Willman 1: An X-ray shot in the dark with Chandra

Abstract

The sterile neutrino is a weakly-interacting particle that emerges in the framework of certain extensions of standard particle physics and that fits naturally with the properties of a warm dark matter particle candidate. We present an analysis of a deep archival Chandra observation of Willman 1, one of the darkest purported ultra-faint dwarf galaxies, that excludes the presence of sterile neutrinos in the 1.6-16 keV mass range within 55 pc of its centre down to the limits of the observation. Spectral analysis of the Chandra data fails to find any non-instrumental spectral feature possibly connected with the radiative decay of a dark matter particle. Accordingly, we establish upper bounds on the sterile neutrino parameter space and discuss it in the context of previous measurements. Regarding the point source population, we identify a total of 26 sources within the central 5 arcminutes to a limiting 0.5-2.0 keV X-ray flux of 6 x 10⁻¹⁶ erg cm-2 s⁻¹. While some of these sources could be formal members of Willman 1, we find no outstanding evidence for either an unusual population of bright X-ray sources or a densely populated cluster core. In fact, the entire X-ray population could be explained by background active galactic nuclei and/or foreground stars unrelated to Willman 1. Finally, possible associations of the X-ray point like population with optical sources from the SDSS DR7 catalogue are discussed.