arXiv subjects

LAT collaboration

Publications and source records attributed to LAT collaboration.

At least 19 recordsLinked to original sources

arXiv2024

Multiwavelength Investigation of $\gamma$-ray Source MGRO J1908+06 Emission Using Fermi-LAT, VERITAS, and HAWC

This paper investigates the origin of the $\gamma$-ray emission from MGRO J1908+06 in the GeV-TeV energy band. By analyzing the data collected by {\it Fermi}-LAT, VERITAS, and HAWC, with the addition of spectral data previously reported by LHAASO, a multiwavelength (MW) study of the morphological and spectral features of MGRO J1908+06 provides insight into the origin of the $\gamma$-ray emission. The mechanism behind the bright TeV emission is studied by constraining the magnetic field strength, the source age and the distance through detailed broadband modeling. Both spectral shape and energy-dependent morphology support the scenario that inverse-Compton (IC) emission of an evolved pulsar wind nebula (PWN) associated with PSR J1907+0602 is responsible for the MGRO J1908+06 $\gamma$-ray emission with a best-fit true age of $T=22\pm 9$ kyr and a magnetic field of $B=5.4 \pm 0.8\ \mu\mathrm{G}$, assuming the distance to the pulsar $d_{\mathrm{PSR}}=3.2$ kpc.

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arXiv2022

The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope -- Data Release 3

An incremental version of the fourth catalog of active galactic nuclei (AGNs) detected by the Fermi-Large Area Telescope is presented. This version (4LAC-DR3) derives from the third data release of the 4FGL catalog based on 12 years of E>50 MeV gamma-ray data, where the spectral parameters, spectral energy distributions (SEDs), yearly light curves, and associations have been updated for all sources. The new reported AGNs include 587 blazar candidates and four radio galaxies. We describe the properties of the new sample and outline changes affecting the previously published one. We also introduce two new parameters in this release, namely the peak energy of the SED high-energy component and the corresponding flux. These parameters allow an assessment of the Compton dominance, the ratio of the Inverse-Compton to the synchrotron peak luminosities, without relying on X-ray data.

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arXiv2022

Incremental Fermi Large Area Telescope Fourth Source Catalog

We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi-LAT catalog of gamma-ray sources. Based on the first twelve years of science data in the energy range from 50 MeV to 1 TeV, it contains 6658 sources. The analysis improves on that used for the 4FGL catalog over eight years of data: more sources are fit with curved spectra, we introduce a more robust spectral parameterization for pulsars, and we extend the spectral points to 1 TeV. The spectral parameters, spectral energy distributions, and associations are updated for all sources. Light curves are rebuilt for all sources with 1 yr intervals (not 2 month intervals). Among the 5064 original 4FGL sources, 16 were deleted, 112 are formally below the detection threshold over 12 yr (but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn. Pulsars are split explicitly between young and millisecond pulsars. Pulsars and binaries newly detected in LAT sources, as well as more than 100 newly classified blazars, are reported. We add three extended sources and 1607 new point sources, mostly just above the detection threshold, among which eight are considered identified, and 699 have a plausible counterpart at other wavelengths. We discuss degree-scale residuals to the global sky model and clusters of soft unassociated point sources close to the Galactic plane, which are possibly related to limitations of the interstellar emission model and missing extended sources.

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arXiv2019

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.

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arXiv2019

Fermi Large Area Telescope Fourth Source Catalog

We present the fourth Fermi Large Area Telescope catalog (4FGL) of gamma-ray sources. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission in the energy range from 50 MeV to 1 TeV, it is the deepest yet in this energy range. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for the Galactic diffuse gamma-ray emission, and two sets of light curves (1-year and 2-month intervals). The 4FGL catalog includes 5064 sources above 4 sigma significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall 358 sources are considered as identified based on angular extent, periodicity or correlated variability observed at other wavelengths. For 1336 sources we have not found plausible counterparts at other wavelengths. More than 3130 of the identified or associated sources are active galaxies of the blazar class, and 239 are pulsars.

