Fermi-LAT Observations of the Gamma-ray Burst GRB 130427A

The observations of the exceptionally bright gamma-ray burst (GRB) 130427A by the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope provide constraints on the nature of such unique astrophysical sources. GRB 130427A had the largest fluence, highest-energy photon (95 GeV), longest $\gamma$-ray duration (20 hours), and one of the largest isotropic energy releases ever observed from a GRB. Temporal and spectral analyses of GRB 130427A challenge the widely accepted model that the non-thermal high-energy emission in the afterglow phase of GRBs is synchrotron emission radiated by electrons accelerated at an external shock.Comment: 30 pages, 6 figures. Accepted for publication in Science. Corresponding authors: S. Zhu ([email protected]); J. Chiang ([email protected]); C. Dermer ([email protected]); N. Omodei ([email protected]); G. Vianello ([email protected]); S. Xiong ([email protected]

Search for Cosmic-Ray Electron and Positron Anisotropies with Seven Years of Fermi Large Area Telescope Data

The Large Area Telescope on board the Fermi Gamma-ray Space Telescope has collected the largest ever sample of high-energy cosmic-ray electron and positron events since the beginning of its operation. Potential anisotropies in the arrival directions of cosmic-ray electrons or positrons could be a signature of the presence of nearby sources. We use almost seven years of data with energies above 42 GeV processed with the Pass~8 reconstruction. The present data sample can probe dipole anisotropies down to a level of $10^{-3}$. We take into account systematic effects that could mimic true anisotropies at this level. We present a detailed study of the event selection optimization of the cosmic-ray electrons and positrons to be used for anisotropy searches. Since no significant anisotropies have been detected on any angular scale, we present upper limits on the dipole anisotropy. The present constraints are among the strongest to date probing the presence of nearby young and middle-aged sources.Comment: 8 pages; 3 figures; Published in Phys. Rev. Lett. 118, 091103; Contact authors: Francesco Costanza [email protected] and M. Nicola Mazziotta [email protected]; Supplemental Material also available on the Ancillary file

Observations of M31 and M33 with the Fermi Large Area Telescope: a galactic center excess in Andromeda?

The Fermi LAT has opened the way for comparative studies of cosmic rays (CRs) and high-energy objects in the Milky Way (MW) and in other, external, star-forming galaxies. Using 2 yr of observations with the Fermi LAT, local Group galaxy M31 was detected as a marginally extended gamma-ray source, while only an upper limit (UL) has been derived for the other nearby galaxy M33. We revisited the gamma-ray emission in the direction of M31 and M33 using more than 7 yr of LAT Pass 8 data in the energy range 0.1-100 GeV, presenting detailed morphological and spectral analyses. M33 remains undetected and we computed an UL for it. This revised UL remains consistent with the observed correlation between gamma-ray luminosity and star-formation rate tracers and implies an average CR density in M33 that is at most half of that of the MW. M31 is detected with a significance of nearly 10 sigma and to be extended with 4 sigma. Its spectrum is consistent with a power law. The spatial distribution of the emission is consistent with a uniform disk with a radius of 0.4 deg and no offset from the center of M31, but nonuniform intensity distributions cannot be excluded. The flux from M31 appears confined to the inner regions of the galaxy and does not fill the disk or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star-formation activity suggesting that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. Alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of M31 by disrupted globular clusters or from the decay or annihilation of dark matter particles, similar to what has been proposed to account for the so-called Galactic Center excess found in Fermi-LAT observations of the MW.Comment: 14 pages, 7 figures, 4 table

Searching for Dark Matter Annihilation from Milky Way Dwarf Spheroidal Galaxies with Six Years of Fermi-LAT Data

The dwarf spheroidal satellite galaxies (dSphs) of the Milky Way are some of the most dark matter (DM) dominated objects known. We report on gamma-ray observations of Milky Way dSphs based on 6 years of Fermi Large Area Telescope data processed with the new Pass 8 event-level analysis. None of the dSphs are significantly detected in gamma rays, and we present upper limits on the DM annihilation cross section from a combined analysis of 15 dSphs. These constraints are among the strongest and most robust to date and lie below the canonical thermal relic cross section for DM of mass $\lesssim$ 100 GeV annihilating via quark and $\tau$-lepton channels.Comment: Accepted by PRL. 18 pages, 8 figures, 2 tables. Includes Supplementary Materials. Corresponding Authors: Brandon Anderson, Alex Drlica-Wagner, Matthew Woo

