Gamma-ray Luminosity Research Articles

THE SECOND CATALOG OF ACTIVE GALACTIC NUCLEI DETECTED BY THEFERMILARGE AREA TELESCOPE

The second catalog of active galactic nuclei (AGNs) detected by the Fermi Large Area Telescope (LAT) in two years of scientific operation is presented. The Second LAT AGN Catalog (2LAC) includes 1017 gamma-ray sources located at high Galactic latitudes (|b|>10{\deg}) that are detected with a test statistic greater than 25 and associated statistically with AGNs. However some of these are affected by analysis issues and some are associated with multiple AGNs. Consequently we define a clean sample which includes 886 AGNs, comprising 395 BL Lacertae objects (BL Lacs), 310 flat-spectrum radio quasars (FSRQs), 157 candidate blazars of unknown type (i.e., with broad-band blazar characteristics but with no optical spectral measurement yet), eight misaligned AGNs, four narrow-line Seyfert 1 (NLS1s), 10 AGNs of other types and two starburst galaxies. Where possible, the blazars have been further classified based on their spectral energy distributions (SEDs) as archival radio, optical, and X-ray data permit. While almost all FSRQs have a synchrotron-peak frequency 10^14 Hz, about half of the BL Lacs have a synchrotron-peak frequency >10^15 Hz. The 2LAC represents a significant improvement relative to the First LAT AGN Catalog (1LAC), with 52% more associated sources. The full characterization of the newly detected sources will require more broad-band data. Various properties, such as gamma-ray fluxes and photon power law spectral indices, redshifts, gamma-ray luminosities, variability, and archival radio luminosities---and their correlations are presented and discussed for the different blazar classes. The general trends observed in 1LAC are confirmed.

FERMILARGE AREA TELESCOPE OBSERVATIONS OF THE CYGNUS LOOP SUPERNOVA REMNANT

We present an analysis of the gamma-ray measurements by the Large Area Telescope(LAT) onboard the \textit{Fermi Gamma-ray Space Telescope} in the region of the supernova remnant(SNR) Cygnus Loop(G74.0$-$8.5). We detect significant gamma-ray emission associated with the SNR in the energy band 0.2--100 GeV. The gamma-ray spectrum shows a break in the range 2--3 GeV. The gamma-ray luminosity is $\sim$ $1 \times 10^{33}$erg s$^{-1}$ between 1--100 GeV, much lower than those of other GeV-emitting SNRs. The morphology is best represented by a ring shape, with inner/outer radii 0$^\circ$.7 $\pm$ 0$^\circ$.1 and 1$^\circ$.6 $\pm$ 0$^\circ$.1. Given the association among X-ray rims, \halpha filaments and gamma-ray emission, we argue that gamma rays originate in interactions between particles accelerated in the SNR and interstellar gas or radiation fields adjacent to the shock regions. The decay of neutral pions produced in nucleon-nucleon interactions between accelerated hadrons and interstellar gas provides a reasonable explanation for the gamma-ray spectrum.

Discovery of gamma- and X-ray pulsations from the young and energetic PSR J1357−6429 withFermiandXMM-Newton