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arXiv2016

Fermi-LAT Observations of the LIGO event GW150914

The Fermi Large Area Telescope (LAT) has an instantaneous field of view covering $\sim 1/5$ of the sky and completes a survey of the full sky every ~3 hours. It provides a continuous, all-sky survey of high-energy gamma-rays, enabling searches for transient phenomena over timescales from milliseconds to years. Among these phenomena could be electromagnetic counterparts to gravitational wave sources. In this paper, we present a detailed study of the LAT observations relevant to Laser Interferometer Gravitational-wave Observatory (LIGO) event GW150904 (Abbott et al. 2016), which is the first direct detection of gravitational waves and has been interpreted as due to coalescence of two stellar-mass black holes. The localization region for GW150904 was outside the LAT field of view at the time of the gravitational wave signal. However, as part of routine survey observations, the LAT observed the entire LIGO localization region within ~70 minutes of the trigger, and thus enabled a comprehensive search for a gamma-ray counterpart to GW150904. The study of the LAT data presented here did not find any potential counterparts to GW150904, but it did provide limits on the presence of a transient counterpart above 100 MeV on timescales of hours to days over the entire GW150904 localization region.

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arXiv2013

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 $γ$-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.

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arXiv2013

The Second Fermi Large Area Telescope Catalog of Gamma-ray Pulsars

This catalog summarizes 117 high-confidence > 0.1 GeV gamma-ray pulsar detections using three years of data acquired by the Large Area Telescope (LAT) on the Fermi satellite. Half are neutron stars discovered using LAT data, through periodicity searches in gamma-ray and radio data around LAT unassociated source positions. The 117 pulsars are evenly divided into three groups: millisecond pulsars, young radio-loud pulsars, and young radio-quiet pulsars. We characterize the pulse profiles and energy spectra and derive luminosities when distance information exists. Spectral analysis of the off-peak phase intervals indicates probable pulsar wind nebula emission for four pulsars, and off-peak magnetospheric emission for several young and millisecond pulsars. We compare the gamma-ray properties with those in the radio, optical, and X-ray bands. We provide flux limits for pulsars with no observed gamma-ray emission, highlighting a small number of gamma-faint, radio-loud pulsars. The large, varied gamma-ray pulsar sample constrains emission models. Fermi's selection biases complement those of radio surveys, enhancing comparisons with predicted population distributions.

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arXiv2013

PSR J2021+4026 in the Gamma Cygni region: the first variable gamma-ray pulsar seen by the Fermi LAT

Long-term monitoring of PSR J2021+4026 in the heart of the Cygnus region with the Fermi Large Area Telescope (LAT) unveiled a sudden decrease in flux above 100 MeV over a time scale shorter than a week. The "jump" was near MJD 55850 (2011 October 16), with the flux decreasing from $(8.33\pm0.08) \times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$ to $(6.86\pm0.13) \times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$. Simultaneously, the frequency spindown rate increased from $(7.8\pm 0.1) \times 10^{-13}$ Hz s$^{-1}$ to $(8.1\pm 0.1) \times 10^{-13}$ Hz s$^{-1}$. Significant (> 5 sigma's) changes in the pulse profile and marginal (< 3 sigma's) changes in the emission spectrum occurred at the same time. There is also evidence for a small, steady flux increase over the three years preceding MJD 55850. This is the first observation at gamma-ray energies of mode changes and intermittent behavior, observed at radio wavelengths for other pulsars. We argue that the change in pulsed gamma-ray emission is due to a change in emission beaming and we speculate that it is precipitated by a shift in the magnetic field structure, leading to a change of either effective magnetic inclination or effective current.

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arXiv2013

Impulsive and Long Duration High-Energy Gamma-ray Emission From the Very Bright 2012 March 7 Solar Flares

The Fermi Large Area Telescope (LAT) observed two bright X-class solar flares on 2012 March 7, and detected gamma-rays up to 4 GeV. We detected gamma-rays both during the impulsive and temporally-extended emission phases, with emission above 100 MeV lasting for approximately 20 hours. Accurate localization of the gamma-ray production site(s) coincide with the solar active region from which X-ray emissions associated with these flares originated. Our analysis of the >100 MeV gamma-ray emission shows a relatively rapid monotonic decrease in flux during the first hour of the impulsive phase, and a much slower, almost monotonic decrease in flux for the next 20 hours. The spectra can be adequately described by a power law with a high energy exponential cutoff, or as resulting from the decay of neutral pions produced by accelerated protons and ions with an isotropic power-law energy distribution. The required proton spectrum has a number index ~3, with minor variations during the impulsive phase, while during the temporally extended phase the spectrum softens monotonically, starting with index ~4. The >30 MeV proton flux and spectra observed near the Earth by the GOES satellites also show a monotonic flux decrease and spectral softening during the extended phase, but with a harder spectrum, with index ~3. Based on the Fermi-LAT and GOES observations of the flux and spectral evolution of these bright flares, we explore the relative merits of prompt and continuous acceleration scenarios, hadronic and leptonic emission processes, and acceleration at the solar corona by the fast Coronal Mass Ejections (CME) as explanations for the observations. We conclude that the most likely scenario is continuous acceleration of protons in the solar corona which penetrate the lower solar atmosphere and produce pions that decay into gamma-rays.

