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Register a new userA Stacked Analysis of Brightest Cluster Galaxies Observed with the Fermi Large Area Telescope
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We present the results of a search for high-energy gamma-ray emission from a large sample of galaxy clusters sharing the properties of three existing Fermi-LAT detections (in Perseus, Virgo and Abell 3392), namely a powerful radio source within their brightest cluster galaxy (BCG). From a parent, X-ray flux-limited sample of clusters, we select 114 systems with a core-dominated BCG radio flux above 50 or 75 mJy, stacking data from the first 45 months of the Fermi mission, to determine statistical limits on the gamma-ray fluxes of the ensemble of candidate sources. For a >300 MeV selection, the distribution of detection significance across the sample is consistent with that across control samples for significances <3 sigma, but has a tail extending to higher values, including three >4 sigma signals which are not associated with previously identified gamma-ray emission. Modelling of the data in these fields results in the detection of four non-2FGL Fermi sources, though none appear to be unambiguously associated with the BCG candidate. A search at energies >3 GeV hints at emission from the BCG in A 2055, which hosts a BL Lac object. There is no evidence for a signal in the stacked data, and the upper limit derived on the gamma-ray flux of an average radio-bright BCG in the sample is an order-of-magnitude more constraining than that calculated for individual objects. F(1 GeV)/F(1.4 GHz) <15, compared with ~120 for NGC 1275 in Perseus, which might indicate a special case for those objects detected at high energies; that beamed emission from member galaxies comprise the dominant bright gamma-ray sources in clusters.
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We present gamma-ray observations with the LAT on board the Fermi Gamma-Ray Telescope of the nearby radio galaxy Centaurus~A. The previous EGRET detection is confirmed, and the localization is improved using data from the first 10 months of Fermi science operation. In previous work, we presented the detection of the lobes by the LAT; in this work, we concentrate on the gamma-ray core of Cen~A. Flux levels as seen by the LAT are not significantly different from that found by EGRET, nor is the extremely soft LAT spectrum ($G=2.67pm0.10_{stat}pm0.08_{sys}$ where the photon flux is $Phipropto E^{-G}$). The LAT core spectrum, extrapolated to higher energies, is marginally consistent with the non-simultaneous HESS spectrum of the source. The LAT observations are complemented by simultaneous observations from Suzaku, the Swift Burst Alert Telescope and X-ray Telescope, and radio observations with the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry (TANAMI) program, along with a variety of non-simultaneous archival data from a variety of instruments and wavelengths to produce a spectral energy distribution (SED). We fit this broadband data set with a single-zone synchrotron/synchrotron self-Compton model, which describes the radio through GeV emission well, but fails to account for the non-simultaneous higher energy TeV emission observed by HESS from 2004-2008. The fit requires a low Doppler factor, in contrast to BL Lacs which generally require larger values to fit their broadband SEDs. This indicates the $g$-ray emission originates from a slower region than that from BL Lacs, consistent with previous modeling results from Cen~A. This slower region could be a slower moving layer around a fast spine, or a slower region farther out from the black hole in a decelerating flow.
The LAT instrument on the Fermi mission will reveal the rich spectral and temporal gamma-ray burst phenomena in the > 100 MeV band. The synergy with Fermis GBM detectors will link these observations to those in the well explored 10-1000 keV range; the addition of the > 100 MeV band observations will resolve theoretical uncertainties about burst emission in both the prompt and afterglow phases. Trigger algorithms will be applied to the LAT data both onboard the spacecraft and on the ground. The sensitivity of these triggers will differ because of the available computing resources onboard and on the ground. Here we present the LATs burst detection methodologies and the instruments GRB capabilities.
Fermi-LAT unidentified sources (unIDs) have proven to be compelling targets for performing indirect dark matter (DM) searches. In a previous work, we found that among the 1235 unIDs in Fermi-LAT catalogs (3FGL, 2FHL and 3FHL) only 44 of those are DM subhalos candidates. We now implement a spectral analysis to test whether these remaining sources are compatible or not with DM origin. This analysis is executed using almost 10 years of Pass 8 Fermi-LAT data. None of the unIDs are found to significantly prefer DM-induced emission compared to other, more conventional, astrophysical sources. In order to discriminate between pulsar and DM sources, we developed a new method which is based on the sources spectral curvature, peak energy, and its detection significance. We also look for spatial extension, which may be a hint for a DM origin according to our N-body simulation studies of the subhalo population. In addition, we used Gaia DR2 data to search for a potential stellar counterpart to our best DM subhalo candidates and, although no firm associations could be found, one of them coincides with the Sagittarius stream. Finally, previous constraints on the DM annihilation cross section are updated with the new number of remaining DM subhalo candidates among unIDs. Our limits now rule out canonical thermal WIMPs up to masses of 10 GeV for $bbar{b}$ and 20 GeV for $tau^+tau^-$ annihilation channels, in this way being as sensitive and complementary to those obtained from other targets and probes.
The $gamma$-ray emission from stars is induced by the interaction of cosmic rays with stellar atmospheres and photon fields. This emission is expected to come in two components: a stellar disk emission, where $gamma$-rays are mainly produced in atmospheric showers generated by hadronic cosmic rays, and an extended halo emission, where the high density of soft photons in the surroundings of stars create a suitable environment for $gamma$-ray production via inverse Compton (IC) scattering by cosmic-ray electrons. Besides the Sun, no other disk or halo from single stars has ever been detected in $gamma$-rays. However, by assuming a cosmic-ray spectrum similar to that observed on Earth, the predicted $gamma$-ray emission of super-luminous stars, like e.g. Betelgeuse and Rigel, could be high enough to be detected by the Fermi Large Area Telescope (LAT) after its first decade of operations. In this work, we use 12 years of Fermi-LAT observations along with IC models to study 9 super-luminous nearby stars, both individually and via stacking analysis. Our results show no significant $gamma$-ray emission, but allow us to restrict the stellar $gamma$-ray fluxes to be on average $<3.3 times 10^{-11}$ ph cm$^{-2}$ s$^{-1}$ at a 3$sigma$ confidence level, which translates to an average local density of electrons in the surroundings of our targets to be less than twice of that observed for the Solar System.
We present a multi-wavelength study of the FSRQ CTA 102 using Fermi-LAT and simultaneous Swift-XRT/UVOT observations. The Fermi-LAT telescope detected one of the brightest flares from this object during Sep, 2016 to Mar, 2017. In the 190 days of observation period the source underwent four major flares. A detailed analysis of the temporal and spectral properties of these flares indicates the flare at MJD 57751.594 has a $gamma$-ray flux of (30.12$pm$4.48)$times 10^{-6}$ ph cm$^{-2}$ s$^{-1}$ (from 90 minutes binning) in the energy range of 0.1--300 GeV. This has been found to be the highest flux ever detected from CTA 102. Time dependent leptonic modelling of the pre-flare, rising state, flares and decaying state has been done. A single emission region of size $6.5times 10^{16}$ cm has been used in our work to explain the multi-wavelength spectral energy distributions. During flares the luminosity in electrons increases nearly seventy times compared to the pre-flare state.
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