjected electron distribution p = 2.5. We conclude that TeV-GeVanticorrelation is due to anisotropic IC scattering and anisotropic?? absorption, and that TeV-X correlation is due to thedependence of IC cooling time on orbital phases. In addition, theconstraint on the inclination angle implies that the compact object inLS5039 is a black hole.
| Gamma-ray absorption and the origin of the gamma-ray flare in Cygnus X-1
Context. The high-mass microquasar Cyg X-1, the best-establishedcandidate for a stellar-mass black hole in the Galaxy, has been detectedin a flaring state at very high energies (VHE), E > 200 GeV, by theAtmospheric Cherenkov Telescope MAGIC. The flare occurred at orbitalphase ? = 0.91, where ? = 1 is the configuration with theblack hole behind the companion high-mass star, when the absorption ofgamma-ray photons by photon-photon annihilation with the stellar fieldis expected to be highest. Aims: We aim to set up a model for thehigh-energy emission and absorption in Cyg X-1 that can explain thenature of the observed gamma-ray flare. Methods: We study thegamma-ray opacity due to pair creation along the whole orbit, and fordifferent locations of the emitter. Then we consider a possiblemechanism for the production of the VHE emission. Results: Wepresent detailed calculations of the gamma-ray opacity and infer fromthese calculations the distance from the black hole where the emittingregion was located. We suggest that the flare was the result of ajet-clump interaction where the decay products of inelastic p - pcollisions dominate the VHE outcome. Conclusions: We are able toreproduce the spectrum of Cyg X-1 during the observed flare underreasonable assumptions. The flare may be the first event of jet-cloudinteraction ever detected at such high energies.
| A search for VHE counterparts of Galactic Fermi bright sources and MeV to TeV spectral characterization
Very high-energy (VHE; E > 100 GeV) ?-rays have been detectedin a wide range of astronomical objects, such as pulsar wind nebulae(PWNe), supernova remnants (SNRs), giant molecular clouds, ?-raybinaries, the Galactic center, active galactic nuclei (AGN), radiogalaxies, starburst galaxies, and possibly star-forming regions. Atlower energies, observations using the Large Area Telescope (LAT)onboard Fermi provide a rich set of data that can be used to study thebehavior of cosmic accelerators in the MeV to TeV energy bands. Inparticular, the improved angular resolution of current telescopes inboth bands compared to previous instruments significantly reduces sourceconfusion and facilitates identification of associated counterparts atlower energies. In this paper, a comprehensive search for VHE?-ray sources that are spatially coincident with GalacticFermi/LAT bright sources is performed, and the available MeV to TeVspectra of coincident sources compared. It is found that bright LAT GeVsources are correlated with TeV sources, in contrast to previous studiesusing EGRET data. Moreover, a single spectral component seems unable todescribe the MeV to TeV spectra of many coincident GeV/TeV sources. Ithas been suggested that ?-ray pulsars may be accompanied by VHE?-ray emitting nebulae, a hypothesis that can be tested with VHEobservations of these pulsars.
| Fermi Large Area Telescope First Source Catalog
We present a catalog of high-energy gamma-ray sources detected by theLarge Area Telescope (LAT), the primary science instrument on the FermiGamma-ray Space Telescope (Fermi), during the first 11 months of thescience phase of the mission, which began on 2008 August 4. The FirstFermi-LAT catalog (1FGL) contains 1451 sources detected andcharacterized in the 100 MeV to 100 GeV range. Source detection wasbased on the average flux over the 11 month period, and the thresholdlikelihood Test Statistic is 25, corresponding to a significance of justover 4?. The 1FGL catalog includes source location regions,defined in terms of elliptical fits to the 95% confidence regions andpower-law spectral fits as well as flux measurements in five energybands for each source. In addition, monthly light curves are provided.Using a protocol defined before launch we have tested for severalpopulations of gamma-ray sources among the sources in the catalog. Forindividual LAT-detected sources we provide firm identifications orplausible associations with sources in other astronomical catalogs.Identifications are based on correlated variability with counterparts atother wavelengths, or on spin or orbital periodicity. For the catalogsand association criteria that we have selected, 630 of the sources areunassociated. Care was taken to characterize the sensitivity of theresults to the model of interstellar diffuse gamma-ray emission used tomodel the bright foreground, with the result that 161 sources at lowGalactic latitudes and toward bright local interstellar clouds areflagged as having properties that are strongly dependent on the model oras potentially being due to incorrectly modeled structure in theGalactic diffuse emission.