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arXiv2013

High-Energy Gamma-Ray Emission From Solar Flares: Summary of Fermi LAT Detections and Analysis of Two M-Class Flares

We present the detections of 19 solar flares detected in high-energy gamma rays (above 100 MeV) with the Fermi Large Area Telescope (LAT) during its first four years of operation. Interestingly, all flares are associated with fairly fast Coronal Mass Ejections (CMEs) and are not all powerful X-ray flares. We then describe the detailed temporal, spatial and spectral characteristics of the first two long-lasting events: the 2011 March 7 flare, a moderate (M3.7) impulsive flare followed by slowly varying gamma-ray emission over 13 hours, and the 2011 June 7 M2.5 flare, which was followed by gamma-ray emission lasting for 2 hours. We compare the Fermi-LAT data with X-ray and proton data measurements from GOES and RHESSI. We argue that a hadronic origin of the gamma rays is more likely than a leptonic origin and find that the energy spectrum of the proton distribution softens after the 2011 March 7 flare, favoring a scenario with continuous acceleration at the flare site. This work suggests that proton acceleration in solar flares is more common than previously thought, occurring for even modest X-ray flares, and for longer durations.

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arXiv2013

Detection of the Characteristic Pion-Decay Signature in Supernova Remnants

Cosmic rays are particles (mostly protons) accelerated to relativistic speeds. Despite wide agreement that supernova remnants (SNRs) are the sources of galactic cosmic rays, unequivocal evidence for the acceleration of protons in these objects is still lacking. When accelerated protons encounter interstellar material, they produce neutral pions, which in turn decay into gamma rays. This offers a compelling way to detect the acceleration sites of protons. The identification of pion-decay gamma rays has been difficult because high-energy electrons also produce gamma rays via bremsstrahlung and inverse Compton scattering. We detected the characteristic pion-decay feature in the gamma-ray spectra of two SNRs, IC 443 and W44, with the Fermi Large Area Telescope. This detection provides direct evidence that cosmic-ray protons are accelerated in SNRs.

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arXiv2013

Constraints on the Galactic Halo Dark Matter from Fermi-LAT Diffuse Measurements

We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope in the Milky Way Halo region searching for a signal from dark matter annihilation or decay. In the absence of a robust dark matter signal, constraints are presented. We consider both gamma rays produced directly in the dark matter annihilation/decay and produced by inverse Compton scattering of the e+e- produced in the annihilation/decay. Conservative limits are derived requiring that the dark matter signal does not exceed the observed diffuse gamma-ray emission. A second set of more stringent limits is derived based on modeling the foreground astrophysical diffuse emission using the GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the distribution of the cosmic-ray sources in the Galaxy, the index of the injection cosmic-ray electron spectrum and the column density of the interstellar gas are taken into account using a profile likelihood formalism, while the parameters governing the cosmic-ray propagation have been derived from fits to local cosmic-ray data. The resulting limits impact the range of particle masses over which dark matter thermal production in the early Universe is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic ray anomalies as annihilation of dark matter.

astro-ph.CO↗

arXiv2012

Fermi LAT study of cosmic-rays and the interstellar medium in nearby molecular clouds

We report an analysis of the interstellar gamma-ray emission from the Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope. They are among the nearest molecular cloud complexes, within ~300 pc from the solar system. The gamma-ray emission produced by interactions of cosmic-rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained gamma-ray emissivities above 250 MeV are (5.9 +/- 0.1(stat) (+0.9/-1.0)(sys)), (10.2 +/- 0.4(stat) (+1.2/-1.7)(sys)), and (9.1 +/- 0.3(stat) (+1.5/-0.6)(sys)) x10^(-27) photons s^(-1) sr^(-1) H-atom^(-1) for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively. Whereas the energy dependences of the emissivities agree well with that predicted from direct CR observations at the Earth, the measured emissivities from 250 MeV to 10 GeV indicate a variation of the CR density by ~20% in the neighborhood of the solar system, even if we consider systematic uncertainties. The molecular mass calibrating ratio, Xco = N(H2)/Wco, is found to be (0.96 +/- 0.06(stat) (+0.15/-0.12)(sys)), (0.99 +/- 0.08(stat) (+0.18/-0.10)(sys)), and (0.63 +/- 0.02(stat) (+0.09/-0.07)(sys)) x10^20 H2-molecule cm^(-2) (K km s^(-1))^(-1) for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, suggesting a variation of Xco in the vicinity of the solar system. From the obtained values of Xco, the masses of molecular gas traced by Wco in the Chamaeleon, R CrA, and Cepheus and Polaris flare regions are estimated to be ~5x10^3, ~10^3, and ~3.3x10^4 Msolar, respectively. A comparable amount of gas not traced well by standard HI and CO surveys is found in the regions investigated.

