Have Atmospheric Cerenkov Telescopes Observed Dark Matter?

TL;DR

This study investigates whether TeV $\gamma$-rays from the Galactic center detected by atmospheric Cherenkov telescopes can be explained by annihilating dark matter. It frames the expected flux as a product of particle-physics inputs (mass and annihilation channels) and an astrophysical J-factor that encodes the dark matter distribution along the line of sight, then compares predictions for different halo profiles (e.g., NFW, Moore, and spikes) with Whipple and CANGAROO-II data while considering GeV-scale constraints from EGRET. The analysis finds that matching the observations generally requires either very cusped or spiked inner halos or unusually large annihilation cross-sections, and it remains challenged by spectral inconsistencies between experiments unless line emission contributes. The authors survey TeV-scale dark matter candidates, notably SUSY neutralinos in the focus-point region and other non-thermal scenarios, and highlight upcoming measurements from HESS and GLAST as crucial for distinguishing a dark matter origin from astrophysical sources, particularly through potential line signatures.

Abstract

Two ground-based experiments have recently independently detected TeV $γ$-rays from the direction of the Galactic center. The observations made by the VERITAS and CANGAROO collaborations are unexpected, although not impossible to interpret in terms of astrophysical sources. Here we examine in detail whether the observed $γ$-rays may arise from the more exotic alternative of annihilations of dark matter particles clustered in the center of the Galaxy.

Figures (7)

• Figure 1: The spectrum of $\gamma$-rays for dark matter annihilation to selected modes --- $b\bar{b}$ (thin full line), $t\bar{t}$ (dotted line), $W^+W^-, ZZ$ (dashed line). The parameterization of Eq. \ref{['para']} is also shown (thick full line).
• Figure 2: Data from the CANGAROO-II experiment compared with the spectrum predicted for dark matter annihilations to gauge bosons (see Eq. \ref{['para']}). The dot-dashed, dotted, solid and dashed lines are for 1, 2, 3 and 5 TeV particles. Normalization was considered a free parameter. Note that the highest energy bin shown (near 2.5 TeV) is less than 1 $\sigma$ in excess of a null result, so should be taken only as an upper limit. Also shown is the flux measured by the VERITAS collaboration, inferred from the integral flux assuming the spectrum of a 5 TeV mass annihilating particle. Note the very different results of the two experiments.
• Figure 3: The integrated flux predicted for CANGAROO-II, HESS (with 250 GeV thresholds) and EGRET if annihilating dark matter is the source of the $\gamma$-rays observed by Whipple. Annihilations primarily to gauge bosons are assumed (using the parameterization of Eq. \ref{['para']}). With CANGAROO-II's integrated flux of $\sim 2 \times 10^{-10} \, \rm{cm}^{-2}\,\rm{s}^{-1}$ above 250 GeV, only a very heavy dark matter particle ($\sim 10$ TeV) is consistent with the CANGAROO-II and Whipple results, assuming continuum emission dominates. If line emission is significant, the mass may be somewhat smaller. If the particle mass is less than 3.5--4 TeV, the continuum emission from annihilations into gauge bosons exceeds the limit placed by EGRET dingus, assuming a negligible line component. If dark matter annihilates mostly to another mode, such as heavy quark pairs, the EGRET limit may be violated for WIMPs as heavy as $\sim5$ TeV.
• Figure 4: The annihilation cross-section and halo profile needed to provide the $\gamma$-ray flux observed by CANGAROO-II (left) and Whipple (right). Contours are shown for several dark matter particle masses.
• Figure 5: A skymap of the Galactic center region. The solid and dashed contours correspond to the regions observed by Whipple and CANGAROO-II, respectively. In these regions the observed significance is greater than 95% for Whipple and 80% for CANGAROO-II. The 95% confidence region for the off-center source observed by EGRET (3EG J1746-2851) is shown as a shaded region. Also shown are a number of selected objects known to be present in the region including Sgr A$^*$ (the dynamical center of the Galaxy and location of the supermassive black hole), two supernova remnants (SNR1 and SNR2, corresponding to Sgr A East and SNR 000.3+00.0, respectively), the Arches and Quintuplet star clusters, the low mass X-ray binary 1E 1743.1-2843 and two $\gamma$-ray sources observed by INTEGRAL (G1 and G2). The boxed area is the region shown in Fig. \ref{['skymapxray']}