Characterizing the population of pulsars in the inner Galaxy with the Fermi Large Area Telescope

An excess of $\gamma$-ray emission from the Galactic Center (GC) region with respect to predictions based on a variety of interstellar emission models and $\gamma$-ray source catalogs has been found by many groups using data from the {\it Fermi} Large Area Telescope (LAT). Several interpretations of this excess have been invoked. In this paper we search for members of an unresolved population of $\gamma$-ray pulsars located in the inner Galaxy that are predicted by the interpretation of the GC excess as being due to a population of such sources. We use cataloged LAT sources to derive criteria that efficiently select pulsars with very small contamination from blazars. We search for point sources in the inner $40^\circ\times40^\circ$ region of the Galaxy, derive a list of approximately 400 sources, and apply pulsar selection criteria to extract pulsar candidates among our source list. We performed the entire data analysis chain with two different interstellar emission models (IEMs), and found a total of 135 pulsar candidates, of which 66 were selected with both IEMs.Comment: Corresponding Authors: Mattia Di Mauro, Eric Charles and Matthew Wood. 15 pages, 14 figures, 2 tables. This version does not include the maximum likelihood analysis section, that was present in the previous version of the paper, since an error was present in that part of the analysis. It includes a new section with a study of the systematics in the search for point source

Fermi-LAT observations of the LIGO/Virgo event GW170817

We present the Fermi Large Area Telescope (LAT) observations of the binary neutron star merger event GW170817 and the associated short gamma-ray burst (SGRB) GRB\,170817A detected by the Fermi Gamma-ray Burst Monitor. The LAT was entering the South Atlantic Anomaly at the time of the LIGO/Virgo trigger ($t_{\rm GW}$) and therefore cannot place constraints on the existence of high-energy (E $>$ 100 MeV) emission associated with the moment of binary coalescence. We focus instead on constraining high-energy emission on longer timescales. No candidate electromagnetic counterpart was detected by the LAT on timescales of minutes, hours, or days after the LIGO/Virgo detection. The resulting flux upper bound (at 95\% C.L.\/) from the LAT is $4.5\times$10$^{-10}$ erg cm$^{-2}$ s$^{-1}$ in the 0.1--1 GeV range covering a period from T0 + 1153 s to T0 + 2027 s. At the distance of GRB\,170817A, this flux upper bound corresponds to a luminosity upper bound of 9.7$\times10^{43}$ erg s$^{-1}$, which is 5 orders of magnitude less luminous than the only other LAT SGRB with known redshift, GRB\,090510. We also discuss the prospects for LAT detection of electromagnetic counterparts to future gravitational wave events from Advanced LIGO/Virgo in the context of GW170817/GRB\,170817A.Comment: Corresponding authors in alphabetical order: Daniel Kocevski ([email protected]), Nicola Omodei ([email protected]), Giacomo Vianello ([email protected]

The Spectrum and Morphology of the Fermi Bubbles

The Fermi bubbles are two large structures in the gamma-ray sky extending to $55^\circ$ above and below the Galactic center. We analyze 50 months of Fermi Large Area Telescope data between 100 MeV and 500 GeV above $10^\circ$ in Galactic latitude to derive the spectrum and morphology of the Fermi bubbles. We thoroughly explore the systematic uncertainties that arise when modeling the Galactic diffuse emission through two separate approaches. The gamma-ray spectrum is well described by either a log parabola or a power law with an exponential cutoff. We exclude a simple power law with more than 7$\sigma$ significance. The power law with an exponential cutoff has an index of $1.9 \pm 0.2$ and a cutoff energy of $110\pm 50$ GeV. We find that the gamma-ray luminosity of the bubbles is $4.4^{+2.4}_{-0.9} \times 10^{37}$ erg s$^{-1}$. We confirm a significant enhancement of gamma-ray emission in the south-eastern part of the bubbles, but we do not find significant evidence for a jet. No significant variation of the spectrum across the bubbles is detected. The width of the boundary of the bubbles is estimated to be $3.4^{+3.7}_{-2.6}$ deg. Both inverse Compton (IC) models and hadronic models including IC emission from secondary leptons fit the gamma-ray data well. In the IC scenario, the synchrotron emission from the same population of electrons can also explain the WMAP and Planck microwave haze with a magnetic field between 5 and 20 $\mu$G.Comment: 66 pages, 48 figures. Accepted for publication in ApJ. Corresponding authors: A. Franckowiak ([email protected]), D. Malyshev ([email protected]), V. Petrosian ([email protected]

