160 research outputs found
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
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 -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 for ALP masses 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 -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
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/
Contemporaneous broadband observations of three high-redshift BL Lac Objects
We have collected broadband spectral energy distributions (SEDs) of three BL
Lac objects, 3FGL J0022.11855 (z=0.689), 3FGL J0630.92406 (z >~ 1.239),
and 3FGL J0811.27529 (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 J0630.92406. While
synchrotron self-Compton models adequately describe the SEDs, the situation may
be complicated by possible external Compton components. For 3FGL
J0811.27529, we also discover a nearby serendipitous source in the X-ray
data, which is almost certainly another lower synchrotron peak frequency
() BL Lac, that may contribute flux in the Fermi band. Since our
sources are unusual high-luminosity, moderate BL Lacs we
compare these quantities and the Compton dominance, the ratio of peak
inverse-Compton to peak synchrotron luminosities (),
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 and
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 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 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
. 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
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