Since the launch of the Fermi satellite, the number of known gamma-ray pulsars has increased tenfold. Most gamma-ray detected pulsars are young and energetic, and many are associated with TeV sources. PSR J1357-6429 is a high spin-down power pulsar (Edot = 3.1 * 10^36 erg/s), discovered during the Parkes multibeam survey of the Galactic plane, with significant timing noise typical of very young pulsars. In the very-high-energy domain, H.E.S.S. has reported the detection of the extended source HESS J1356-645 (intrinsic Gaussian width of 12') whose centroid lies 7' from PSR J1357-6429. Using a rotational ephemeris obtained with 74 observations made with the Parkes telescope at 1.4 GHz, we phase-fold more than two years of gamma-ray data acquired by Fermi-LAT as well as those collected with XMM-Newton, and perform gamma-ray spectral modeling. Significant gamma and X-ray pulsations are detected from PSR J1357-6429. The light curve in both bands shows one broad peak. Gamma-ray spectral analysis of the pulsed emission suggests that it is well described by a simple power-law of index 1.5 +/- 0.3stat +/- 0.3syst with an exponential cut-off at 0.8 +/- 0.3stat +/- 0.3syst GeV and an integral photon flux above 100 MeV of (6.5 +/- 1.6stat +/- 2.3syst) * 10^-8 cm^-2 s^-1. The X-ray spectra obtained from the new data provide results consistent with those reported by Zavlin (2007). Upper limits on the gamma-ray emission from its potential pulsar wind nebula (PWN) are also reported. Assuming a distance of 2.4 kpc, the Fermi LAT energy flux yields a gamma-ray luminosity for PSR J1357-6429 of L_gamma = (2.13 +/- 0.25stat +/- 0.83syst) * 10^34 erg/s, consistent with an L_gamma \propto sqrt(Edot) relationship. The Fermi non-detection of the pulsar wind nebula associated with HESS J1356-645 provides new constraints on the electron population responsible for the extended TeV emission.

SUB-LUMINOUS γ-RAY PULSARS

Most pulsars observed by the Fermi LAT have gamma-ray luminosities scaling with spindown power Edot as L_gamma (Edot x 10^33 erg/s)^{1/2}. However, there exist one detection and several upper limits an order of magnitude or more fainter than this trend. We describe these `sub-luminous' gamma-ray pulsars, and discuss the case for this being an orientation effect. Of the 12 known young radio pulsars with Edot>10^34 erg/s and d<2kpc several are substantially sub-luminous. The limited available geometrical constraints favor aligned geometries for these pulsars, although no one case for alignment is compelling. In this scenario GeV emission detected from such sub-luminous pulsars can be due to a lower altitude, lower-power accelerator gap.

THE EFFECT OF BLAZAR SPECTRAL BREAKS ON THE BLAZAR CONTRIBUTION TO THE EXTRAGALACTIC GAMMA-RAY BACKGROUND

The spectral shapes of the contributions of different classes of unresolved gamma-ray emitters can provide insight into their relative contributions to the extragalactic gamma-ray background (EGB) and the natures of their spectra at GeV energies. We calculate the spectral shapes of the contributions to the EGB arising from BL Lacertae type objects (BL Lacs) and flat-spectrum radio quasars (FSRQs) assuming blazar spectra can be described as broken power laws. We fit the resulting total blazar spectral shape to the Fermi Large Area Telescope measurements of the EGB, finding that the best-fit shape reproduces well the shape of the Fermi EGB for various break scenarios. We conclude that a scenario in which the contribution of blazars is dominant cannot be excluded on spectral grounds alone, even if spectral breaks are shown to be common among Fermi blazars. We also find that while the observation of a featureless (within uncertainties) power-law EGB spectrum by Fermi does not necessarily imply a single class of contributing unresolved sources with featureless individual spectra, such an observation and the collective spectra of the separate contributing populations determine the ratios of their contributions. As such, a comparison with studies including blazar gamma-ray luminosity functions could have profound implications for the blazar contribution to the EGB, blazar evolution, and blazar gamma-ray spectra and emission.

Implications of automatic photon quenching on compact gamma-ray sources

Aims: We investigate photon quenching in compact non-thermal sources. This involves photon-photon annihilation and lepton synchrotron radiation in a network that can become non-linear. As a result the gamma-ray luminosity of a source cannot exceed a critical limit that depends only on the radius of the source and on the magnetic field. Methods: We perform analytic and numerical calculations that verify previous results and extend them so that the basic properties of photon quenching are investigated. Results: We apply the above to the 2006 TeV observations of quasar 3C279 and obtain the parameter space of allowed values for the radius of the emitting source, its magnetic field strength and the Doppler factor of the flow. We argue that the TeV observations favour either a modest Doppler factor and a low magnetic field or a high Doppler factor and a high magnetic field.

