arXiv subjects

LAT Collaboration

Publications and source records attributed to LAT Collaboration.

At least 37 records · Page 2Linked to original sources

arXiv2016

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 $γ$-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 $γ$-ray opacity of the Universe.

astro-ph.HE↗

arXiv2016

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

We present results from γ-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 γ-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 1.8σ). 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 γ-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.

astro-ph.HE↗

arXiv2016

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$-$1855 (z=0.689), 3FGL J0630.9$-$2406 (z >~ 1.239), and 3FGL J0811.2$-$7529 (z=0.774), detected by Fermi with relatively flat GeV spectra. By observing simultaneously in the near-IR to hard X-ray band, we can well characterize the high end of the synchrotron component of the SED. Thus, fitting the SEDs to synchro-Compton models of the dominant emission from the relativistic jet, we can constrain the underlying particle properties and predict the shape of the GeV Compton component. Standard extragalactic background light (EBL) models explain the high-energy absorption well, with poorer fits for high UV models. The fits show clear evidence for EBL absorption in the Fermi spectrum of our highest redshift source 3FGL J0630.9$-$2406. While synchrotron self-Compton models adequately describe the SEDs, the situation may be complicated by possible external Compton components. For 3FGL J0811.2$-$7529, we also discover a nearby serendipitous source in the X-ray data, which is almost certainly another lower synchrotron peak frequency ($ν^{sy}_{pk}$) BL Lac, that may contribute flux in the Fermi band. Since our sources are unusual high-luminosity, moderate $ν^{sy}_{pk}$ 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.

astro-ph.HE↗

arXiv2016

Deep morphological and spectral study of the SNR RCW 86 with Fermi-LAT

RCW 86 is a young supernova remnant (SNR) showing a shell-type structure at several wavelengths and is thought to be an efficient cosmic-ray (CR) accelerator. Earlier \textit{Fermi} Large Area Telescope results reported the detection of $γ$-ray emission coincident with the position of RCW 86 but its origin (leptonic or hadronic) remained unclear due to the poor statistics. Thanks to 6.5 years of data acquired by the \textit{Fermi}-LAT and the new event reconstruction Pass 8, we report the significant detection of spatially extended emission coming from RCW 86. The spectrum is described by a power-law function with a very hard photon index ($Γ= 1.42 \pm 0.1_{\rm stat} \pm 0.06_{\rm syst}$) in the 0.1--500 GeV range and an energy flux above 100 MeV of ($2.91$ $\pm$ $0.8_{\rm stat}$ $\pm$ $0.12_{\rm syst}$) $\times$ $10^{-11}$ erg cm$^{-2}$ s$^{-1}$. Gathering all the available multiwavelength (MWL) data, we perform a broadband modeling of the nonthermal emission of RCW 86 to constrain parameters of the nearby medium and bring new hints about the origin of the $γ$-ray emission. For the whole SNR, the modeling favors a leptonic scenario in the framework of a two-zone model with an average magnetic field of 10.2 $\pm$ 0.7 $μ$G and a limit on the maximum energy injected into protons of 2 $\times$ 10$^{49}$ erg for a density of 1 cm$^{-3}$. In addition, parameter values are derived for the North-East (NE) and South-West (SW) regions of RCW 86, providing the first indication of a higher magnetic field in the SW region.

astro-ph.HE↗

arXiv2015

Fermi-LAT Observations of High-Energy Gamma-Ray Emission Toward the Galactic Center

The Fermi Large Area Telescope (LAT) has provided the most detailed view to date of the emission towards the Galactic centre (GC) in high-energy gamma-rays. This paper describes the analysis of data taken during the first 62 months of the mission in the energy range 1-100 GeV from a $15^\circ \times 15^\circ$ region about the direction of the GC, and implications for the interstellar emissions produced by cosmic ray (CR) particles interacting with the gas and radiation fields in the inner Galaxy and for the point sources detected. Specialised interstellar emission models (IEMs) are constructed that enable separation of the gamma-ray emission from the inner $\sim 1$ kpc about the GC from the fore- and background emission from the Galaxy. Based on these models, the interstellar emission from CR electrons interacting with the interstellar radiation field via the inverse Compton (IC) process and CR nuclei inelastically scattering off the gas producing gamma-rays via $π^0$ decays from the inner $\sim 1$ kpc is determined. The IC contribution is found to be dominant in the region and strongly enhanced compared to previous studies. A catalog of point sources for the $15^\circ \times 15^\circ$ region is self-consistently constructed using these IEMs: the First Fermi-LAT Inner Galaxy point source Catalog (1FIG). After subtracting the interstellar emission and point-source contributions from the data a residual is found that is a sub-dominant fraction of the total flux. If spatial templates that peak toward the GC are used to model the positive residual and included in the total model for the $15^\circ \times 15^\circ$ region, the agreement with the data improves, but none of the additional templates account for all of the residual structure. The spectrum of the positive residual modelled with these templates has a strong dependence on the choice of IEM. [Abridged]

