Sensitivity of HAWC to high-mass dark matter annihilations

The High Altitude Water Cherenkov (HAWC) observatory is a wide field-of-view detector sensitive to gamma rays of 100 GeV to a few hundred TeV. Located in central Mexico at 19° North latitude and 4100 m above sea level, HAWC will observe gamma rays and cosmic rays with an array of water Cherenkov detectors. The full HAWC array is scheduled to be operational in Spring 2015. In this paper, we study the HAWC sensitivity to the gamma-ray signatures of high-mass (multi-TeV) dark matter annihilation. The HAWC observatory will be sensitive to diverse searches for dark matter annihilation, including annihilation from extended dark matter sources, the diffuse gamma-ray emission from dark matter annihilation, and gamma-ray emission from nonluminous dark matter subhalos. Here we consider the HAWC sensitivity to a subset of these sources, including dwarf galaxies, the M31 galaxy, the Virgo cluster, and the Galactic center. We simulate the HAWC response to gamma rays from these sources in several well-motivated dark matter annihilation channels. If no gamma-ray excess is observed, we show the limits HAWC can place on the dark matter cross section from these sources. In particular, in the case of dark matter annihilation into gauge bosons, HAWC will be able to detect a narrow range of dark matter masses to cross sections below thermal. HAWC should also be sensitive to nonthermal cross sections for masses up to nearly 1000 TeV. The constraints placed by HAWC on the dark matter cross section from known sources should be competitive with current limits in the mass range where HAWC has similar sensitivity. HAWC can additionally explore higher dark matter masses than are currently constrained.

Figure 1

The Sommerfeld enhancement to the dark matter cross section from the exchange of $W$ bosons. The calculation uses the formalism of Ref. [38] and assumes weak-scale coupling of 1/35. The dark matter cross section is shown for 4 dark matter velocities: $3\mathrm{km}{\mathrm{s}}^{-1}$ (magenta), $30\mathrm{km}{\mathrm{s}}^{-1}$ (blue), $300\mathrm{km}{\mathrm{s}}^{-1}$ (red), and $3000\mathrm{km}{\mathrm{s}}^{-1}$ (black). The unenhanced thermal cross section ($v=3\times {10}^5\mathrm{km}{\mathrm{s}}^{-1}$) is shown for comparison (purple). In this work, we assume $v_{\mathrm{rel}}\sim 300\mathrm{km}{\mathrm{s}}^{-1}$, thought to be the relative velocity of Galactic dark matter.

Figure 2

The projected dark matter limits from dwarf galaxies for HAWC after five years, for the $b\overline{b}$, $t\overline{t}$, ${\mu }^{+}{\mu }^{-}$, ${\tau }^{+}{\tau }^{-}$, and $W^{+}{W}^{-}$ dark matter annihilation channels. From top to bottom, the curves are for Draco (blue), Coma Berenices (red), and Segue 1 (black). The solid curves are the dark matter limits for just the prompt gamma-ray emission. In the $W^{+}{W}^{-}$ plot, the dashed curves are the limit on the early-universe annihilation cross section when natural Sommerfeld enhancement is included in the cross section today (with $v_{\mathrm{rel}}\sim 300\mathrm{km}{\mathrm{s}}^{-1}$). We show the 157.9-hour MAGIC dark matter exclusion limits from Segue 1 as the purple dot-dashed curve for comparison [40]. Note that Sommerfeld enhancement improves the MAGIC results similarly to those of HAWC. The solid purple line shows the expected dark matter thermal cross section. All limits are at 95% CL.

Figure 3

The projected dark matter limits from the Virgo cluster and the galaxy M31 for HAWC after five years, for the $b\overline{b}$, $t\overline{t}$, ${\mu }^{+}{\mu }^{-}$, ${\tau }^{+}{\tau }^{-}$, and $W^{+}{W}^{-}$ dark matter annihilation channels. From top to bottom, the curves are for the Virgo cluster with a smooth (sm) NFW profile (blue), M31 with a smooth (sm) NFW profile (red), the substructure-boosted (bst) Virgo cluster (magenta), and the substructure-boosted (bst) M31 (black). The triple-dot-dashed purple line is the limit from the H.E.S.S. observatory observations of the Fornax cluster [41], boosted (bst) using the substructure boost model of Ref. [31]. The dot-dashed purple line is the limit from the Fermi-LAT observations of the Virgo cluster [36], boosted (bst) using the substructure boost model of Ref. [31]. For the ${\mu }^{+}{\mu }^{-}$ channel, both the H.E.S.S. Fornax limit and the Fermi-LAT Virgo limit are for a combination of prompt emission and IC emission. Here we employ the substructure boost of 35 for the Virgo cluster, 15 for M31, and 29 for the Fornax cluster, based on Ref. [31]. The solid curves are the dark matter limits for just the prompt gamma-ray emission, and the dot-dashed curves are the limits considering both the prompt gamma-ray mission and the IC emission from electrons and positrons scattering on the CMB. In the $W^{+}{W}^{-}$ plot, the dashed curves are the limit on the early-universe annihilation cross section when natural Sommerfeld enhancement is included in the cross section today (with $v_{\mathrm{rel}}\sim 300\mathrm{km}{\mathrm{s}}^{-1}$). The width of the gray bands above the smooth Virgo cluster lines to the right of the figure indicate the range in the dark matter limit for all masses due to possible uncertainty from the point-source subtraction in the analysis. The solid purple line shows the expected dark matter thermal cross section. All limits are at 95% CL.

Figure 4

The projected dark matter limits from the GC for HAWC after five years, for the $b\overline{b}$, $t\overline{t}$, ${\mu }^{+}{\mu }^{-}$, ${\tau }^{+}{\tau }^{-}$, and $W^{+}{W}^{-}$ dark matter annihilation channels. From top to bottom, the curves are for the GC assuming an NFW profile (red) and assuming an Einasto profile (black). The solid curves are the dark matter limits for just the prompt gamma-ray emission, and the dot-dashed curves are the limits considering both the prompt gamma-ray mission and the IC emission from electrons and positrons scattering on the CMB. In the $W^{+}{W}^{-}$ plot, the dashed curves are the limit on the early-universe annihilation cross section when natural Sommerfeld enhancement is included in the cross section today (with $v_{\mathrm{rel}}\sim 300\mathrm{km}{\mathrm{s}}^{-1}$). The width of the gray bands on the right of the figure indicate the range in the dark matter limit for all masses due to the combined uncertainty from the HAWC sensitivity near the edge of its field of view and the possible uncertainty due to point-source subtraction in the analysis. We show the H.E.S.S. 112-hour dark matter exclusion limits from the GC as the purple dot-dashed curve for comparison and in the $W^{+}{W}^{-}$ plot, the H.E.S.S. limit with Sommerfeld enhancement is shown as the purple dashed curve [42, 43]. The solid purple line shows the expected dark matter thermal cross section. All limits are at 95% CL.