A MULTIWAVELENGTH STUDY ON THE HIGH-ENERGY BEHAVIOR OF THEFERMI/LAT PULSARS

Using archival as well as freshly acquired data, we assess the X-ray behaviour of the Fermi/LAT gamma-ray pulsars listed in the First Fermi source catalog. After revisiting the relationships between the pulsars' rotational energy losses and their X and gamma-ray luminosities, we focus on the distance-indipendent gamma to X-ray flux ratios. When plotting our Fgamma/Fx values as a function of the pulsars' rotational energy losses, one immediately sees that pulsars with similar energetics have Fgamma/Fx spanning 3 decades. Such spread, most probably stemming from vastly different geometrical configurations of the X and gamma-ray emitting regions, defies any straightforward interpretation of the plot. Indeed, while energetic pulsars do have low Fgamma/Fx values, little can be said for the bulk of the Fermi neutron stars. Dividing our pulsar sample into radio-loud and radio-quiet subsamples, we find that, on average, radio-quiet pulsars do have higher values of Fgamma/Fx, implying an intrinsec faintness of their X-ray emission and/or a different geometrical configuration. Moreover, despite the large spread mentioned above, statistical tests show a lower scatter in the radio-quiet dataset with respect to the radio-loud one, pointing to a somewhat more constrained geometry for the radio-quiet objects with respect to the radio-loud ones.

CONTRIBUTION OF GAMMA-RAY-LOUD RADIO GALAXIES’ CORE EMISSIONS TO THE COSMIC MeV AND GeV GAMMA-RAY BACKGROUND RADIATION

The Fermi gamma-ray satellite has recently detected gamma-ray emissions from radio galaxy cores. From these samples, we first examine the correlation between the luminosities at 5 GHz, L_{5GHz}, and at 0.1-10 GeV, L_{gamma}, of these gamma-ray loud radio galaxies. We find that the correlation is significant with L_{gamma} \propto L_{5GHz}^{1.16} based on a partial correlation analysis. Using this correlation and the radio luminosity function (RLF) of radio galaxies, we further explore the contribution of gamma-ray loud radio galaxies to the unresolved extragalactic gamma-ray background (EGRB). The gamma-ray luminosity function is obtained by normalizing the RLF to reproduce the source count distribution of the Fermi gamma-ray loud radio galaxies. We find that gamma-ray loud radio galaxies will explain ~25% of the unresolved Fermi EGRB flux above 100 MeV and will also make a significant contribution to the EGRB in the 1-30 MeV energy band. Since blazars explain 22% of the EGRB above 100 MeV, radio loud active galactic nuclei (AGNs) population explains ~47% of the unresolved EGRB. We further make an interpretation on the origin of the EGRB. The observed EGRB spectrum at 0.2-100 GeV does not show an absorption signature by the extragalactic background light. Thus, the dominant population of the origin of EGRB at very high energy (>30 GeV) might be nearby gamma-ray emitting sources or sources with very hard gamma-ray spectrum.

The transition between BL Lac objects and flat spectrum radio quasars

We study the BL Lac objects detected in the one year all sky survey of the Fermi satellite, with a energy spectral slope alpha_gamma in the [0.1-100 GeV] band greater than 1.2. In the alpha_gamma vs gamma-ray luminosity plane, these BL Lacs occupy the region populated by Flat Spectrum Radio Quasars (FSRQs). Studying the properties of their spectral energy distributions (SED) and of their emitting lines, we find that several of these BL Lacs have a SED similar to FSRQs and that they do have broad lines of large equivalent width, and should be reclassified as FSRQs even adopting the current phenomenological definition (i.e. equivalent width EW of the emitting line greater than 5 A). In other cases, even if the EW width is small, the emitting lines can be as luminous as in quasars, and again their SED is similar to the SED of FSRQs. Sources classified as BL Lacs with a SED appearing as intermediate between BL Lacs and FSRQs also have relatively weak broad emission lines and small EW, and can be considered as transition sources. These properties are confirmed also by model fitting, that allows to derive the relevant intrinsic jet parameters and the jet power. This study leads us to propose a physical distinction between the two classes of blazars, based on the luminosity of the broad line region measured in Eddington units. The dividing line is of the order of L_BLR/L_Edd ~ 5e-4, in good agreement with the idea that the presence of strong emitting lines is related to a transition in the accretion regime, becoming radiatively inefficient below a disk luminosity of the order of one per cent of the Eddington one.