| Relativistic Doppler-boosted emission in gamma-ray binaries
Context. Gamma-ray binaries could be compact pulsar wind nebulae formedwhen a young pulsar orbits a massive star. The pulsar wind is containedby the stellar wind of the O or Be companion, creating a relativisticcomet-like structure accompanying the pulsar along its orbit. Aims: The X-ray and the very high energy (>100 GeV, VHE) gamma-rayemission from the binary LS 5039 are modulated on the orbital period ofthe system. Maximum and minimum flux occur at the conjunctions of theorbit, suggesting that the explanation is linked to the orbitalgeometry. The VHE modulation has been proposed to be due to the combinedeffect of Compton scattering and pair production on stellar photons,both of which depend on orbital phase. The X-ray modulation could be dueto relativistic Doppler boosting in the comet tail where both the X-rayand VHE photons would be emitted. Methods: Relativisticaberrations change the seed stellar photon flux in the comoving frame soDoppler boosting affects synchrotron and inverse Compton emissiondifferently. The dependence with orbital phase of relativisticDoppler-boosted (isotropic) synchrotron and (anisotropic) inverseCompton emission is calculated, assuming that the flow is orientedradially away from the star (LS 5039) or tangentially to the orbit (LS I+61°303, PSR B1259-63). Results: Doppler boosting of thesynchrotron emission in LS 5039 produces a lightcurve whose shapecorresponds to the X-ray modulation. The observations imply an outflowvelocity of 0.15-0.33c consistent with the expected flow speed atthe pulsar wind termination shock. In LS I +61°303, the calculatedDoppler boosted emission peaks in phase with the observed VHE and X-raymaximum. Conclusions: Doppler boosting is not negligible ingamma-ray binaries, even for mildly relativistic speeds. The boostedmodulation reproduces the X-ray modulation in LS 5039 and could alsoprovide an explanation for the puzzling phasing of the VHE peak in LS I+61°303.
| Feasibility study of Lense-Thirring precession in LS I +61°303
Context. Very recent analysis of the radio spectral index and highenergy observations have shown that the two-peak accretion/ejectionmicroquasar model applies for LS I +61°303. Aims: The fastvariations of the position angle observed with MERLIN and confirmed byconsecutive VLBA images must therefore be explained in the context ofthe microquasar scenario. Methods: We calculate what could be theprecessional period for the accretion disk in LS I +61°303 undertidal forces of the Be star (Ptidal-forces) or under theeffect of frame dragging produced by the rotation of the compact object(PLense-Thirring). Results: Ptidal-forcesis more than one year. PLense-Thirring depends on thetruncated radius of the accretion disk, Rtr. We determinedRtr = 300 rg for observed QPO at 2 Hz. This valueis much above the few rg, where the Bardeen-Petterson effectshould align the midplane of the disk. For this truncated radius of theaccretion disk PLense-Thirring for a slow rotator results ina few days. Conclusions: Lense-Thirring precession induced by aslowly rotating compact object could be compatible with the dailyvariations of the ejecta angle observed in LS I +61°303.
| The intriguing nature of the high-energy gamma ray source XSS J12270-4859
Context. The nature of the hard X-ray source XSS J12270-4859 is stillunclear. It was claimed to be a possible magnetic cataclysmic variableof the Intermediate Polar type from its optical spectrum and a possible860 s X-ray periodicity in RXTE data. However, recent observations donot support the latter variability, leaving this X-ray source stillunclassified. Aims: To investigate its nature we present abroad-band X-ray and gamma ray study of this source based on a recentXMM-Newton observation and archival INTEGRAL and RXTE data. Using theFermi/LAT 1-year point source catalogue, we tentatively associate XSSJ12270-4859 with 1FGL J1227.9-4852, a source of high-energy gamma rayswith emission up to 10 GeV. We further complement the study with UVphotometry from XMM-Newton and ground-based optical and near-IRphotometry. Methods: We have analysed both timing and spectralproperties in the gamma rays, X-rays, UV and optical/near-IR bands ofXSS J12270-4859. Results: The X-ray emission is highly variable,showing flares and intensity dips. The flares consist of flare-dippairs. Flares are detected in both X-rays and the UV range, while thesubsequent dips are present only in the X-ray band. Further aperiodicdipping behaviour is observed during X-ray quiescence, but not in theUV. The broad-band 0.2–100 keV X-ray/soft gamma ray spectrum isfeatureless and well described by a power law model with ? = 1.7.The high-energy spectrum from 100 MeV to 10 GeV is represented by apower law index of 2.45. The luminosity ratio between 0.1–100 GeVand 0.2–100 keV is ~0.8, indicating that the GeV emission is asignificant component of the total energy output. Furthermore, the X-rayspectrum does not greatly change during flares, quiescence and the dipsseen in quiescence. The X-ray spectrum however hardens during thepost-flare dips, where a partial covering absorber is also required tofit the spectrum. Optical photometry acquired at different epochsreveals a period of 4.32 hr that could be ascribed to the binary orbitalperiod. Near-IR, possibly ellipsoidal, variations are detected. Largeamplitude variability on shorter (tens mins) timescales is found to benon-periodic. Conclusions: The observed variability at allwavelengths together with the spectral characteristics strongly favour alow-mass atypical low-luminosity X-ray binary and are against a magneticcataclysmic variable nature. The association with a Fermi/LAThigh-energy gamma ray source further strengths this interpretation.Based on observations obtained with XMM-Newton and INTEGRAL, ESA sciencemissions with instruments and contributions directly funded by ESAMember States and NASA, with Fermi a NASA mission with contributionsfrom France, Germany, Italy, Japan, Sweden and USA and with the REMTelescope INAF at ESO, La Silla, Chile
