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Register a new userSearch for extended gamma-ray emission from the Virgo galaxy cluster with Fermi-LAT
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Galaxy clusters are one of the prime sites to search for dark matter (DM) annihilation signals. Depending on the substructure of the DM halo of a galaxy cluster and the cross sections for DM annihilation channels, these signals might be detectable by the latest generation of $gamma$-ray telescopes. Here we use three years of Fermi Large Area Telescope (LAT) data, which are the most suitable for searching for very extended emission in the vicinity of nearby Virgo galaxy cluster. Our analysis reveals statistically significant extended emission which can be well characterized by a uniformly emitting disk profile with a radius of 3deg that moreover is offset from the cluster center. We demonstrate that the significance of this extended emission strongly depends on the adopted interstellar emission model (IEM) and is most likely an artifact of our incomplete description of the IEM in this region. We also search for and find new point source candidates in the region. We then derive conservative upper limits on the velocity-averaged DM pair annihilation cross section from Virgo. We take into account the potential $gamma$-ray flux enhancement due to DM sub-halos and its complex morphology as a merging cluster. For DM annihilating into $boverline{b}$, assuming a conservative sub-halo model setup, we find limits that are between 1 and 1.5 orders of magnitude above the expectation from the thermal cross section for $m_{mathrm{DM}}lesssim100,mathrm{GeV}$. In a more optimistic scenario, we exclude $langle sigma v ranglesim3times10^{-26},mathrm{cm^{3},s^{-1}}$ for $m_{mathrm{DM}}lesssim40,mathrm{GeV}$ for the same channel. Finally, we derive upper limits on the $gamma$-ray-flux produced by hadronic cosmic-ray interactions in the inter cluster medium. We find that the volume-averaged cosmic-ray-to-thermal pressure ratio is less than $sim6%$.
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We report on a search for monochromatic $gamma$-ray features in the spectra of galaxy clusters observed by the emph{Fermi} Large Area Telescope. Galaxy clusters are the largest structures in the Universe that are bound by dark matter (DM), making them an important testing ground for possible self-interactions or decays of the DM particles. Monochromatic $gamma$-ray lines provide a unique signature due to the absence of astrophysical backgrounds and are as such considered a smoking-gun signature for new physics. An unbinned joint likelihood analysis of the sixteen most promising clusters using five years of data at energies between 10 and 400 GeV revealed no significant features. For the case of self-annihilation, we set upper limits on the monochromatic velocity-averaged interaction cross section. These limits are compatible with those obtained from observations of the Galactic Center, albeit weaker due to the larger distance to the studied clusters.
In the work we search for the $gamma$-ray signal from M33, one of the biggest galaxies in the Local Group, by using the Pass 8 data of Fermi Large Area Telescope (LAT). No statistically significant gamma-ray emission has been detected in the direction of M33 and we report a new upper limit of high energy ($>100,rm MeV$) photon flux of $2.3times 10^{-9},rm ph,cm^{-2},s^{-1}$, which is more strict than previous constrains and implies a cosmic ray density of M33 lower than that speculated previously. Nevertheless the current limit is still in agreement with the correlation of star formation rate and $gamma$-ray luminosity inferred from the Local group galaxies and a few nearby starburst galaxies.
Context: Very-high-energy (VHE; E>100 GeV) {gamma}-ray emission from blazars inevitably gives rise to electron-positron pair production through the interaction of these {gamma}-rays with the Extragalactic Background Light (EBL). Depending on the magnetic fields in the proximity of the source, the cascade initiated from pair production can result in either an isotropic halo around an initially beamed source or a magnetically broadened cascade flux. Aims: Both extended pair halo (PH) and magnetically broadened cascade (MBC) emission from regions surrounding the blazars 1ES 1101-232, 1ES 0229+200 and PKS 2155-304 were searched for, using VHE {gamma}-ray data taken with the High Energy Stereoscopic System (H.E.S.S.), and high energy (HE; 100 MeV<E<100 GeV) {gamma}-ray data with the Fermi Large Area Telescope (LAT). Methods: By comparing the angular distributions of the reconstructed gamma-ray events to the angular profiles calculated from detailed theoretical models, the presence of PH and MBC was investigated. Results: Upper limits on the extended emission around 1ES 1101-232, 1ES 0229+200 and PKS 2155-304 are found to be at a level of few percent of the Crab nebula flux above 1 TeV, depending on the assumed photon index of the cascade emission. Assuming strong Extra-Galactic Magnetic Field (EGMF) values, > 10$^{-12}$G, this limits the production of pair halos developing from electromagnetic cascades. For weaker magnetic fields, in which electromagnetic cascades would result in magnetically broadened cascades, EGMF strengths in the range (0.3 - 3)$times 10^{-15}$G were excluded for PKS 2155-304 at the 99% confidence level, under the assumption of a 1 Mpc coherence length.
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.
Fast radio bursts (FRBs) are one of the most exciting new mysteries of astrophysics. Their origin is still unknown, but recent observations seems to link them to Soft Gamma Repeaters and, in particular, to magnetar giant flares (MGFs). The recent detection of a MGF at GeV energies by the textit{Fermi} Large Area Telescope (LAT) motivated the search for GeV counterparts to the >100 currently known FRBs. Taking advantage of more than 12 years of textit{Fermi}-LAT data, we perform a search for gamma-ray emission from all the reported repeating and non-repeating FRBs. We analyse on different-time scales the textit{Fermi}-LAT data of each individual source separately, including a cumulative analysis on the repeating ones. In addition, we perform the first stacking analysis at GeV energies of this class of sources in order to constrain the gamma-ray properties of the FRBs that are undetected at high energies. The stacking analysis is a powerful method that allow a possible detection from below-threshold FRBs providing important information on these objects. In this talk we present the preliminary results of our study and we discuss their implications for the predictions of gamma-ray emission from this class of sources
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