Understanding hadronic gamma-ray emission from supernova remnants

We aim to test the plausibility of a theoretical framework in which the gamma-ray emission detected from supernova remnants may be of hadronic origin, i.e., due to the decay of neutral pions produced in nuclear collisions involving relativistic nuclei. In particular, we investigate the effects induced by magnetic field amplification on the expected particle spectra, outlining a phenomenological scenario consistent with both the underlying Physics and the larger and larger amount of observational data provided by the present generation of gamma experiments, which seem to indicate rather steep spectra for the accelerated particles. In addition, in order to study to study how pre-supernova winds might affect the expected emission in this class of sources, the time-dependent gamma-ray luminosity of a remnant with a massive progenitor is worked out. Solid points and limitations of the proposed scenario are finally discussed in a critical way.

γ-rays in flat-spectrum AGN: revisiting the fast jet hypothesis with the CJF sample

The recent release of the First Fermi-LAT Source Catalog solidified the predominant association of extragalactic gamma-ray emitters to active galaxies, in particular blazars. A tight connection between AGN jet kinematics and gamma-ray properties has been argued for, attributing the energetic emission from active galaxies to their highly relativistic outflows. We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample to study the connection between AGN jet kinematics and their gamma-ray properties. The high number of sources included in the sample, in addition to the excellent kinematic data available, allows us to investigate the origin of gamma-ray emission in AGN. We identify the CJF sources detected in gamma-rays (by Fermi-LAT and EGRET). We use gamma-ray luminosities and the available VLBI kinematic data to look for correlations between gamma-ray and kinematic properties, as well as for differences between AGN classes (quasars, BL Lacs, radio galaxies). We also check the kinematics of the TeV sources in the CJF. 21.8% of the CJF has been detected in the gamma-rays. We find the detectability of BL Lacs significantly higher compared to quasars. gamma-detected sources show a wider apparent jet velocity distribution compared to the non-detected ones, but the maxima of both distributions are at similar values. No strong link between gamma-ray detectability and fast apparent jet speeds is found. A tentative correlation is found between gamma-ray luminosity and maximum apparent jet speeds, stronger for BL Lac and gamma-variable sources. We find non-radial jet motions to be important to gamma-ray emission. We suggest two-zone, spine-sheath, models as a possible explanation to our results. We find 2 out of 4 CJF TeV sources show superluminal jet speeds, in contrast to previous studies.

GAMMA-RAY EMISSION FROM THE GLOBULAR CLUSTERS LILLER 1, M80, NGC 6139, NGC 6541, NGC 6624, AND NGC 6752

Globular clusters (GCs) are emerging as a new class of gamma-ray emitters, thanks to the data obtained from the Fermi Gamma-ray Space Telescope. By now, eight GCs are known to emit gamma-rays at energies >100~MeV. Based on the stellar encounter rate of the GCs, we identify potential gamma-ray emitting GCs out of all known GCs that have not been studied in details before. In this paper, we report the discovery of a number of new gamma-ray GCs: Liller 1, NGC 6624, and NGC 6752, and evidence for gamma-ray emission from M80, NGC 6139, and NGC 6541, in which gamma-rays were found within the GC tidal radius. With one of the highest metallicity among all GCs in the Milky Way, the gamma-ray luminosity of Liller 1 is found to be the highest of all known gamma-ray GCs. In addition, we confirm a previous report of significant gamma-ray emitting region next to NGC 6441. We briefly discuss the observed offset of gamma-rays from some GC cores. The increasing number of known gamma-ray GCs at distances out to ~10 kpc is important for us to understand the gamma-ray emitting mechanism and provides an alternative probe to the underlying millisecond pulsar populations of the GCs.

