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Register a new userDissecting the inner Galaxy with $gamma$-ray pixel count statistics
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We combine adaptive template fitting and pixel count statistics in order to assess the nature of the Galactic center excess in Fermi-LAT data. We reconstruct the flux distribution of point sources in the inner Galaxy well below the Fermi-LAT detection threshold, and measure their radial and longitudinal profiles. Point sources and diffuse emission from the Galactic bulge each contributes $mathcal{O}$(10%) of the total emission therein, disclosing a sub-threshold point-source contribution to the Galactic center excess.
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We analyze about 12 years of Fermi-LAT data in the direction of the Andromeda galaxy (M31). We robustly characterize its spectral and morphological properties against systematic uncertainties related to the modeling of the Galactic diffuse emission. We perform this work by adapting and exploiting the potential of the skyFACT adaptive template fitting algorithm. We reconstruct the gamma-ray image of M31 in a template-independent way, and we show that flat spatial models are preferred by data, indicating an extension of the $gamma$-ray emission of about 0.3-0.4 degree for the bulge of M31. This study also suggests that a second component, extending to at least 1 degree, contributes to the observed total emission. We quantify systematic uncertainties related to mis-modeling of Galactic foreground emission at the level of 2.9%.
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.
Different forms of long gamma-ray bursts (GRBs) Luminosity Functions are considered on the basis of an explicit physical model. The inferred flux distributions are compared with the observed ones from two samples of GRBs, Swift and Fermi GBM. The best fit parameters of the Luminosity functions are found and the physical interpretations are discussed. The results are consistent with the observation of a comparable number of flat phase afterglows and monotonic decreasing ones.
The jet composition and radiative efficiency of GRBs are poorly constrained from the data. If the jet composition is matter-dominated (i.e. a fireball), the GRB prompt emission spectra would include a dominant thermal component originating from the fireball photosphere, and a non-thermal component presumably originating from internal shocks whose radii are greater than the photosphere radius. We propose a method to directly dissect the GRB fireball energy budget into three components and measure their values by combining the prompt emission and early afterglow data. The measured parameters include the initial dimensionless specific enthalpy density ($eta$), bulk Lorentz factors at the photosphere radius ($Gamma_{rm ph}$) and before fireball deceleration ($Gamma_0$), the amount of mass loading ($M$), as well as the GRB radiative efficiency ($eta_gamma$). All the parameters can be derived from the data for a GRB with a dominant thermal spectral component, a deceleration bump feature in the early afterglow lightcurve, and a measured redshift. The results only weakly depend on the density $n$ of the interstellar medium when the composition ${cal Y}$ parameter (typically unity) is specified.
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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