Constraints to dark matter annihilation from high-latitude HAWC unidentified sources

Abstract

The $Λ$CDM cosmological framework predicts the existence of thousands of subhalos in our own Galaxy not massive enough to retain baryons and become visible. Yet, some of them may shine in gamma rays provided that the dark matter (DM) is made of weakly interacting massive particles (WIMPs), that would self-annihilate and would appear as unidentified gamma-ray sources (unIDs) in gamma-ray catalogs. Indeed, unIDs have proven to be competitive targets for DM searches with gamma rays. In this work, we focus on the three high-latitude ($|b|\geq 10^\circ$) sources present in the 2HWC catalog of the High Altitude Water Cherenkov (HAWC) observatory with no associations at other wavelenghts. Indeed, only one of these sources, 2HWC J1040+308, is found to be above the HAWC detection threshold when considering 760 days of data, a factor 1.5 more exposure time than in the original 2HWC catalog. Other instruments such as Fermi-LAT or VERITAS at lower energies do not detect this source. Also, this unID is reported as spatially extended, making it even more interesting in a DM search context. While waiting for more data that may shed further light on the nature of this source, we set competitive upper limits on the annihilation cross section by comparing this HAWC unID to expectations based on state-of-the-art N-body cosmological simulations of the Galactic subhalo population. We find these constraints to be particularly competitive for heavy WIMPs, i.e., masses above $\sim 25$ (40) TeV in the case of the $b\bar{b}$ ($τ^+τ^-$) annihilation channel, reaching velocity-averaged cross section values of $2\cdot10^{-25}$ ($5\cdot10^{-25}$) $cm^3s^{-1}$. Although far from the thermal relic cross section value, the obtained limits are independent and nicely complementary to those from radically different DM analyses and targets, demonstrating again the high potential of this DM search approach.