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 Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope

The fourth catalog of active galactic nuclei (AGNs) detected by the Fermi Gamma-ray Space Telescope Large Area Telescope (4LAC) between 2008 August 4 and 2016 August 2 contains 2863 objects located at high Galactic latitudes (|b|>10{\deg}). It includes 85% more sources than the previous 3LAC catalog based on 4 years of data. AGNs represent at least 79% of the high-latitude sources in the fourth Fermi-Large Area Telescope Source Catalog (4FGL), which covers the energy range from 50 MeV to 1 TeV. In addition, 344 gamma-ray AGNs are found at low Galactic latitudes. Most of the 4LAC AGNs are blazars (98%), while the remainder are other types of AGNs. The blazar population consists of 24% Flat Spectrum Radio Quasars (FSRQs), 38% BL Lac-type objects (BL Lacs), and 38% blazar candidates of unknown types (BCUs). On average, FSRQs display softer spectra and stronger variability in the gamma-ray band than BL Lacs do, confirming previous findings. All AGNs detected by ground-based atmospheric Cherenkov telescopes are also found in the 4LAC.Comment: Fits tables can be found at https://www.ssdc.asi.it/fermi4lac/table_4LAC.fits and https://www.ssdc.asi.it/fermi4lac/table_lowlat_sample.fits. About 200 counterpart names have changed relative to the earlier version and match the entries in the 4FGL-DR2 catalog (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/10yr_catalog/

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

Investigating the Nature of Late-Time High-Energy GRB Emission Through Joint Fermi\Swift Observations

We use joint observations by the Neil Gehrels Swift X-ray Telescope (XRT) and the Fermi Large Area Telescope (LAT) of gamma-ray burst (GRB) afterglows to investigate the nature of the long-lived high-energy emission observed by Fermi LAT. Joint broadband spectral modeling of XRT and LAT data reveal that LAT nondetections of bright X-ray afterglows are consistent with a cooling break in the inferred electron synchrotron spectrum below the LAT and/or XRT energy ranges. Such a break is sufficient to suppress the high-energy emission so as to be below the LAT detection threshold. By contrast, LAT-detected bursts are best fit by a synchrotron spectrum with a cooling break that lies either between or above the XRT and LAT energy ranges. We speculate that the primary difference between GRBs with LAT afterglow detections and the non-detected population may be in the type of circumstellar environment in which these bursts occur, with late-time LAT detections preferentially selecting GRBs that occur in low wind-like circumburst density profiles. Furthermore, we find no evidence of high-energy emission in the LAT-detected population significantly in excess of the flux expected from the electron synchrotron spectrum fit to the observed X-ray emission. The lack of excess emission at high energies could be due to a shocked external medium in which the energy density in the magnetic field is stronger than or comparable to that of the relativistic electrons behind the shock, precluding the production of a dominant synchrotron self-Compton (SSC) component in the LAT energy range. Alternatively, the peak of the SSC emission could be beyond the 0.1-100 GeV energy range considered for this analysis.Comment: 25 pages, 7 figures, and 2 table

Search for Spectral Irregularities due to Photon-Axionlike-Particle Oscillations With the Fermi Large Area Telescope

We report on the search for spectral irregularities induced by oscillations between photons and axionlike-particles (ALPs) in the $\gamma$-ray spectrum of NGC 1275, the central galaxy of the Perseus cluster. Using six years of Fermi Large Area Telescope data, we find no evidence for ALPs and exclude couplings above $5\times10^{-12}\,\mathrm{GeV}^{-1}$ for ALP masses $0.5 \lesssim m_a \lesssim 5$ neV at 95% confidence. The limits are competitive with the sensitivity of planned laboratory experiments, and, together with other bounds, strongly constrain the possibility that ALPs can reduce the $\gamma$-ray opacity of the Universe.Comment: Accepted by PRL. Includes Supplemental Material. 8+10 pages, 2+7 figures, 1+2 tables. References updated. Matches published version. Corresponding Authors: Manuel Meyer, Jan Conrad, Miguel Sanchez-Cond

Contemporaneous broadband observations of three high-redshift BL Lac Objects

We have collected broadband spectral energy distributions (SEDs) of three BL Lac objects, 3FGL J0022.1$-$1855 (z=0.689), 3FGL J0630.9$-$2406 (z >~ 1.239), and 3FGL J0811.2$-$7529 (z=0.774), detected by Fermi with relatively flat GeV spectra. By observing simultaneously in the near-IR to hard X-ray band, we can well characterize the high end of the synchrotron component of the SED. Thus, fitting the SEDs to synchro-Compton models of the dominant emission from the relativistic jet, we can constrain the underlying particle properties and predict the shape of the GeV Compton component. Standard extragalactic background light (EBL) models explain the high-energy absorption well, with poorer fits for high UV models. The fits show clear evidence for EBL absorption in the Fermi spectrum of our highest redshift source 3FGL J0630.9$-$2406. While synchrotron self-Compton models adequately describe the SEDs, the situation may be complicated by possible external Compton components. For 3FGL J0811.2$-$7529, we also discover a nearby serendipitous source in the X-ray data, which is almost certainly another lower synchrotron peak frequency ($\nu^{sy}_{pk}$) BL Lac, that may contribute flux in the Fermi band. Since our sources are unusual high-luminosity, moderate $\nu^{sy}_{pk}$ BL Lacs we compare these quantities and the Compton dominance, the ratio of peak inverse-Compton to peak synchrotron luminosities ($L^{IC}_{pk}/L^{sy}_{pk}$), with those of the full Fermi BL Lac population.Comment: 12 pages, 6 figures, 5 tables, accepted for publication in Ap

