A New Signature of Dark Matter Annihilations: Gamma-Rays from Intermediate-Mass Black Holes

TL;DR

The paper investigates indirect detection of dark matter through gamma-rays produced by annihilation in mini-spikes around intermediate-mass black holes (IMBHs). It compares two IMBH formation pathways and computes the resulting DM density enhancements and gamma-ray fluxes, employing two benchmark DM models to assess detectability with GLAST and ground-based telescopes. A key finding is that many IMBH mini-spikes could be detectable, and a population of sources with identical spectral cutoffs at the DM particle mass would constitute a smoking-gun signature of DM annihilation. The work implies a potential boost to the gamma-ray background and offers a novel probe of IMBH/SMBH formation, with practical implications for multi-instrument follow-ups and cross-checks in nearby galaxies like Andromeda.

Abstract

We study the prospects for detecting gamma-rays from Dark Matter (DM) annihilations in enhancements of the DM density (mini-spikes) around intermediate-mass black holes with masses in the range $10^2 \lsim M / \msun \lsim 10^6$. Focusing on two different IMBH formation scenarios, we show that, for typical values of mass and cross section of common DM candidates, mini-spikes, produced by the adiabatic growth of DM around pregalactic IMBHs, would be bright sources of gamma-rays, which could be easily detected with large field-of-view gamma-ray experiments such as GLAST, and further studied with smaller field-of-view, larger-area experiments like Air Cherenkov Telescopes CANGAROO, HESS, MAGIC and VERITAS. The detection of many gamma-ray sources not associated with a luminous component of the Local Group, and with identical cut-offs in their energy spectra at the mass of the DM particle, would provide a potential smoking-gun signature of DM annihilations and shed new light on the nature of intermediate and supermassive Black Holes.

Figures (5)

• Figure 1: Mass function of unmerged IMBHs in the scenario B, for a Milky Way Halo at z=0. The distribution is based on an average of $200$ Monte Carlo realizations of a halo of virial mass $M_v = 10^{12.1} h^{-1}{\rm\,M_\odot}$, roughly the size of the halo of the Milky Way.
• Figure 2: Cumulative radial distribution of unmerged IMBHs in the scenario A (red) and B (black), for a Milky Way Halo at z=0. The mean and error are based on $200$ Monte Carlo realizations of IMBH populations in Milky Way-sized halos. Notice that unlike subhalo populations, IMBHs do not exhibit a significant anti-bias with respect to the DM. Rather, they are slightly biased toward being found near the halo center.
• Figure 3: Energy spectra of photons per annihilation for different annihilation channels. The solid and dotted lines both correspond to the $b \bar{b}$ annihilation channel, the differences are due to different parametrizations of quark fragmentation and different DM particle mass scales. The solid line shows the parameterization of Ref. Fornengo:2004kj with $m_{\chi}=1$ TeV, while the dotted line shows that of Ref. Bertone:2002ms with $m_{\chi}=100$ GeV. The short-dashed line corresponds to the spectra for annihilation through the WW and ZZ channels. In particular, we show the fit from Ref. Bergstrom:1997fj. Lastly, the long-dashed shows the spectrum, summed over contributing channels, for annihilation of Kaluza-Klein DM from Ref. Bergstrom:2004cy.
• Figure 4: IMBHs integrated luminosity function, i.e. number of black holes producing a gamma-ray flux larger than a given flux, as a function of the flux, for our scenario B (i.e. for IMBHs with mass $\sim 10^5 {\rm\,M_\odot}$). The upper (lower) line corresponds to $m_\chi=100$ GeV, $\sigma v=3\times 10 ^{-26}$ cm$^3$ s$^{-1}$ ($m_\chi=1$ TeV, $\sigma v= 10 ^{-29}$ cm$^3$ s$^{-1}$). For each curve we also show the 1-$\sigma$ scatter among different realizations of Milky Way-sized host DM halos. The figure can be interpreted as the number of IMBHs that can be detected from experiments with point source sensitivity $\Phi$ (above 1 GeV), as a function of $\Phi$. We show for comparison the 5$\sigma$ point source sensitivity above $1$ GeV of EGRET and GLAST (1 year).
• Figure 5: IMBHs integrated luminosity function in scenario A (i.e. for IMBHs with mass $\sim 10^2 {\rm\,M_\odot}$). The upper (lower) line corresponds to $m_\chi=100$ GeV, $\sigma v=3\times 10 ^{-26}$ cm$^3$ s$^{-1}$ ( $m_\chi=1$ TeV, $\sigma v= 10 ^{-29}$ cm$^3$ s$^{-1}$). For each curve we also show the 1-$\sigma$ scatter among realizations of Milky Way-sized halos. For the sake of comparison, we also show the point source sensitivity above $1$ GeV for EGRET and GLAST.