Prospects of future MeV telescopes in probing weak-scale Dark Matter

TL;DR

This paper assesses how upcoming MeV gamma-ray telescopes can probe weak-scale Dark Matter by exploiting secondary photons from DM-induced $e^{\pm}$ via inverse Compton scattering and bremsstrahlung in the Galactic Center. Using an NFW DM halo and modeling both prompt and secondary emissions, the authors forecast DM sensitivity with two methods (SNR and Fisher) for Amego, e-ASTROGAM, and Mast over a ~3-year observing window. They find that secondary emissions dramatically enhance the MeV signal, enabling exploration of thermal DM up to the TeV scale and GeV DM with cross-sections 2–3 orders of magnitude below current bounds, particularly for leptonic channels. Overall, MeV telescopes provide a valuable complement to high-energy γ-ray searches, extending indirect-detection reach into mass and cross-section regimes that are challenging for existing instruments, albeit with uncertainties from DM halo, ISRF, and propagation modeling.

Abstract

Galactic weak-scale Dark Matter (DM) particles annihilating into lepton-rich channels not only produce gamma-rays via prompt radiation but also generate abundant energetic electrons and positrons, which subsequently emit through bremsstrahlung or inverse Compton scattering (collectively called `secondary-radiation photons'). While the prompt gamma-rays concentrate at high-energy, the secondary emission falls in the MeV range, which a number of upcoming experiments (AMEGO, E-ASTROGAM, MAST...) will probe. We investigate the sensitivity of these future telescopes for weak-scale DM, focusing for definiteness on observations of the galactic center. We find that they have the potential of probing a wide region of the DM parameter space which is currently unconstrained. Namely, in rather optimistic configurations, future MeV telescopes could probe thermally-produced DM with a mass up to the TeV range, or GeV DM with an annihilation cross section 2 to 3 orders of magnitude smaller than the current bounds, precisely thanks to the significant leverage provided by their sensitivity to secondary emissions. We comment on astrophysical and methodological uncertainties, and compare with the reach of high-energy gamma ray experiments.

Figures (13)

• Figure 1: Different types of DM-induced photon fluxes discussed in eqs. \ref{['eq:prompt_gamma']}, \ref{['eq:ICS_flux']} and \ref{['eq:brem_flux']}, averaged over a disk of $10^{\circ}$ around the GC. The four panels are for four different values of the DM mass: $m_{\rm DM} = 1, 10, 10^2$ and $10^3$ GeV. Here, for definiteness, we consider ${\rm DM \, DM} \rightarrow \mu^+\mu^-$ annihilations with $\langle \sigma v \rangle = 3\times 10^{-26}$$\rm cm^3 s^{-1}$. In addition, different diffuse photon backgrounds and the Fermi-Lat GC $\gamma$-ray data are also shown (see the text for details). This figure shows that, in our energy range of interest, the secondary photon flux such as the ICS can be a very important component of the DM signal for different DM masses.
• Figure 2: Effective areas of the future telescopes Amego, e-Astrogam and Mast, for the range of the photon energy considered in this work.
• Figure 3: Upper bounds and projected sensitivities on the annihilation cross-section $\langle \sigma v \rangle$ as a function of $m_{\rm DM}$, for $\mu^+\mu^-$ (upper panels) and $e^+e^-$ (lower panels) annihilation channels. Notice the different vertical ranges for different channels. For each channel, the projected sensitivities are shown considering the two statistical approaches discussed in sections \ref{['sec:SNR_method']} (left column) and \ref{['sec:Fisher_method']} (right column). In each case, our conservative upper bounds are shown with a red solid line, and the existing bounds from the literature with green shaded areas (see the text for details). The projected sensitivities of the upcoming MeV telescopes Amego, e-Astrogam and Mast are shown by blue, orange and magenta curves, respectively, for an observation time of $10^8$ sec ($\simeq 3$ yrs). The solid and dashed black curves show the projections of Cta for the observations towards GC and dwarf galaxies (when available), respectively. Finally, the solid gray thin band in each plot indicates the value of $\langle \sigma v \rangle$ corresponding to the observed relic abundance for thermal DM, assuming $s$-wave annihilation Steigman:2012nb.
• Figure 4: Same as fig. \ref{['fig:sv_mx_limits_1']}, but considering $b\bar{b}$ (upper panels) and $W^+W^-$ (lower panels) annihilation channels. Notice the different horizontal and vertical ranges for different channels. The solid and dashed black curves show the projections of Cta for the observations towards GC and dwarf galaxies, respectively, and the dark blue curve shows the projection for Swgo, when available.
• Figure 5: Illustration of the importance of considering the secondary signals of DM in the case of the leptonic annihilation channels. The comparison between the Fisher projections (for Amego with $t_{\rm obs} = 10^8$ sec) obtained considering and not considering the secondary signals are shown for the $\mu^+\mu^-$, $e^+e^-$ (left panel) and the $b\bar{b}$, $W^+W^-$ (right panel) channels. The colored solid curves are the projections obtained considering the secondary signals and are the same as the ones presented in figs. \ref{['fig:sv_mx_limits_1']} and \ref{['fig:sv_mx_limits_2']}, while the dashed curves show the projections obtained not considering the secondary signals.