| Optical depths for gamma-rays in the radiation field of a star heated by an external X-ray source in LMXBs. Application to Hercules X-1 and Scorpius X-1
The surface of a low mass star inside a compact low mass X-ray binarysystem (LMXB) can be heated by an external X-ray source apparentlycaused by the accretion of mass onto a companion compact object (aneutron star or a black hole). As a result, the surface temperature ofthe star can become significantly higher than it is in the normal stateresulting from thermonuclear burning. We investigate whether high energyelectrons and gamma-rays, injected within the binary system, canefficiently interact with this enhanced radiation field. To decide this,we calculate the optical depths of the gamma-ray photons in theradiation field of such an irradiated star as a function of the phase ofthe binary system. Based on these calculations, we conclude that compactlow mass X-ray binary systems may also become sources of high energygamma-rays since conditions for the interaction of electrons and?-rays are quite similar to those found within high mass TeV?-ray binaries such as LS 5039 and LSI 303 +61. However, becauseof differences in the soft radiation field, the expected ?-raylight curves can differ significantly between low mass and high massX-ray binaries. As an example, we apply these calculations to two wellknown LMXBs Her X-1 and Sco X-1. It is concluded that electronsaccelerated to high energies inside these binaries should collide with asufficient number of soft photon targets from the companion star toachieve efficient ?-ray production.
| A compact pulsar wind nebula model of the ?-ray-loud binary LS I +61?303
We study a model of LS I +61°303 in which its radio to TeV emissionis due to interaction of a relativistic wind from a young pulsar withthe wind from its companion Be star. The detailed structure of thestellar wind plays a critical role in explaining the properties of thesystem. We assume the fast polar wind is clumpy, which is typical forradiatively driven winds. The clumpiness and some plasma instabilitiescause the two winds to mix. The relativistic electrons from the pulsarwind are retained in the moving clumps by inhomogeneities of themagnetic field, which explains the X-ray variability observed ontime-scales much shorter than the orbital period. We calculate detailedinhomogeneous spectral models reproducing the average broad-bandspectrum from radio to TeV. Given the uncertainties on the magneticfield within the wind and the form of the distribution of relativisticelectrons, the X-ray spectrum could be dominated by either Compton orsynchrotron emission. The recent Fermi observations constrain thehigh-energy cut-off in the electron distribution to be at the Lorentzfactor of 2 × 104 or ~108 in the former andlatter model, respectively. We provide formulae comparing the losses ofthe relativistic electrons due to Compton, synchrotron and Coulombprocesses versus the distance from the Be star. We calculate the opticaldepth of the wind to free-free absorption, showing that it will suppressmost of the radio emission within the orbit, including the pulsed signalof the rotating neutron star. We point out the importance of Compton andCoulomb heating of the stellar wind within and around the ?-rayemitting region. Then, we find the most likely mechanism explaining theorbital modulation at TeV energies is anisotropy of emission, withrelativistic electrons accelerated along the surface of equal rampressure of the two winds. Pair absorption of the TeV emissionsuppresses one of the two maxima expected in an orbit.
| 3D simulations of wind-jet interaction in massive X-ray binaries
Context. High-mass microquasars may produce jets that will stronglyinteract with surrounding stellar winds on binary system spatial scales. Aims: We study the dynamics of the collision between a mildlyrelativistic hydrodynamical jet of supersonic nature and the wind of anOB star. Methods: We performed numerical 3D simulations of jetsthat cross the stellar wind with the code Ratpenat. Results: Thejet head generates a strong shock in the wind, and strong recollimationshocks occur due to the initial overpressure of the jet with itsenvironment. These shocks can accelerate particles up to TeV energiesand produce gamma-rays. The recollimation shock also strengthens jetasymmetric Kelvin-Helmholtz instabilities produced in the wind/jetcontact discontinuity. This can lead to jet disruption even for jetpowers of several times 1036 erg s-1. Conclusions: High-mass microquasar jets likely suffer a strongrecollimation shock that can be a site of particle acceleration up tovery high energies, but also eventually lead to the disruption of thejet.
