The Andromeda galaxy is the closest spiral galaxy to us and has been the subject of numerous studies. It harbors a massive dark matter halo, which may span up to ∼600 kpc across and comprises ∼90% of ...the galaxy's total mass. This halo size translates into a large diameter of 42° on the sky, for an M31-Milky Way (MW) distance of 785 kpc, but its presumably low surface brightness makes it challenging to detect with γ-ray telescopes. Using 7.6 yr of Fermi Large Area Telescope (Fermi-LAT) observations, we make a detailed study of the γ-ray emission between 1-100 GeV toward M31's outer halo, with a total field radius of 60° centered at M31, and perform an in-depth analysis of the systematic uncertainties related to the observations. We use the cosmic-ray propagation code GALPROP to construct specialized interstellar emission models to characterize the foreground γ-ray emission from the MW, including a self-consistent determination of the isotropic component. We find evidence for an extended excess that appears to be distinct from the conventional MW foreground, having a total radial extension upward of ∼120-200 kpc from the center of M31. We discuss plausible interpretations of the excess emission, but emphasize that uncertainties in the MW foreground-and in particular, modeling of the H i-related components-have not been fully explored and may impact the results.
The presence of an excess γ-ray signal toward the Galactic Center (GC) has now been well established, and is known as the GC excess. Leading explanations for the signal include mismodeling of the ...Galactic diffuse emission along the line of sight, an unresolved population of millisecond pulsars, and/or the annihilation of dark matter (DM). Recently, evidence for another excess γ-ray signal has been reported toward the outer halo of M31. In this work we interpret the excess signals from both the GC and outer halo of M31 in the framework of DM annihilation, and show that the two spectra are consistent with a DM origin once J-factors are taken into account. We further compare the excesses to models of DM annihilation, and determine the corresponding best-fit parameters. We find good fits to the spectrum both in two body and four body annihilation modes.
Abstract Recent observations of high-energy neutrinos by IceCube and gamma rays by the Fermi Large Area Telescope (LAT) and the MAGIC telescope have suggested that neutrinos are produced in gamma-ray ...opaque environments in the vicinity of supermassive black holes. In this work, we present 20 MeV–1 TeV spectra of three Seyfert galaxies whose nuclei are predicted to be active in neutrinos, NGC 4151, NGC 4945, and the Circinus galaxy, using 14.4 yr of Fermi LAT data. In particular, we find evidence of sub-GeV excess emission that can be attributed to gamma rays from NGC 4945, as was also seen in NGC 1068. These spectral features are consistent with predictions of the magnetically powered corona model, and we argue that NGC 4945 is among the brightest neutrino active galaxies detectable for KM3Net and Baikal-GVD. On the other hand, in contrast to other reported results, we do not detect gamma rays from NGC 4151, which constrains neutrino emission from the accretion shock model. Future neutrino detectors such as IceCube-Gen2 and MeV gamma - ray telescopes such as AMEGO-X will be crucial for discriminating among the theoretical models.
An excess
-ray signal toward the outer halo of M31 has recently been reported. Although other explanations are plausible, the possibility that it arises from dark matter (DM) is valid. In this work ...we interpret the excess in the framework of DM annihilation, using as our representative case WIMP DM annihilating to bottom quarks, and we perform a detailed study of the systematic uncertainty in the
-factor for the M31 field. We find that the signal favors a DM particle with a mass of ~45-72 GeV. While the mass is well constrained, the systematic uncertainty in the cross section spans 3 orders of magnitude, ranging from ~5 × 10
-5 × 10
cm
s
. This high uncertainty is due to two main factors, namely, an uncertainty in the substructure nature and geometry of the DM halos for both M31 and the Milky Way (MW), and correspondingly, an uncertainty in the contribution to the signal from the MW's DM halo along the line of sight. However, under the conditions that the minimum subhalo mass is ≲10
and the actual contribution from the MW's DM halo along the line of sight is at least ~30% of its total value, we show that there is a large overlap with the DM interpretations of both the Galactic center (GC) excess and the antiproton excess, while also being compatible with the limits for the MW dwarf spheroidals. More generally, we summarize the results from numerous complementary DM searches in the energy range 10 GeV-300 GeV corresponding to the GC excess and identify a region in parameter space that still remains viable for discovery of the DM particle.
Abstract
In 2016, the Compton Spectrometer and Imager (COSI) had a successful 46 day flight on board NASA’s Super Pressure Balloon platform. In this work, we report measurements of the Galactic ...diffuse continuum emission (GDCE) observed toward the inner Galaxy during the flight, which in the COSI energy band (0.2–5 MeV) is primarily generated from inverse Compton radiation. Within uncertainties, we find overall good agreement with previous measurements from INTEGRAL/SPI and COMPTEL. Based on these initial findings, we discuss the potential for further probing the GDCE with the 2016 COSI balloon data, as well as prospects for the upcoming satellite mission.
An excess γ-ray signal coming from the direction of the Galactic center (GC) has been detected by the Fermi Large Area Telescope (Fermi-LAT). Possible explanations for the excess include inaccuracies ...in the foreground/background model, an unresolved population of millisecond pulsars, and/or dark matter (DM) annihilation. However, the GC is a complicated region, and the true nature of the excess currently remains uncertain.M31 is the closest spiral galaxy to us and has been the subject of numerous studies. Because it is both massive and relatively nearby, M31 is an ideal target for studying galaxies, and historically it has played a major role in the development of our understanding of the Universe. γ-ray radiation coming from the inner region of M31 was first detected in 2010 by Fermi-LAT. Since the initial detection, all studies to date have focused on the innermost region, where the galactic disk can be observed. However, M31’s galactic disk only amounts to roughly 10% of the galaxy’s total mass, with the other 90% being in the form of DM, which extends well beyond the inner region of the galaxy.In this dissertation, a detailed study of the γ-ray emission towards M31's outer halo is made. Using the cosmic ray propagation code GALPROP, specialized interstellar emission models are constructed to characterize the foreground emission from the Milky Way (MW), including a self-consistent determination of the isotropic component, and an in-depth analysis of the systematic uncertainties related to the observations. Evidence is found for an extended excess that appears to be distinct from the conventional MW foreground, having a total radial extension upwards of ~120–200 kpc from the center of M31. A DM interpretation is found to provide a good description of the observed emission and is consistent with the GC excess DM interpretation. However, uncertainties in the MW foreground, and in particular, modeling of the H I-related components, have not been fully explored and may impact the results.
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