astro-ph.HE↗

arXiv2015

Deep view of the Large Magellanic Cloud with 6 years of Fermi-LAT observations

The nearby Large Magellanic Cloud (LMC) provides a rare opportunity of a spatially resolved view of an external star-forming galaxy in gamma-rays. The LMC was detected at 0.1-100GeV as an extended source with CGRO/EGRET and using early observations with the Fermi-LAT. The emission was found to correlate with massive star-forming regions and to be particularly bright towards 30 Doradus. Studies of the origin and transport of cosmic rays (CRs) in the Milky Way are frequently hampered by line-of-sight confusion and poor distance determination. The LMC offers a complementary way to address these questions by revealing whether and how the gamma-ray emission is connected to specific objects, populations of objects, and structures in the galaxy. We revisited the gamma-ray emission from the LMC using about 73 months of Fermi-LAT P7REP data in the 0.2-100GeV range. We developed a complete spatial and spectral model of the LMC emission, for which we tested several approaches: a simple geometrical description, template-fitting, and a physically driven model for CR-induced interstellar emission. In addition to identifying PSR J0540-6919 through its pulsations, we find two hard sources positionally coincident with plerion N 157B and supernova remnant N 132D, which were also detected at TeV energies with H.E.S.S. We detect an additional soft source that is currently unidentified. Extended emission dominates the total flux from the LMC. It consists of an extended component of about the size of the galaxy and additional emission from 3 to 4 regions with degree-scale sizes. If it is interpreted as CRs interacting with interstellar gas, the large-scale emission implies a large-scale population of ~1-100GeV CRs with a density of ~30% of the local Galactic value. On top of that, the 3 to 4 small-scale emission regions would correspond to enhancements of the CR density by factors 2 to 6 or higher (Abridged).

astro-ph.HE↗

arXiv2015

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 $τ$-lepton channels.

astro-ph.HE↗

arXiv2015

Resolving the Extragalactic $γ$-ray Background above 50 GeV with Fermi-LAT

The Fermi Large Area Telescope (LAT) Collaboration has recently released a catalog of 360 sources detected above 50 GeV (2FHL). This catalog was obtained using 80 months of data re-processed with Pass 8, the newest event-level analysis, which significantly improves the acceptance and angular resolution of the instrument. Most of the 2FHL sources at high Galactic latitude are blazars. Using detailed Monte Carlo simulations, we measure, for the first time, the source count distribution, $dN/dS$, of extragalactic $γ$-ray sources at $E>50$ GeV and find that it is compatible with a Euclidean distribution down to the lowest measured source flux in the 2FHL ($\sim8\times 10^{-12}$ ph cm$^{-2}$ s$^{-1}$). We employ a one-point photon fluctuation analysis to constrain the behavior of $dN/dS$ below the source detection threshold. Overall the source count distribution is constrained over three decades in flux and found compatible with a broken power law with a break flux, $S_b$, in the range $[8 \times 10^{-12},1.5 \times 10^{-11}]$ ph cm$^{-2}$ s$^{-1}$ and power-law indices below and above the break of $α_2 \in [1.60,1.75]$ and $α_1 = 2.49 \pm 0.12$ respectively. Integration of $dN/dS$ shows that point sources account for at least $86^{+16}_{-14}\%$ of the total extragalactic $γ$-ray background. The simple form of the derived source count distribution is consistent with a single population (i.e. blazars) dominating the source counts to the minimum flux explored by this analysis. We estimate the density of sources detectable in blind surveys that will be performed in the coming years by the Cherenkov Telescope Array.