| Fermi Observations of Cassiopeia and Cepheus: Diffuse Gamma-ray Emission in the Outer Galaxy
We present the analysis of the interstellar ?-ray emissionmeasured by the Fermi Large Area Telescope toward a region in the secondGalactic quadrant at 100° <= l <= 145° and -15° <=b <= +30°. This region encompasses the prominent Gould Beltclouds of Cassiopeia, Cepheus, and the Polaris flare, as well as atomicand molecular complexes at larger distances, like that associated withNGC 7538 in the Perseus arm. The good kinematic separation in velocitybetween the local, Perseus, and outer arms, and the presence of massivecomplexes in each of them, make this region well suited to probe cosmicrays (CRs) and the interstellar medium beyond the solar circle. The?-ray emissivity spectrum of the gas in the Gould Belt isconsistent with expectations based on the locally measured CR spectra.The ?-ray emissivity decreases from the Gould Belt to the Perseusarm, but the measured gradient is flatter than expectations for CRsources peaking in the inner Galaxy as suggested by pulsars. The XCO = N(H2)/W CO conversion factor isfound to increase from (0.87 ± 0.05) × 1020cm-2 (K km s-1)-1 in the Gould Belt to(1.9 ± 0.2) × 1020 cm-2 (K kms-1)-1 in the Perseus arm. We derive masses forthe molecular clouds under study. Dark gas, not properly traced by radioand microwave surveys, is detected in the Gould Belt through acorrelated excess of dust and ?-ray emission: its mass amounts to~50% of the CO-traced mass.
| The Palermo Swift-BAT hard X-ray catalogue. II. Results after 39 months of sky survey
Aims: We present the Palermo Swift-BAT hard X-ray catalogueobtained from the analysis of data acquired during the first 39 monthsof the Swift mission. Methods: We developed a dedicated softwareto perform the data reduction, mosaicking, and source detection of theBAT survey data. We analyzed the BAT dataset in three energy bands(14-150 keV, 14-30 keV, 14-70 keV), obtaining a list of 962 detectionsabove a significance threshold of 4.8 standard deviations. Theidentification of the source counterparts was pursued using threestrategies: cross-correlation with published hard X-ray catalogues,analysis of field observations of soft X-ray instruments, andcross-correlation with SIMBAD databases. Results: The surveycovers 90% of the sky down to a flux limit of 2.5×10-11 erg cm-2 s-1 and 50% ofthe sky down to a flux limit of 1.8 ×10-11 ergcm-2 s-1 in the 14-150 keV band. We derived acatalogue of 754 identified sources, of which ˜69% areextragalactic, ˜27% are Galactic objects, and ˜4% arealready known X-ray or gamma ray emitters, whose nature has yet to bedetermined. The integrated flux of the extragalactic sample is ˜1%of the cosmic X-ray background in the 14-150 keV range.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/510/A48
| Monitoring the hard X-ray sky with SuperAGILE
Context. SuperAGILE is the hard X-ray monitor of the AGILE gamma raymission, in orbit since 23 April 2007. It is an imaging experiment basedon a set of four independent silicon strip detectors, equipped withone-dimensional coded masks, operating in the nominal energy range 18-60keV. Aims: The main goal of SuperAGILE is the observation ofcosmic sources simultaneously with the main gamma-ray AGILE experiment,the Gamma Ray Imaging Detector (GRID). Given its ~steradian-wide fieldof view and its ~15 mCrab day-sensitivity, SuperAGILE is also wellsuited to the long-term monitoring of Galactic compact objects and thedetection of bright transients. Methods: The SuperAGILE detectorproperties and design allow for a 6 arcmin angular resolution in each ofthe two independent orthogonal projections of the celestial coordinates.Photon by photon data are continuously available by means of experimenttelemetry, and are used to derive images and fluxes of individualsources, with integration times depending on the source intensity andposition in the field of view. Results: We report on the mainscientific results achieved by SuperAGILE over its first two years inorbit, until April 2009. The scientific observations started in mid-July2007, with the science verification phase, continuing during thecomplete AGILE Cycle 1 and the first ~half of Cycle 2. Despite thelargely non-uniform sky coverage, due to the pointing strategy of theAGILE mission, a few tens of Galactic sources were monitored, sometimesfor unprecedently long continuous periods, leading to the detection alsoof several bursts and outbursts. Approximately one gamma ray burst permonth was detected and localized, allowing for prompt multiwavelengthobservations. A few extragalactic sources in bright states wereoccasionally detected as well. The light curves of sources measured bySuperAGILE are made publicly available on the web in almost real-time.To enable a proper scientific use of these, we provide the reader withthe relevant scientific and technical background.