Search for gamma-ray emission from the Coma Cluster with six years of Fermi-LAT data

We present results from {\gamma}-ray observations of the Coma cluster incorporating 6 years of Fermi-LAT data and the newly released {\emph{Pass 8}} event-level analysis. Our analysis of the region reveals low-significance residual structures within the virial radius of the cluster that are too faint for a detailed investigation with the current data. Using a likelihood approach that is free of assumptions on the spectral shape we derive upper limits on the {\gamma}-ray flux that is expected from energetic particle interactions in the cluster. We also consider a benchmark spatial and spectral template motivated by models in which the observed radio halo is mostly emission by secondary electrons. In this case, the median expected and observed upper limits for the flux above 100 MeV are $1.7\times10^{-9}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ and $5.2\times10^{-9}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ respectively (the latter corresponds to residual emission at the level of 1.8{\sigma}). These bounds are comparable to or higher than predicted levels of hadronic gamma-ray emission in cosmic-ray models with or without reacceleration of secondary electrons, although direct comparisons are sensitive to assumptions regarding the origin and propagation mode of cosmic rays and magnetic field properties. The minimal expected {\gamma}-ray flux from radio and star-forming galaxies within the Coma cluster is roughly an order of magnitude below the median sensitivity of our analysis.Comment: 9 pages, 4 figures, 2 tables, minor revised version accepted for publication; corresponding authors: S. Zimmer, J. Conrad, O. Reimer & Y. Rephael

Search for Gamma-Ray Emission from Local Primordial Black Holes with the Fermi Large Area Telescope

Black holes with masses below approximately $10^{15}$ g are expected to emit gamma rays with energies above a few tens of MeV, which can be detected by the Fermi Large Area Telescope (LAT). Although black holes with these masses cannot be formed as a result of stellar evolution, they may have formed in the early Universe and are therefore called Primordial Black Holes (PBHs). Previous searches for PBHs have focused on either short timescale bursts or the contribution of PBHs to the isotropic gamma-ray emission. We show that, in case of individual PBHs, the Fermi LAT is most sensitive to PBHs with temperatures above approximately 16 GeV and masses $6\times 10^{11}$ g, which it can detect out to a distance of about 0.03 pc. These PBHs have a remaining lifetime of months to years at the start of the Fermi mission. They would appear as potentially moving point sources with gamma-ray emission that becomes spectrally harder and brighter with time until the PBH completely evaporates. In this paper, we develop a new algorithm to detect the proper motion of a gamma-ray point sources, and apply it to 318 unassociated point sources at high galactic latitude in the third Fermi-LAT source catalog (3FGL). None of unassociated point sources with spectra consistent with PBH evaporation show significant proper motion. Using the non-detection of PBH candidates, we derive a 99\% confidence limit on PBH evaporation rate in the vicinity of the Earth $\dot{\rho}_{\rm PBH} < 7.2 \times 10^3\: {\rm {pc}^{-3} {yr}^{-1}}$. This limit is similar to the limits obtained with ground-based gamma-ray observatories.Comment: Accepted by ApJ. Corresponding authors: Christian Johnson, Dmitry Malyshev, Steve Ritz, Stefan Fun

Fermi Large Area Telescope Third Source Catalog

We present the third Fermi Large Area Telescope source catalog (3FGL) of sources in the 100 MeV-300 GeV range. Based on the first four years of science data from the Fermi Gamma-ray Space Telescope mission, it is the deepest yet in this energy range. Relative to the 2FGL catalog, the 3FGL catalog incorporates twice as much data as well as a number of analysis improvements, including improved calibrations at the event reconstruction level, an updated model for Galactic diffuse gamma-ray emission, a refined procedure for source detection, and improved methods for associating LAT sources with potential counterparts at other wavelengths. The 3FGL catalog includes 3033 sources above 4 sigma significance, with source location regions, spectral properties, and monthly light curves for each. Of these, 78 are flagged as potentially being due to imperfections in the model for Galactic diffuse emission. Twenty-five sources are modeled explicitly as spatially extended, and overall 232 sources are considered as identified based on angular extent or correlated variability (periodic or otherwise) observed at other wavelengths. For 1009 sources we have not found plausible counterparts at other wavelengths. More than 1100 of the identified or associated sources are active galaxies of the blazar class; several other classes of non-blazar active galaxies are also represented in the 3FGL. Pulsars represent the largest Galactic source class. From source counts of Galactic sources we estimate the contribution of unresolved sources to the Galactic diffuse emission is ~3% at 1 GeV.Comment: 101 pages, 26 figures, accepted for publication in Astrophysical Journal Supplement Series. The ancillary files are PDFs of the full versions of Tables 4 and 8 and a FITS version of Table 11. v3 has corrected Table 6 and minor edits. The 3FGL catalog is available at http://fermi.gsfc.nasa.gov/ssc/data/access/lat/4yr_catalo