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Register a new userSearch for cosmic-ray induced gamma-ray emission in Galaxy Clusters
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Current theories predict relativistic hadronic particle populations in clusters of galaxies in addition to the already observed relativistic leptons. In these scenarios hadronic interactions give rise to neutral pions which decay into $gamma$ rays, that are potentially observable with the Large Area Telescope (LAT) on board the Fermi space telescope. We present a joint likelihood analysis searching for spatially extended $gamma$-ray emission at the locations of 50 galaxy clusters in 4 years of Fermi-LAT data under the assumption of the universal cosmic-ray model proposed by Pinzke & Pfrommer (2010). We find an excess at a significance of $2.7sigma$ which upon closer inspection is however correlated to individual excess emission towards three galaxy clusters: Abell 400, Abell 1367 and Abell 3112. We discuss these cases in detail and conservatively attribute the emission to unmodeled background (for example, radio galaxies within the clusters). Through the combined analysis of 50 clusters we exclude hadronic injection efficiencies in simple hadronic models above 21% and establish limits on the cosmic-ray to thermal pressure ratio within the virial radius, $R_{200}$, to be below 1.2-1.4% depending on the morphological classification. In addition we derive new limits on the $gamma$-ray flux from individual clusters in our sample.
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We report evidence for extended gamma-ray emission from the Virgo, Fornax and Coma clusters based on a maximum-likelihood analysis of the 3-year Fermi-LAT data. For all three clusters, excess emission is observed within three degrees of the center, peaking at the GeV scale. This emission cannot be accounted for by known Fermi sources or by the galactic and extragalactic backgrounds. If interpreted as annihilation emission from supersymmetric dark matter (DM) particles, the data prefer models with a particle mass in the range 20-60 GeV annihilating into the b-bbar channel, or 2-10 GeV and >1 TeV annihilating into mu-mu final states. Our results are consistent with those obtained by Hooper and Linden from a recent analysis of Fermi-LAT data in the region of the Galactic Centre. An extended DM annihilation profile dominated by emission from substructures is preferred over a simple point source model. The significance of DM detection is 4.4 sigma in Virgo and lower in the other two clusters. We also consider the possibility that the excess emission arises from cosmic ray (CR) induced gamma-rays, and infer a CR level within a factor of three of that expected from analytical models. However, the significance of a CR component is lower than the significance of a DM component, and there is no need for such a CR component in the presence of a DM component in the preferred DM mass range. We also set flux and cross-section upper limits for DM annihilation into the b-bbar and mu-mu channels in all three clusters.
Galaxy clusters are predicted to produce gamma-rays through cosmic ray interactions and/or dark matter annihilation, potentially detectable by the Fermi Large Area Telescope (Fermi-LAT). We present a new, independent stacking analysis of Fermi-LAT photon count maps using the 78 richest nearby clusters (z<0.12) from the Two Micron All-Sky Survey (2MASS) cluster catalog. We obtain the lowest limit on the photon flux to date, 2.3e-11 ph/s/cm^2 (95% confidence) per cluster in the 0.8-100 GeV band, which corresponds to a luminosity limit of 3.5e44 ph/s. We also constrain the emission limits in a range of narrower energy bands. Scaling to recent cosmic ray acceleration and gamma-ray emission models, we find that cosmic rays represent a negligible contribution to the intra-cluster energy density and gas pressure.
Multiwavelength observations suggest that clusters are reservoirs of vast amounts relativistic electrons and positrons that are either injected into and accelerated directly in the intra-cluster medium, or produced as secondary pairs by cosmic ray ions scattering on ambient protons. In these possible scenarios gamma rays are produced either through electrons upscattering low-energy photons or by decay of neutral pions produced by hadronic interactions. In addition, the high mass-to-light ratios in clusters in combination with considerable Dark Matter (DM) overdensities makes them interesting targets for indirect DM searches with gamma rays. The resulting signals are different from known point sources or from diffuse emission and could possibly be detected with the Fermi-LAT. Both WIMP annihilation/decay spectra and cosmic ray induced emission are determined by universal parameters, which make a combined statistical likelihood analysis feasible. We present initial results of this analysis leading to limits on the DM annihilation cross section or decay time and on the hadron injection efficiency.
Globular clusters (GCs) are established emitters of high-energy (HE, 100 MeV<E<100 GeV) gamma-ray radiation which could originate from the cumulative emission of the numerous millisecond pulsars (msPSRs) in the clusters cores or from inverse Compton (IC) scattering of relativistic leptons accelerated in the GC environment. These stellar clusters could also constitute a new class of sources in the very-high-energy (VHE, E>100 GeV) gamma-ray regime, judging from the recent detection of a signal from the direction of Terzan 5 with the H.E.S.S. telescope array. We searched for point-like and extended VHE gamma-ray emission from 15 GCs serendipitously covered by H.E.S.S observations and also performed a stacking analysis combining the data from all GCs to investigate the hypothesis of a population of faint emitters. Assuming IC emission as the origin of the VHE gamma-ray signal from the direction of Terzan 5, we calculated the expected gamma-ray flux from each of the 15 GCs, based on their number of millisecond pulsars, their optical brightness and the energy density of background photon fields. We did not detect significant VHE gamma-ray emission from any of the 15 GCs in either of the two analyses. Given the uncertainties related to the parameter determinations, the obtained flux upper limits allow to rule out the simple IC/msPSR scaling model for NGC 6388 and NGC 7078. The upper limits derived from the stacking analyses are factors between 2 and 50 below the flux predicted by the simple leptonic scaling model, depending on the assumed source extent and the dominant target photon fields. Therefore, Terzan 5 still remains exceptional among all GCs, as the VHE gamma-ray emission either arises from extra-ordinarily efficient leptonic processes, or from a recent catastrophic event, or is even unrelated to the GC itself.
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.
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