Constraints to Dark Matter Annihilation from High-Latitude HAWC Unidentified Sources

Abstract

The ΛCDM 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 outshine in gamma rays provided that the dark matter 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 dark matter searches with gamma rays. In this work, we focus on the three high-latitude (|b| ≥ 10 o ) sources present in the 2HWC catalog of the High Altitude Water Cherenkov (HAWC) observatory with no clear 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, i.e., a factor 1.5 more exposure time than in the original 2HWC catalog. Other gamma-ray instruments such as Fermi-LAT or VERITAS at lower energies do not detect the source. Also, this unID is reported as spatially extended, making it even more interesting in a dark matter 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 ∼ 25 (40) TeV in the case of the bb (τ + τ − ) annihilation channel, reaching velocity-averaged cross section values of 2 · 10 −25 (5 · 10 −25 ) cm 3 s −1 . Although far from testing the thermal relic cross section value, the obtained limits are independent and nicely complementary to those from radically different DM analyses and targets, demonstrating once again the high potential of this DM search approach.

Sensitivity of the Cherenkov Telescope Array to dark subhalos

“…In terms of limits, this work also complements previous works on dark subhalo detection for other gamma-ray telescopes, namely the Fermi-LAT [36] and the HAWC [34] observatory. Indeed, the synergy between the three mentioned instruments, LAT, HAWC and CTA, is evident: LAT dominates the sensitivity for energies below few hundred of GeV and HAWC above few tens of TeV, with CTA bridging the gap in the region in between.…”

“…Therefore, the respective weight of the limits at present may change with time: LAT results will probably slowly improve, dominating for long time the constraints below the TeV. In the meantime, HAWC will have several years of data to close the gap from 5, but showing only the EXPO constraints and the comparison with previous works, namely the Fermi-LAT unIDs [36], HAWC unIDs [34], and the previous work on unIDs detection with CTA [58]. higher energies.…”

“…In Figure 6, we put our results into context by showing a selection of exclusion limits obtained by other instruments that can be compared to ours, as they aim for setting constraints with unidentified sources (unIDs), namely the Fermi-LAT [36], HAWC [34], and the previous work on unIDs detection with CTA [58]. For the sake of clarity, only the best limits, i.e., the ones obtained with the EXPO strategy, are plotted.…”

“…The LAT-HAWC-CTA constraints from [36,34] and this work, as shown in Figure 6, have the advantage that they were obtained with the exact same methodology and adopting the same N-body simulation results. This means that these limits are fully consistent one another.…”

“…Evidence of their passage may be obtained from their gravitational interaction on e.g., the surrounding stellar fields or by the imprint on the stellar velocity distribution [9,43,25,26,60]. Yet, despite their unknown location, they can be excellent candidates for gamma-ray DM searches given their typical number densities, masses and distances [38,78,20,41,24,56] and, indeed, several groups already searched for dark subhalos in gamma-ray data and catalogs, and were able to set competitive constraints in the absence of them [22,11,68,87,5,88,14,15,74,67,2,35,36,33]. In principle, CTA is less suitable than e.g.…”