CONTRIBUTION FROM STAR-FORMING GALAXIES TO THE COSMIC GAMMA-RAY BACKGROUND RADIATION

We present a new theoretical calculation of the contribution to the extragalactic gamma-ray background radiation (EGRB) from star-forming galaxies, based on a state-of-the-art model of hierarchical galaxy formation that is in quantitative agreement with a variety of observations of local and high-redshift galaxies. Gamma-ray luminosity ($L_\gamma$) and spectrum of galaxies are related to star formation rate ($\psi$), gas mass ($M_gas$), and star formation mode (quiescent or starburst) of model galaxies using latest observed data of nearby galaxies. We try the two limiting cases about gamma-ray production: the escape limit ($L_\gamma \propto \psi M_gas$) and the calorimetric limit ($L_\gamma \propto \psi$), and our standard model predicts 7 and 4% contribution from star-forming galaxies to the total EGRB flux (including bright resolved sources) recently reported by the Fermi Gamma-Ray Space Telescope. Systematic uncertainties do not allow us to determine the EGRB flux better than by a factor of ~2. The predicted number of nearby galaxies detectable by Fermi is consistent with the observation. Intergalactic absorption by pair-production attenuates the EGRB flux only by a modest factor of ~1.3 at the highest Fermi energy band, and the reprocessed cascade emission does not significantly alter EGRB at lower photon energies. The sum of the known contributions from AGNs and star-forming galaxies can explain a large part of EGRB, with a remarkable agreement between the predicted model spectrum and observation.

Twelve-hour spikes from the Crab Pevatron

Aims. The Crab nebula displayed a large gamma-ray flare on September 18, 2010. To more closely understand the origin of this phenomenon, we analyze the INTEGRAL (20-500 keV) and FERMI (0.1-300 GeV) data collected almost simultaneously during the flare. Methods. We divide the available data into three different sets, corresponding to the pre-flare period, the flare, and the subsequent quiescence. For each period, we perform timing and spectral analyses to differentiate between the contributions of the pulsar and from the surrounding nebula to the gamma-ray luminosity. Results. No significant variations in the pulse profile and spectral characteristics are detected in the hard X-ray domain. In contrast, we identify three separate enhancements in the gamma-ray flux lasting for about 12 hours and separated by an interval of about two days from each other. The spectral analysis shows that the flux enhancement, confined below ~ 1 GeV, can be modelled by a power-law with a high energy exponential cut-off, where either the cut-off energy or the model normalization increased by a factor of ~ 5 relative to the pre-flare emission. We also confirm that the gamma-ray flare is not pulsed. Conclusions. The timing and spectral analysis indicate that the gamma-ray flare is due to synchrotron emission from a very compact Pevatron located in the region of interaction between the pulsar wind and the surrounding nebula. These are the highest electron energies ever measured in a cosmic accelerator. The spectral properties of the flare are interpreted in the framework of a relativistically moving emitter and/or a harder emitting electron population.

ERRATUM: “THE BLAZAR SEQUENCE AND THE COSMIC GAMMA-RAY BACKGROUND RADIATION IN THEFERMIERA” (2009, ApJ, 702, 523)

