gamma-rays from annihilating dark matter in galaxy clusters: stacking vs single source analysis
Résumé
Clusters of galaxies are potentially important targets for indirect searches for dark matter annihilation. Here, we reassess the detection prospects for annihilation in massive halos, based on a statistical investigation of 1743 clusters from the recent MCXC meta-catalogue. We derive a new data-driven limit for the extra-galactic DM annihilation background Jextra-gal>JGal/5 and consider a source-stacking approach. The number of clusters scales with their brightness (boosted by DM substructures) to the power of -2 for an integration angle 0.1deg. It suggests that stacking may provide a significant improvement over a single target analysis for gamma-ray observations at high-energies where the angular resolution achievable is comparable to this angle. In our study the mean angle containing 80% of the dark-matter signal for the entire sample (assuming an NFW DM profile) is 0.15deg. It indicates that instruments with this angular resolution or better would be optimal for a cluster annihilation search based on stacking. A detailed study based on realistic Fermi-LAT performance and position-dependent background suggests, however, that stacking is likely to result in only modest (a factor >1.7) sensitivity improvement, in comparison to the analysis of the most promising single source in our study (Virgo). This is a consequence of (i) the relatively poor resolution of Fermi-LAT in the energy range where most photon statistics are available, and (ii) the larger angles subtended by bright, nearby objects such as Virgo. Based on the expected performance of CTA, we find no improvement with stacking, due to the requirement for pointed observations. We note that several potentially important targets, Ophiuchus, A2199, A3627 (Norma) or CIZAJ1324.7-5736 may be disfavoured due to a poor contrast with respect to the Galactic DM signal [abridged]