Prospects for annihilating dark matter in the inner galactic halo by the Cherenkov Telescope Array

We compute the sensitivity to dark matter annihilations for the forthcoming large Cherenkov Telescope Array (CTA) in several primary channels and over a range of dark matter masses from 50 GeV up to 80 TeV. For all channels, we include inverse Compton scattering of $e^{\pm }$ by dark matter annihilations on the ambient photon background, which yields substantial contributions to the overall $\gamma$-ray flux. We improve the analysis over previous work by: (i) implementing a spectral and morphological analysis of the $\gamma$-ray emission; (ii) taking into account the most up-to-date cosmic ray background obtained from a full CTA Monte Carlo simulation and a description of the diffuse astrophysical emission; and (iii) including the systematic uncertainties in the rich observational CTA data sets. We find that our spectral and morphological analysis improves the CTA sensitivity by roughly a factor 2. For the hadronic channels, CTA will be able to probe thermal dark matter candidates over a broad range of masses if the systematic uncertainties in the data sets will be controlled better than the percent level. For the leptonic modes, the CTA sensitivity will be well below the thermal value of the annihilation cross-section. In this case, even with larger systematics, thermal dark matter candidates up to masses of a few TeV will be easily studied.

Figure 1

Several example of $\gamma$-rays fluxes for 2 TeV DM candidates annihilating with thermal cross-section ($⟨\sigma \mathrm{v}⟩=3\times {10}^{-26}{\mathrm{cm}}^3/\mathrm{s}$) into leptonic channels (first row) and hadronic ones (second row). In all plots, the different hashing shows the predicted signal in two benchmark RoIs with apertures ${\overline{\theta }}_1$ (dotted line), and ${\overline{\theta }}_5$ (solid line). The red, blue and thick black lines denote instead the spectral features of the prompt, ICSs and total fluxes, respectively. See the text for further details.

Figure 2

Spectral and spatial morphology of typical DM signals together with the irreducible backgrounds. Left panel: Expected DM differential fluxes in the first RoI, considering 2 TeV thermal DM candidates annihilating into $b\overline{b}$ (black solid line) and $e^{+}{e}^{-}$ (red solid line) primary channels. For a sake of illustration, we show a spectrum for the CR hadrons ($\text{protons}+\text{nuclei}$) background (dashed green line) multiplied by a rejection factor $\epsilon =10^{-2}$ and a log-normal distribution for the electron spectrum (green dotted line) without rejection factor (${\epsilon }_e=1$). The parametrizations of the irreducible backgrounds are given in Table 3 of Ref. [42]. Notice that the energy $E$ in the $y$-axis, refers to the “true” energy of the particle which initiates the shower. The GDE flux detected by FERMI-LAT extrapolated at high energy is also shown as dashed blue line. Central panel: Expected differential count rate in the first RoI considering 2 TeV DM candidates with thermal cross-section into $b\overline{b}$ (black solid line) and $e^{+}{e}^{-}$ (red solid line) primary channels. The most up-to-date CR background from a CTA Monte Carlo simulation (solid green line) and the GDE (dashed blue line) detected by Fermi-LAT in the same region are also presented for the sake of comparison. Right panel: Expected total rate summed over all energy bins in the $i$-esime RoI and for the same benchmark DM candidates (red filled triangle for annihilating DM into $e^{+}{e}^{-}$ pairs, black filled square for $b\overline{b}$ primary channel). The irreducible CR hadrons background in the $i$-esime RoI is identified as green filled circles, while our “extreme” isotropic GDE as blue-filled stars. See the text for further details.

Figure 3

CTA sensitivity to DM annihilation in the ($m_{\mathrm{DM}},⟨\sigma v⟩$) plane. Unless otherwise stated, our morphological analysis employs five adjacent RoIs, the exposure in each RoIs is 500 h, the energy threshold is 30 GeV and only the statistical uncertainties are taken into account. Upper-left panel: Improvement of the CTA sensitivity for the DM $\mathrm{DM}\to \overline{b}b$ channel due to our multibin morphological method (two, three, five and eight RoIs) compared to the case where only the energy spectral information either in the first (solid black line) or in the second (dashed black line) RoI is taken into account. Upper-right panel: CTA sensitivity for the DM $\mathrm{DM}\to \overline{b}b$ channel when only the spectral information in the individual RoIs (RoI 1 to RoI 5) is considered. For a sake of comparison, we also show how our multibin morphological method (solid red line) ameliorates the CTA sensitivity. Bottom-left panel: Degradation of the CTA sensitivity for annihilating DM into $\overline{b}b$ pairs due to possible systematic errors in the rich observational data sets. Observation times of 100 h (black lines) and 500 h (red lines) assuming different values of systematics: 0.3% (dashed lines), 3% (dotted lines), only statistical fluctuations (solid lines) are taken into account. Bottom-right panel: Impact of an “extreme” GDE added on top of the CR background, on the CTA sensitivity. The sensitivity is shown for annihilating DM into $\overline{b}b$ (back lines) and $e^{+}{e}^{-}$ (red lines) pairs with (dotted lines) and without (solid lines) the irreducible GDE background. See the text for further details.

Figure 4

Unless otherwise stated, the most optimistic limits in the ($m_{\mathrm{DM}},⟨\sigma v⟩$) plane in which neither the systematic uncertainties in the data sets nor the GDE are taken into account. Left panel: CTA sensitivity to DM annihilation for various primary channels (DM $\mathrm{DM}\to b\overline{b}$ in black, $\overline{t}t$ in blue, $W^{+}{W}^{-}$ in yellow, ${\tau }^{+}{\tau }^{-}$ in green, ${\mu }^{+}{\mu }^{-}$ in red and $e^{+}{e}^{-}$ in dashed black) together with the reference value of the thermal cross-section (long-dashed black line). The sensitivity is computed for a 500 h flat exposure over the five RoIs used in our analysis assuming a 100% branching ratio in each annihilation channel. Right panel: CTA sensitivity to DM annihilation for the DM $\mathrm{DM}\to b\overline{b}$ channel (black line) compared to the most constraining limits to date. In particular, we report the H.E.S.S. limit for 112h of observations of the GC region (solid red line) and the Fermi-LAT limit coming from 5 years of observations of 15 dSphs (solid blue line). An estimate of the projected sensitivity of Fermi-LAT for 45 dSphs and 10 years of observation time is also shown (dotted blue line). For the sake of comparison, we also report the CTA sensitivity assuming a reasonable 0.3% of systematics (dashed black line). See the text for further details.