Separating astrophysical sources from indirect dark matter signals

Indirect searches for products of dark matter annihilation and decay face the challenge of identifying an uncertain and subdominant signal in the presence of uncertain backgrounds. Two valuable approaches to this problem are (1) using analysis methods which take advantage of different features in the energy spectrum and angular distribution of the signal and backgrounds, and (2) more accurate characterization of backgrounds, which allows for more robust identification of possible signals. These two approaches are complementary and can be significantly strengthened when used together. I review the status of indirect searches with gamma rays using two promising targets, the Inner Galaxy and the Isotropic Gamma-Ray Background. For both targets, uncertainties in the properties of backgrounds is a major limitation to the sensitivity of indirect searches. I then highlight approaches which can enhance the sensitivity of indirect searches using these targets.Comment: 7 pages, 4 figures. Contributed to the National Academy of Sciences' Dark Matter Sackler Colloquiu

A review of indirect searches for particle dark matter

The indirect detection of dark matter annihilation and decay using observations of photons, charged cosmic rays, and neutrinos offers a promising means of identifying the particle nature of this elusive component of the universe. The last decade has seen substantial advances in observational data sets, complemented by new insights from numerical simulations, which together have enabled for the first time strong constraints on dark matter particle models, and have revealed several intriguing hints of possible signals. This review provides an introduction to indirect detection methods and an overview of recent results in the field.Comment: 32 pages, 6 figures; invited review, accepted to Contemporary Physic

Joint anisotropy and source count constraints on the contribution of blazars to the diffuse gamma-ray background

We place new constraints on the contribution of blazars to the large-scale isotropic gamma-ray background (IGRB) by jointly analyzing the measured source count distribution (logN-logS) of blazars and the measured intensity and anisotropy of the IGRB. We find that these measurements point to a consistent scenario in which unresolved blazars make less than 20% of the IGRB intensity at 1-10 GeV while accounting for the majority of the measured anisotropy in that energy band. These results indicate that the remaining fraction of the IGRB intensity is made by a component with a low level of intrinsic anisotropy. We determine upper limits on the anisotropy from non-blazar sources, adopting the best-fit parameters of the measured source count distribution to calculate the unresolved blazar anisotropy. In addition, we show that the anisotropy measurement excludes some recently proposed models of the unresolved blazar population.Comment: 7 pages, 4 figures. v2: new section (Sec.III) and 2 figures added. Expanded discussions in the other sections. Results and conclusions unchanged. New Section III is also a reply to the comment of Harding & Abazajian arXiv:1204.3870 on this wor

Dark matter subhalos and unidentified sources in the Fermi 3FGL source catalog

If dark matter consists of weakly interacting massive particles (WIMPs), dark matter subhalos in the Milky Way could be detectable as gamma-ray point sources due to WIMP annihilation. In this work, we perform an updated study of the detectability of dark matter subhalos as gamma-ray sources with the Fermi Large Area Telescope (Fermi LAT). We use the results of the Via Lactea II simulation, scaled to the Planck 2015 cosmological parameters, to predict the local dark matter subhalo distribution. Under optimistic assumptions for the WIMP parameters --- a 40 GeV particle annihilating to $b\bar{b}$ with a thermal cross-section, as required to explain the Galactic center GeV excess --- we predict that at most $\sim 10$ subhalos might be present in the third Fermi LAT source catalog (3FGL). This is a smaller number than has been predicted by prior studies, and we discuss the origin of this difference. We also compare our predictions for the detectability of subhalos with the number of subhalo candidate sources in 3FGL, and derive upper limits on the WIMP annihilation cross-section as a function of the particle mass. If a dark matter interpretation could be excluded for all 3FGL sources, our constraints would be competitive with those found by indirect searches using other targets, such as known Milky Way satellite galaxies.Comment: 17 pages, 8 figure

Signatures of LCDM substructure in tidal debris

In the past decade, surveys of the stellar component of the Galaxy have revealed a number of streams from tidally disrupted dwarf galaxies and globular clusters. Simulations of hierarchical structure formation in LCDM cosmologies predict that the dark matter halo of a galaxy like the Milky Way contains hundreds of subhalos with masses of ~10^8 solar masses and greater, and it has been suggested that the existence of coherent tidal streams is incompatible with the expected abundance of substructure. We investigate the effects of dark matter substructure on tidal streams by simulating the disruption of a self-gravitating satellite on a wide range of orbits in different host models both with and without substructure. We find that the halo shape and the specific orbital path more strongly determine the overall degree of disruption of the satellite than does the presence or absence of substructure, i.e., the changes in the large-scale properties of the tidal debris due to substructure are small compared to variations in the debris from different orbits in a smooth potential. Substructure typically leads to an increase in the degree of clumpiness of the tidal debris in sky projection, and in some cases a more compact distribution in line-of-sight velocity. Substructure also leads to differences in the location of sections of debris compared to the results of the smooth halo model, which may have important implications for the interpretation of observed tidal streams. A unique signature of the presence of substructure in the halo which may be detectable by upcoming surveys is identified. We conclude, however, that predicted levels of substructure are consistent with a detection of a coherent tidal stream from a dwarf galaxy.Comment: 15 pages, 13 figures, accepted for publication in ApJ. Matches accepted versio

Constraints on Axions and Axionlike Particles from Fermi Large Area Telescope Observations of Neutron Stars

We present constraints on the nature of axions and axionlike particles (ALPs) by analyzing gamma--ray data from neutron stars using the Fermi Large Area Telescope. In addition to axions solving the strong CP problem of particle physics, axions and ALPs are also possible dark matter candidates. We investigate axions and ALPs produced by nucleon--nucleon bremsstrahlung within neutron stars. We derive a phenomenological model for the gamma--ray spectrum arising from subsequent axion decays. By analyzing 5 years of gamma-ray data (between 60 MeV and 200 MeV) for a sample of 4 nearby neutron stars, we do not find evidence for an axion or ALP signal, thus we obtain a combined 95\% confidence level upper limit on the axion mass of 7.9$\times 10^{-2}$ eV, which corresponds to a lower limit for the Peccei-Quinn scale $f_a$ of 7.6$\times 10^7$ GeV. Our constraints are more stringent than previous results probing the same physical process, and are competitive with results probing axions and ALPs by different mechanisms.Comment: Accepted by Physical Review D (http://journals.aps.org/prd