| The Fourth IBIS/ISGRI Soft Gamma-ray Survey Catalog
In this paper, we report on the fourth soft gamma-ray source catalogobtained with the IBIS gamma-ray imager on board the INTEGRAL satellite.The scientific data set is based on more than 70 Ms of high-qualityobservations performed during the first five and a half years of theCore Program and public observations. Compared to previous IBIS surveys,this catalog includes a substantially increased coverage ofextragalactic fields, and comprises more than 700 high-energy sourcesdetected in the energy range 17-100 keV, including both transients andfaint persistent objects that can only be revealed with longer exposuretimes. A comparison is provided with the latest Swift/BAT surveyresults.Based on observations with INTEGRAL, an ESA project with instruments andScience Data Centre funded by ESA member states (especially the PIcountries: Denmark, France, Germany, Italy, Switzerland, Spain), CzechRepublic, and Poland, and with the participation of Russia and the USA.
| Probing the Nature of the Unidentified TeV Gamma-Ray Source Hess J0632+057 with Swift
New generation TeV gamma-ray telescopes have discovered many newsources, including several enigmatic unidentified TeV objects. HESSJ0632+057 is a particularly interesting unidentified TeV source since:it is a point source, it has a possible hard-spectrum X-ray counterpartand a positionally consistent Be star, it has evidence of long-term veryhigh energy gamma-ray flux variability, and it is postulated to be anewly detected TeV/X-ray binary. We have obtained Swift X-ray telescopeobservations of this source from MJD 54857 to 54965, in an attempt toascertain its nature and to investigate the hypothesis that it is apreviously unknown X-ray/TeV binary. Variability and spectral propertiessimilar to those of the other three known X-ray/TeV binaries have beenobserved, with measured flux increases by factors of ~3. X-rayvariability is present on multiple timescales including days to months;however, no clear signature of periodicity is present on the timescalesprobed by these data. If binary modulation is present and dominating themeasured variability, then the period of the orbit is likely to be>=54 days (half of this campaign), or it has a shorter period with avariable degree of flux modulation on successive high states. If the twohigh states measured to date are due to binary modulation, then thefavored period is approximately 35-40 days. More observations arerequired to determine if this object is truly a binary system and todetermine the extent that the measured variability is due to inter-orbitflaring effects or periodic binary modulation.
| Soft gamma repeaters
The observational properties of the soft gamma repeaters are reviewedbriefly, starting with the time histories and energy spectra of theirbursts. The short bursts and giant flares are compared. Their quiescentemission is presented, and the context of the magnetar model isdiscussed.
| MAGIC highlights.
The single-dish configuration of the MAGIC Cherenkov telescope has beenin operation since 2004 , with an operational window of 0.03-30 TeV. Thestereo configuration, made possible by the recently completed seconddish, is due to start a second phase of MAGIC operation soon. In thispaper we offer a review of MAGIC phase-1 (mono) results.
| On the Apparent Lack of Be X-Ray Binaries with Black Holes
In our Galaxy there are 64 Be X-ray binaries known to date. Out ofthese, 42 host a neutron star (NS), and for the remainder the nature ofthe companion is unknown. None, so far, are known to host a black hole(BH). There seems to be no apparent mechanism that would preventformation or detection of Be stars with BHs. This disparity is referredto as a missing Be-BH X-ray binary problem. We point out that currentevolutionary scenarios that lead to the formation of Be X-ray binariespredict that the ratio of binaries with NSs to the ones with BHs israther high, F NStoBH ~ 10-50, with the more likely formationmodels providing the values at the high end. The ratio is a naturaloutcome of (1) the stellar initial mass function that produces more NSsthan BHs and (2) common envelope evolution (i.e., a major mechanisminvolved in the formation of interacting binaries) that naturallyselects progenitors of Be X-ray binaries with NSs (binaries withcomparable mass components have more likely survival probabilities) overones with BHs (which are much more likely to be common envelopemergers). A comparison of this ratio (i.e., F NStoBH ~ 30)with the number of confirmed Be-NS X-ray binaries (42) indicates thatthe expected number of Be-BH X-ray binaries is of the order of only~0-2. This is entirely consistent with the observed Galactic sample.
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