astro-ph.CO↗

arXiv2015

2FHL: The Second Catalog of Hard Fermi-LAT Sources

We present a catalog of sources detected above 50 GeV by the {\it Fermi}-Large Area Telescope (LAT) in 80 months of data. The newly delivered Pass 8 event-level analysis allows the detection and characterization of sources in the 50 GeV--2 TeV energy range. In this energy band, {\it Fermi}-LAT has detected 360 sources, which constitute the second catalog of hard {\it Fermi}-LAT sources (2FHL). The improved angular resolution enables the precise localization of point sources ($\sim$1.7$'$ radius at 68 % C.~L.) and the detection and characterization of spatially extended sources. We find that 86 % of the sources can be associated with counterparts at other wavelengths, of which the majority (75 %) are active galactic nuclei and the rest (11 %) are Galactic sources. Only 25 % of the 2FHL sources have been previously detected by Cherenkov telescopes, implying that the 2FHL provides a reservoir of candidates to be followed up at very high energies. This work closes the energy gap between the observations performed at GeV energies by {\it Fermi}-LAT on orbit and the observations performed at higher energies by Cherenkov telescopes from the ground.

astro-ph.HE↗

arXiv2015

Search for Gamma-Ray Emission from DES Dwarf Spheroidal Galaxy Candidates with Fermi-LAT Data

Due to their proximity, high dark-matter content, and apparent absence of non-thermal processes, Milky Way dwarf spheroidal satellite galaxies (dSphs) are excellent targets for the indirect detection of dark matter. Recently, eight new dSph candidates were discovered using the first year of data from the Dark Energy Survey (DES). We searched for gamma-ray emission coincident with the positions of these new objects in six years of Fermi Large Area Telescope data. We found no significant excesses of gamma-ray emission. Under the assumption that the DES candidates are dSphs with dark matter halo properties similar to the known dSphs, we computed individual and combined limits on the velocity-averaged dark matter annihilation cross section for these new targets. If the estimated dark-matter content of these dSph candidates is confirmed, they will constrain the annihilation cross section to lie below the thermal relic cross section for dark matter particles with masses < 20 GeV annihilating via the b-bbar or tau+tau- channels.

astro-ph.HE↗

arXiv2015

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.

astro-ph.HE↗

arXiv2015

Updated Search for Spectral Lines from Galactic Dark Matter Interactions with Pass 8 Data from the Fermi Large Area Telescope

Dark matter in the Milky Way may annihilate directly into gamma rays, producing a monoenergetic spectral line. Therefore, detecting such a signature would be strong evidence for dark matter annihilation or decay. We search for spectral lines in the Fermi Large Area Telescope observations of the Milky Way halo in the energy range 200 MeV to 500 GeV using analysis methods from our most recent line searches. The main improvements relative to previous works are our use of 5.8 years of data reprocessed with the Pass 8 event-level analysis and the additional data resulting from the modified observing strategy designed to increase exposure of the Galactic center region. We searched in five sky regions selected to optimize sensitivity to different theoretically-motivated dark matter scenarios and find no significant detections. In addition to presenting the results from our search for lines, we also investigate the previously reported tentative detection of a line at 133 GeV using the new Pass 8 data.

astro-ph.HE↗

arXiv2014

Fermi Establishes Classical Novae as a Distinct Class of Gamma-Ray Sources

A classical nova results from runaway thermonuclear explosions on the surface of a white dwarf that accretes matter from a low-mass main-sequence stellar companion. In 2012 and 2013, three novae were detected in gamma rays and stood in contrast to the first gamma-ray detected nova V407 Cygni 2010, which belongs to a rare class of symbiotic binary systems. Despite likely differences in the compositions and masses of their white dwarf progenitors, the three classical novae are similarly characterized as soft spectrum transient gamma-ray sources detected over 2-3 week durations. The gamma-ray detections point to unexpected high-energy particle acceleration processes linked to the mass ejection from thermonuclear explosions in an unanticipated class of Galactic gamma-ray sources.

astro-ph.HE↗

arXiv2014

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$σ$ 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 $μ$G.

astro-ph.HE↗

arXiv2014

Inferred cosmic-ray spectrum from ${\it Fermi}$-LAT $γ$-ray observations of the Earth's limb