Fermi LAT Stacking Analysis of Swift Localized Gamma-ray Bursts

We perform a comprehensive stacking analysis of data collected by the Fermi Large Area Telescope (LAT) of gamma-ray bursts (GRB) localized by the Swift spacecraft, which were not detected by the LAT but which fell within the instrument's field of view at the time of trigger. We examine a total of 79 GRBs by comparing the observed counts over a range of time intervals to that expected from designated background orbits, as well as by using a joint likelihood technique to model the expected distribution of stacked counts. We find strong evidence for subthreshold emission at MeV to GeV energies using both techniques. This observed excess is detected during intervals that include and exceed the durations typically characterizing the prompt emission observed at keV energies and lasts at least 2700 s after the co-aligned burst trigger. By utilizing a novel cumulative likelihood analysis, we find that although a burst's prompt gamma-ray and afterglow X-ray flux both correlate with the strength of the subthreshold emission, the X-ray afterglow flux measured by Swift's X-ray Telescope (XRT) at 11 hr post trigger correlates far more significantly. Overall, the extended nature of the subthreshold emission and its connection to the burst's afterglow brightness lend further support to the external forward shock origin of the late-time emission detected by the LAT. These results suggest that the extended high-energy emission observed by the LAT may be a relatively common feature but remains undetected in a majority of bursts owing to instrumental threshold effects.Comment: Contact Authors: D. Kocevski, [email protected]; J. Chiang, [email protected]; J. Racusin, [email protected]; 39 page, 13 figures, 1 Table, Accepted to Ap

Fermi Large Area Telescope Detection of Extended Gamma-Ray Emission from the Radio Galaxy Fornax A

We report the Fermi Large Area Telescope detection of extended gamma-ray emission from the lobes of the radio galaxy Fornax A using 6.1 years of Pass 8 data. After Centaurus A, this is now the second example of an extended gamma-ray source attributed to a radio galaxy. Both an extended flat disk morphology and a morphology following the extended radio lobes were preferred over a point-source description, and the core contribution was constrained to be < 14% of the total gamma-ray flux. A preferred alignment of the gamma-ray elongation with the radio lobes was demonstrated by rotating the radio lobes template. We found no significant evidence for variability on ~0.5 year timescales. Taken together, these results strongly suggest a lobe origin for the gamma rays. With the extended nature of the > 100 MeV gamma-ray emission established, we model the source broadband emission considering currently available total lobe radio and millimeter flux measurements, as well as X-ray detections attributed to inverse Compton (IC) emission off the cosmic microwave background (CMB). Unlike the Centaurus A case, we find that a leptonic model involving IC scattering of CMB and extragalactic background light (EBL) photons underpredicts the gamma-ray fluxes by factors of about ~ 2 - 3, depending on the EBL model adopted. An additional gamma-ray spectral component is thus required, and could be due to hadronic emission arising from proton-proton collisions of cosmic rays with thermal plasma within the radio lobes.Comment: Accepted for publication in The Astrophysical Journal. --Corresponding authors: J. D. Magill (jmagill_at_umd.edu), W. McConville (wmcconvi_at_umd.edu), M. Georganopoulos (georgano_at_umbc.edu), \L. Stawarz (stawarz_at_oa.uj.edu.pl), C. C. Cheung ([email protected]

The Fermi Galactic Center GeV Excess and Implications for Dark Matter

The region around the Galactic center (GC) is now well established to be brighter at energies of a few GeV than expected from conventional models of diffuse gamma-ray emission and catalogs of known gamma-ray sources. We study the GeV excess using 6.5 years of data from the Fermi Large Area Telescope. We characterize the uncertainty of the GC excess spectrum and morphology due to uncertainties in cosmic-ray source distributions and propagation, uncertainties in the distribution of interstellar gas in the Milky Way, and uncertainties due to a potential contribution from the Fermi bubbles. We also evaluate uncertainties in the excess properties due to resolved point sources of gamma rays. The Galactic center is of particular interest as it would be expected to have the brightest signal from annihilation of weakly interacting massive dark matter particles. However, control regions along the Galactic plane, where a dark-matter signal is not expected, show excesses of similar amplitude relative to the local background. Based on the magnitude of the systematic uncertainties, we conservatively report upper limits for the annihilation cross section as function of particle mass and annihilation channel.Comment: Contact authors: A. Albert, E. Charles, A. Franckowiak, D. Malyshev, L. Tibaldo. 63 pages, 34 figures. Published in Ap