(Abridged) We present a new model of the blazar gamma-ray luminosity function (GLF) and the spectrum of the extragalactic gamma-ray background (EGRB), which is consistent with the observed distributions of EGRET blazars. The unified sequence of blazar spectral energy distribution (SED) is taken into account to make a non-trivial prediction for the EGRB spectrum and more realistic comparison with the data than previous studies. We then try to explain the EGRB data by the two AGN populations: one is blazars, and the other is non-blazar AGNs that are responsible for the EGRB in the MeV band. We find that ~80% of the EGRB photon flux at > 100 MeV can be explained by the sum of the two populations, while ~45% can be accounted for only by blazars. The predicted EGRB spectrum is in agreement with a wide range of the observed data from X-ray to GeV, within the systematic uncertainties in the EGRB determination by EGRET. These results indicate that AGNs including blazars are the primary source of EGRB. Blazars are dominant in EGRB at higher energy bands of >100 MeV, while non-blazar AGNs dominate at <100 MeV. Almost all of the EGRB flux from blazars will be resolved into discrete sources by the Fermi Gamma-ray Space Telescope, while that from non-blazar AGNs will largely remain unresolved. Therefore, comparison between the integrated source counts and diffuse EGRB flux as a function of photon energy will give a simple and clear test of our model. Various quantitative predictions for Fermi observations are also made. Especially, our model predicts 600-1200 blazars in all sky down to 2e-9 photons/cm^-2/s (>100 MeV), which is considerably smaller than most of previous studies.

THE RADIATIVE X-RAY AND GAMMA-RAY EFFICIENCIES OF ROTATION-POWERED PULSARS

We present a statistical analysis of the X-ray luminosity of rotation powered pulsars and their surrounding nebulae using the sample of Kargaltsev & Pavlov (2008) and we complement this with an analysis of the gamma-ray-emission of Fermi detected pulsars. We report a strong trend in the efficiency with which spin-down power is converted to X-ray and gamma-ray emission with characteristic age: young pulsars and their surrounding nebulae are efficient X-ray emitters, whereas in contrast old pulsars are efficient gamma-ray emitters. We divided the X-ray sample in a young (Tau < 1.7x10^4 yr) and old sample and used linear regression to search for correlations between the logarithm of the X-ray and gamma-ray luminosities and the logarithms of the periods and period derivatives. The X-ray emission from young pulsars and their nebulae are both consistent with L_X ~ Pdot^3/P^6. For old pulsars and their nebulae the X-ray luminosity is consistent with a more or less constant efficiency eta = L_X/Edot = ~ 8x10^-5. For the gamma-ray luminosity we confirm that L_gamma ~ Edot^(1/2). We discuss these findings in the context of pair production inside pulsar magnetospheres and the striped wind model. We suggest that the striped wind model may explain the similarity between the X-ray properties of the pulsar wind nebulae and the pulsars themselves, which according to the striped wind model may both find their origin outside the light cylinder, in the pulsar wind zone.

PSRs J0248+6021 and J2240+5832: young pulsars in the northern Galactic plane

Pulsars PSR J0248+6021 (rotation period P=217 ms and spin-down power Edot = 2.13E35 erg/s) and PSR J2240+5832 (P=140 ms, Edot = 2.12E35 erg/s) were discovered in 1997 with the Nancay radio telescope during a northern Galactic plane survey, using the Navy-Berkeley Pulsar Processor (NBPP) filter bank. GeV gamma-ray pulsations from both were discovered using the Fermi Large Area Telescope. Twelve years of radio and polarization data allow detailed investigations. The two pulsars resemble each other both in radio and in gamma-ray data. Both are rare in having a single gamma-ray pulse offset far from the radio peak. The high dispersion measure for PSR J0248+6021 (DM = 370 pc cm^-3) is most likely due to its being within the dense, giant HII region W5 in the Perseus arm at a distance of 2 kpc, not beyond the edge of the Galaxy as obtained from models of average electron distributions. Its high transverse velocity and the low magnetic field along the line-of-sight favor this small distance. Neither gamma-ray, X-ray, nor optical data yield evidence for a pulsar wind nebula surrounding PSR J0248+6021. The gamma-ray luminosity for PSR J0248+6021 is L_ gamma = (1.4 \pm 0.3)\times 10^34 erg/s. For PSR J2240+5832, we find either L_gamma = (7.9 \pm 5.2) \times 10^34 erg/s if the pulsar is in the Outer arm, or L_gamma = (2.2 \pm 1.7) \times 10^34 erg/s for the Perseus arm. These luminosities are consistent with an L_gamma ~ sqrt(Edot) rule. Comparison of the gamma-ray pulse profiles with model predictions, including the constraints obtained from radio polarization data, favor emission in the far magnetosphere. These two pulsars differ mainly in their inclination angles and acceleration gap widths, which in turn explains the observed differences in the gamma-ray peak widths.