Recent accurate measurements of cosmic-ray (CR) species by ATIC-2, CREAM, and PAMELA reveal an unexpected hardening in the proton and He spectra above a few hundred GeV, a gradual softening of the spectra just below a few hundred GeV, and a harder spectrum of He compared to that of protons. These newly-discovered features may offer a clue to the origin of high-energy CRs. We use the ${\it Fermi}$ Large Area Telescope observations of the $γ$-ray emission from the Earth's limb for an indirect measurement of the local spectrum of CR protons in the energy range $\sim 90~$GeV-$6~$TeV (derived from a photon energy range $15~$GeV-$1~$TeV). Our analysis shows that single power law and broken power law spectra fit the data equally well and yield a proton spectrum with index $2.68 \pm 0.04$ and $2.61 \pm 0.08$ above $\sim 200~$GeV, respectively.

astro-ph.HE↗

arXiv2014

Dark Matter Constraints from Observations of 25 Milky Way Satellite Galaxies with the Fermi Large Area Telescope

The dwarf spheroidal satellite galaxies of the Milky Way are some of the most dark-matter-dominated objects known. Due to their proximity, high dark matter content, and lack of astrophysical backgrounds, dwarf spheroidal galaxies are widely considered to be among the most promising targets for the indirect detection of dark matter via gamma rays. Here we report on gamma-ray observations of 25 Milky Way dwarf spheroidal satellite galaxies based on 4 years of Fermi Large Area Telescope (LAT) data. None of the dwarf galaxies are significantly detected in gamma rays, and we present gamma-ray flux upper limits between 500 MeV and 500 GeV. We determine the dark matter content of 18 dwarf spheroidal galaxies from stellar kinematic data and combine LAT observations of 15 dwarf galaxies to constrain the dark matter annihilation cross section. We set some of the tightest constraints to date on the the annihilation of dark matter particles with masses between 2 GeV and 10 TeV into prototypical Standard Model channels. We find these results to be robust against systematic uncertainties in the LAT instrument performance, diffuse gamma-ray background modeling, and assumed dark matter density profile.

astro-ph.HE↗

arXiv2013

The First Fermi-LAT Catalog of Sources Above 10 GeV

We present a catalog of gamma-ray sources at energies above 10 GeV based on data from the Large Area Telescope (LAT) accumulated during the first three years of the Fermi Gamma-ray Space Telescope mission. The first Fermi-LAT catalog of >10GeV sources (1FHL) has 514 sources. For each source we present location, spectrum, a measure of variability, and associations with cataloged sources at other wavelengths. We found that 449 (87%) could be associated with known sources, of which 393 (76% of the 1FHL sources) are active galactic nuclei. Of the 27 sources associated with known pulsars, we find 20 (12) to have significant pulsations in the range >10GeV (>25GeV). In this work we also report that, at energies above 10 GeV, unresolved sources account for 27+/-8 % of the isotropic gamma-ray background, while the unresolved Galactic population contributes only at the few percent level to the Galactic diffuse background. We also highlight the subset of the 1FHL sources that are best candidates for detection at energies above 50-100 GeV with current and future ground-based gamma-ray observatories.

astro-ph.IM↗

arXiv2013

Search for Gamma-ray Spectral Lines with the Fermi Large Area Telescope and Dark Matter Implications

Weakly Interacting Massive Particles (WIMPs) are a theoretical class of particles that are excellent dark matter candidates. WIMP annihilation or decay may produce essentially monochromatic gamma rays detectable by the Fermi Large Area Telescope (LAT) against the astrophysical gamma-ray emission of the Galaxy. We have searched for spectral lines in the energy range 5--300 GeV using 3.7 years of data, reprocessed with updated instrument calibrations and an improved energy dispersion model compared to the previous Fermi-LAT Collaboration line searches. We searched in five regions selected to optimize sensitivity to different theoretically-motivated dark matter density distributions. We did not find any globally significant lines in our a priori search regions and present 95% confidence limits for annihilation cross sections of self-conjugate WIMPs and decay lifetimes. Our most significant fit occurred at 133 GeV in our smallest search region and had a local significance of 3.3 standard deviations, which translates to a global significance of 1.5 standard deviations. We discuss potential systematic effects in this search, and examine the feature at 133 GeV in detail. We find that both the use of reprocessed data and of additional information in the energy dispersion model contribute to the reduction in significance of the line-like feature near 130 GeV relative to significances reported in other works. We also find that the feature is narrower than the LAT energy resolution at the level of 2 to 3 standard deviations, which somewhat disfavors the interpretation of the 133 GeV feature as a real WIMP signal.

astro-ph.HE↗