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arXiv2011

Fermi-LAT Observations of Markarian 421: the Missing Piece of its Spectral Energy Distribution

We report on the gamma-ray activity of the high-synchrotron-peaked BL Lacertae object Mrk 421 during the first 1.5 years of Fermi operation, from 2008 August 5 to 2010 March 12. We find that the Large Area Telescope (LAT) gamma-ray spectrum above 0.3 GeV can be well-described by a power-law function with photon index Gamma=1.78 +/- 0.02 and average photon flux F(>0.3 GeV)=(7.23 +/- 0.16) x 10^{-8} ph cm^{-2} s^{-1}. Over this time period, the Fermi-LAT spectrum above 0.3 GeV was evaluated on 7-day-long time intervals, showing significant variations in the photon flux (up to a factor ~3 from the minimum to the maximum flux), but mild spectral variations. The variability amplitude at X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially larger than that in gamma-rays measured by Fermi-LAT, and these two energy ranges are not significantly correlated. We also present the first results from the 4.5-month-long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign (2009 January 19 to 2009 June 1). During this campaign, Mrk 421 showed a low activity at all wavebands. The extensive multi-instrument (radio to TeV) data set provides an unprecedented, complete look at the quiescent spectral energy distribution (SED) for this source. The broad band SED was reproduced with a leptonic (one-zone Synchrotron Self-Compton) and a hadronic model (Synchrotron Proton Blazar). Both frameworks are able to describe the average SED reasonably well, implying comparable jet powers but very different characteristics for the blazar emission site.

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arXiv2011

Fermi Gamma-ray Space Telescope Observations of the Gamma-ray Outburst from 3C 454.3 in November 2010

The flat-spectrum radio quasar 3C 454.3 underwent an extraordinary 5-day gamma-ray outburst in November 2010 where the daily flux measured with the Fermi Large Area Telescope (LAT) at photon energies E>100 MeV reached (66+/-2) x 10^-6 ph cm^-2 s^-1. This is a factor of 3 higher than its previous maximum flux recorded in December 2009 and ~5 times brighter than the Vela pulsar, which is normally the brightest source in the gamma-ray sky. The 3-hr peak flux was (85+/-5) x 10^-6 ph cm^-2 s^-1, corresponding to an apparent isotropic luminosity of 2.1+/-0.2 10^50 erg s^-1, the highest ever recorded for a blazar. In this paper, we investigate the features of this exceptional event in the gamma-ray band of the Fermi-LAT. In contrast to previous flares of the same source observed with the Fermi-LAT, clear spectral changes are observed during the flare.

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arXiv2010

Fermi Large Area Telescope observations of Local Group galaxies: Detection of M31 and search for M33

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.

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arXiv2010

Insights Into the High-Energy Gamma-ray Emission of Markarian 501 from Extensive Multifrequency Observations in the Fermi Era

We report on the gamma-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average LAT gamma-ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 +/- 0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of 2), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 +/- 0.14, and the softest one is 2.51 +/- 0.20. These unexpected spectral changes do not correlate with the measured flux variations above 0.3GeV. In this paper, we also present the first results from the 4.5-month-long multifrequency campaign (2009 March 15 - August 1) on Mrk 501, which included the VLBA, Swift, RXTE, MAGIC and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign. The average spectral energy distribution of Mrk 501 is well described by the standard one-zone synchrotron self-Compton model. In the framework of this model, we find that the dominant emission region is characterized by a size <~ 0.1 pc (comparable within a factor of few to the size of the partially-resolved VLBA core at 15-43 GHz), and that the total jet power (~10^{44} erg s^{-1}) constitutes only a small fraction (~10^{-3}) of the Eddington luminosity. The energy distribution of the freshly-accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3GeV-10TeV, with spectral indices 2.2 and 2.7 below and above the break energy of 20GeV. We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic, proton-mediated shocks.

astro-ph.HE↗