Resurrecting Hitomi for Decaying Dark Matter and Forecasting Leading Sensitivity for XRISM

The Hitomi x-ray satellite mission carried unique high-resolution spectrometers that were set to revolutionize the search for sterile neutrino dark matter (DM) by looking for narrow x-ray lines arising from DM decays. Unfortunately, the satellite was lost shortly after launch, and to date the only analysis using Hitomi for DM decay used data taken towards the Perseus cluster. In this work we present a significantly more sensitive search from an analysis of archival Hitomi data towards blank sky locations, searching for DM decaying in our own Milky Way. The recently launched XRISM satellite has nearly identical soft-x-ray spectral capabilities to Hitomi; we project the full-mission sensitivity of XRISM for analyses of their future blank-sky data, and we find that XRISM will have the leading sensitivity to decaying DM for masses between roughly 1 to 18 keV, with important implications for sterile neutrino and heavy axionlike particle DM scenarios.

Irreducible Axion Background

Searches for dark matter decaying into photons constrain its lifetime to be many orders of magnitude larger than the age of the Universe. A corollary statement is that the abundance of any particle that can decay into photons over cosmological timescales is constrained to be much smaller than the cold dark-matter density. We show that an irreducible freeze-in contribution to the relic density of axions is in violation of that statement in a large portion of the parameter space. This allows us to set stringent constraints on axions in the mass range 100 eV-100 MeV. At 10 keV our constraint on a photophilic axion is g_{aγγ}≲8.1×10^{-14}  GeV^{-1}, almost 3 orders of magnitude stronger than the bounds established using horizontal branch stars; at 100 keV our constraint on a photophobic axion coupled to electrons is g_{aee}≲8.0×10^{-15}, almost 4 orders of magnitude stronger than the present results. Although we focus on axions, our argument is more general and can be extended to, for instance, sterile neutrinos.

Dark matter interpretation of the Fermi-LAT observations toward the outer halo of M31

An excess γ-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 J-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 × 10-27-5 × 10-24 cm3 s-1. 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 ≲10-6 M ⊙ 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.

Fermi-LAT Observations of γ-Ray Emission toward the Outer Halo of M31

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.

Unresolved Gamma-Ray Sky through its Angular Power Spectrum

The gamma-ray sky has been observed with unprecedented accuracy in the last decade by the Fermi -large area telescope (LAT), allowing us to resolve and understand the high-energy Universe. The nature of the remaining unresolved emission [unresolved gamma-ray background (UGRB)] below the LAT source detection threshold can be uncovered by characterizing the amplitude and angular scale of the UGRB fluctuation field. This Letter presents a measurement of the UGRB autocorrelation angular power spectrum based on eight years of Fermi-LAT Pass 8 data products. The analysis is designed to be robust against contamination from resolved sources and noise systematics. The sensitivity to subthreshold sources is greatly enhanced with respect to previous measurements. We find evidence (with ∼3.7σ significance) that the scenario in which two classes of sources contribute to the UGRB signal is favored over a single class. A double power law with exponential cutoff can explain the anisotropy energy spectrum well, with photon indices of the two populations being 2.55±0.23 and 1.86±0.15.

Search for Dark Matter Annihilation in Galaxy Groups

We use 413 weeks of publicly available Fermi Pass 8 gamma-ray data combined with recently developed galaxy group catalogs to search for evidence of dark matter annihilation in extragalactic halos. In our study, we use luminosity-based mass estimates and mass-to-concentration relations to infer the J factors and associated uncertainties for hundreds of galaxy groups within a redshift range z≲0.03. We employ a conservative substructure boost factor model, which only enhances the sensitivity by an O(1) factor. No significant evidence for dark matter annihilation is found, and we exclude thermal relic cross sections for dark matter masses below ∼30  GeV to 95% confidence in the bb[over ¯] annihilation channel. These bounds are comparable to those from Milky Way dwarf spheroidal satellite galaxies. The results of our analysis increase the tension but do not rule out the dark matter interpretation of the Galactic Center excess. We provide a catalog of the galaxy groups used in this study and their inferred properties, which can be broadly applied to searches for extragalactic dark matter.