Unidentified gamma-ray sources as targets for indirect dark matter detection with theFermi-Large Area Telescope

Abstract

One of the predictions of the ΛCDM cosmological framework is the hierarchical formation of structure, giving rise to dark matter (DM) halos and subhalos. When the latter are massive enough they retain gas (i.e., baryons) and become visible. This is the case of the dwarf satellite galaxies in the Milky Way (MW). Below a certain mass, halos may not accumulate significant amounts of baryons and remain completely dark. However, if DM particles are Weakly Interacting Massive Particles (WIMPs), we expect them to annihilate in subhalos, producing gamma rays which can be detected with the Fermi satellite. Using the three most recent point-source Fermi Large Area Telescope (LAT) catalogs (3FGL, 2FHL and 3FHL), we search for DM subhalo candidates among the unidentified sources, i.e., sources with no firm association to a known astrophysical object. We apply several selection criteria based on the expected properties of the DM-induced emission from subhalos, which allow us to significantly reduce the list of potential candidates. Then, by characterizing the minimum detection flux of the instrument and comparing our sample to predictions from the Via Lactea II (VL-II) N-body cosmological simulation, we place conservative and robust constraints on the σv − m DM parameter space. For annihilation via the τ + τ − channel, we put an upper limit of 4 × 10 −26 (5 × 10 −25 ) cm 3 s −1 for a mass of 10 (100) GeV. A critical improvement over previous treatments is the repopulation we made to include low-mass subhalos below the VL-II mass resolution. With more advanced subhalo candidate filtering the sensitivity reach of our method can potentially improve these constraints by a factor 3 (2) for τ + τ − (bb) channel.are an example of the most massive members of this population [11]. Yet, these dSphs are exceptional objects, in that they are massive enough to retain baryons (i.e., gas) and form stars. Conversely, the vast majority of the Galactic DM subhalos are not expected to host baryons and therefore remain completely dark [12]. Given their much larger number density, many of these small subhalos will be much closer to the Earth than the bigger ones, making them potentially interesting for dark matter searches.Should the Weakly Interacting Massive Particle (WIMP) DM model be correct (see, e.g., [13, 14] for a review), these objects may be detectable in the gamma-ray data. WIMPs can achieve the correct relic DM abundance (the so-called "WIMP miracle") through selfannihilation in the early Universe. Self-annihilation of WIMPs gives rise to a Standard Model (SM) particle-antiparticle pair which, among other possible subsequent by-products, typically yields gamma-ray photons. The ongoing self-annihilation of WIMPs in subhalos could be bright enough to be detectable.Since its launch in 2008, the Large Area Telescope on board the NASA Fermi Gammaray Space Telescope (Fermi-LAT) has been surveying the sky searching for gamma-ray sources [15]. The Fermi-LAT is a pair conversion telescope designed to observe the energy band from 20 MeV to gre...

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

“…While waiting for more data that can help to shed further light on the true nature of 2HWC J1040+308, we proceed in Section 3 and set limits on the DM annihilation cross section by conservatively assuming that this source is actually a subhalo. To do so, we follow the methodology in [23] and compare the HAWC unID observations with predictions for the Galactic subhalo population as derived from N-body cosmological simulations of a galaxy like our own. In particular, we follow the simulation work in [23,24], where the VL-II DM-only simulation was repopulated to include subhalos with masses down to 10 3 M .…”

“…Related to this, we note that the simplest and most robust way to improve our DM constraints is to also associate 2HWC J1040+308 to a known astrophysical source. In fact, a simple detection of variability or multi-wavelength emission would be enough to discard it as a DM subhalo (see several examples of unID rejections in [23]). As HAWC accumulates exposure time, also its minimum detectable flux will be lowered, thus allowing for better DM limits for a fixed number of unIDs.…”

“…Then, we rescale the obtained F min by differences in exposure times, i.e., a factor √ 507/760 = 0.82. 5 This is so because, by default, this quantity is computed for a source spectrum with a power law index Γ ∼ 2, while the DM is better parametrized by a "Power Law with SuperExponential Cutoff" [23], where the index and cutoff energy vary over the annihilation channel and WIMP mass. 6 This sensitivity is computed for the Crab, which is located at DEC∼ 22 • , while our source is at ∼ 30 • instead.…”

“…Actually, hundreds of realizations of the parent simulation are performed that allow to derive statistical meaningful J-factor results. More precisely, as done in [23], in order to derive 95% C.L. upper limits on the DM annihilation cross section, we obtain the distribution of J-factor values for the N th subhalo under consideration and then as reference J-factor the one above which 95% of the J-factor distribution is contained; see Figure 3.…”

“…Interestingly, an important fraction of objects (typically between 30 − 40%, depending on the catalog) in the very high energy (VHE, E > 100 GeV) sky are unidentified sources (unIDs), i.e., objects with no clear single association or counterpart. Some of these unIDs may actually be DM subhalos [22,23,[25][26][27][28][29][30][31][32][33]. In our work, we will explore this possibility focusing on the unIDs recently detected in the VHE regime by the High Altitude Water Cherenkov Observatory (HAWC) [34,35].…”