Searching for Dark Matter with X‐Ray Observations of Local Dwarf Galaxies

Abstract

A generic feature of weakly interacting massive particle (WIMP) dark matter models is the emission of photons over a broad energy band resulting from the stable yields of dark matter pair annihilation. Inverse Compton scattering off cosmic microwave background photons of energetic electrons and positrons produced in dark matter annihilation is expected to produce significant diffuse X-ray emission. Dwarf galaxies are ideal targets for this type of dark matter search technique, being nearby, dark matter dominated systems free of any astrophysical diffuse X-ray background. In this paper, we present the first systematic study of X-ray observations of local dwarf galaxies aimed at the search for WIMP dark matter. We outline the optimal energy and angular ranges for current telescopes and analyze the systematic uncertainties connected to electron/positron diffusion. We do not observe any significant X-ray excess, and we translate this null result into limits on the mass and pair annihilation cross section for particle dark matter. Our results indicate that X-ray observations of dwarf galaxies currently constrain dark matter models at the same level as or even more strongly than gamma-ray observations of the same systems, although at the expenses of introducing additional assumptions and related uncertainties in the modeling of diffusion and energy loss processes. The limits we find constrain portions of the supersymmetric parameter space, particularly if the effect of dark matter substructures is included. Finally, we comment on the role of future X-ray satellites (e.g., Constellation-X, XEUS ) and on their complementarity with GLAST and other gamma-ray telescopes in the quest for particle dark matter.

Local Group dSph radio survey with ATCA – II. Non-thermal diffuse emission

“…In the right-hand panel, p f in = 3 and Gaussian spatial profile are assumed. The upper lines show the corresponding X-ray bounds for the Carina and Fornax dSphs obtained following (Jeltema & Profumo 2008). Circles show the expected CR density from the dSph SFR reported in column 8 of Table 3.…”

“…9, we compare the bounds that can be obtained on the non-thermal electron density from current X-ray data with the constraints derived in this work. For the former, we consider the analysis of (Jeltema & Profumo 2008), which made use of XMM-Newton archival data for the Carina and Fornax dSph targets (on top of Ursa Minor which however is not part of our dSph sample). The derived flux limits in the 0.5-8 keV band are, respectively, 2.1 · 10 −5 and 10 −5 photons cm −2 s −1 for an aperture of 6' radius (Jeltema & Profumo 2008).…”

“…For the former, we consider the analysis of (Jeltema & Profumo 2008), which made use of XMM-Newton archival data for the Carina and Fornax dSph targets (on top of Ursa Minor which however is not part of our dSph sample). The derived flux limits in the 0.5-8 keV band are, respectively, 2.1 · 10 −5 and 10 −5 photons cm −2 s −1 for an aperture of 6' radius (Jeltema & Profumo 2008). Following this analysis, we can constrain U el for the models discussed above.…”

A New Target Object for Constraining Annihilating Dark Matter

“…Consequently, most of the high-energy electrons escape from the dwarf galaxy without losing most of their energy and the resulting x-ray signal is suppressed. For example, Colafrancesco, Profumo & Ullio (2007); Jeltema & Profumo (2008) study the x-ray constraints for the local dwarf galaxies and find that the upper bounds of the annihilation cross sections are quite loose.…”

“…is the total cooling rate, R = 2.7 kpc is the isophotal radius of NGC 2976 (Kennicutt et al 2003), and we have used a conservative diffusion coefficient D 0 = 10 27 cm 2 s −1 (Jeltema & Profumo 2008). Therefore, we ensure that most of the high-energy positrons and electrons produced would loss most of their energy before escaping the galaxy.…”