A population of gamma-ray emitting globular clusters seen with theFermiLarge Area Telescope

Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. We use 546 days of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular clusters. Steady point-like high-energy gamma-ray emission has been significantly detected towards 8 globular clusters. Five of them (47 Tucanae, Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices $(0.7 < \Gamma <1.4)$ and clear evidence for an exponential cut-off in the range 1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral indices $(1.0 < \Gamma < 1.7)$, however the presence of an exponential cut-off can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC 6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral properties. From the observed gamma-ray luminosities, we estimate the total number of MSPs that is expected to be present in these globular clusters. We show that our estimates of the MSP population correlate with the stellar encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters, commensurate with previous estimates. The observation of high-energy gamma-ray emission from a globular cluster thus provides a reliable independent method to assess their millisecond pulsar populations that can be used to make constraints on the original neutron star X-ray binary population, essential for understanding the importance of binary systems in slowing the inevitable core collapse of globular clusters.

Searching for the most distant blazars with the Fermi Gamma-ray Space Telescope

We investigate the prospects for discovering blazars at very high-redshifts (z>3-6) with the Fermi Gamma-Ray Space Telescope (Fermi), employing a model for the evolving gamma-ray luminosity function (GLF) of the blazar population. Our previous GLF model is used as a basis, which features luminosity-dependent density evolution implied from X-ray data on active galactic nuclei, as well as the blazar sequence paradigm for their spectral energy distribution, and which is consistent with EGRET and current Fermi observations of blazars.Here we augment the high-redshift evolution of this model by utilizing the luminosity function of quasars from the Sloan Digital Sky Survey (SDSS), which is well-constrained up to z~5. We find that Fermi may discover a few blazars up to z~6 in the entire sky during its 5-year survey. We further discuss how such high-redshift blazar candidates may be efficiently selected in future Fermi data.

FermiLarge Area Telescope observations of Local Group galaxies: detection of M 31 and search for M 33

Cosmic rays (CRs) can be studied through the galaxy-wide gamma-ray emission that they generate when propagating in the interstellar medium. The comparison of the diffuse signals from different systems may inform us about the key parameters in CR acceleration and transport. We aim to determine and compare the properties of the CR-induced gamma-ray emission of several Local Group galaxies. We use 2 years of nearly continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to search for gamma-ray emission from M31 and M33. We compare the results with those for the Large Magellanic Cloud, the Small Magellanic Cloud, the Milky Way, and the starburst galaxies M82 and NGC253. We detect a gamma-ray signal at 5sigma significance in the energy range 200 MeV-20 GeV that is consistent with originating from M31. The integral photon flux above 100MeV amounts to 9.1 +/- 1.9 (stat) +/- 1.0 (sys) x10e-9 ph/cm2/s. We find no evidence for emission from M33 and derive an upper limit on the photon flux >100MeV of 5.1 x10e-9 ph/cm2/s (2sigma). Comparing these results to the properties of other Local Group galaxies, we find indications for a correlation between star formation rate and gamma-ray luminosity that also holds for the starburst galaxies. The gamma-ray luminosity of M31 is about half that of the Milky Way, which implies that the ratio between the average CR densities in M31 and the Milky Way amounts to 0.35 +/- 0.25. The observed correlation between gamma-ray luminosity and star formation rate suggests that the flux of M33 is not far below the current upper limit from the LAT observations.