Black Hole Window into $p$-Wave Dark Matter Annihilation

We present a new method to measure or constrain $p$-wave-suppressed cross sections for dark matter (DM) annihilations inside the steep density spikes induced by supermassive black holes. We demonstrate that the high DM densities, together with the increased velocity dispersion, within such spikes combine to make thermal $p$-wave annihilation cross sections potentially visible in $\gamma$-ray observations of the Galactic center (GC). The resulting DM signal is a bright central point source with emission originating from DM annihilations in the absence of a detectable spatially extended signal from the halo. We define two simple reference theories of DM with a thermal $p$-wave annihilation cross section and establish new limits on the combined particle and astrophysical parameter space of these models, demonstrating that Fermi Large Area Telescope is currently sensitive to thermal $p$-wave DM over a wide range of possible scenarios for the DM distribution in the GC.

Figure 1

Spectra of three possible candidates to contain emission from a BH-induced DM density spike [39, 40]. In black are two example predictions of thermal HSHP DM with different particle masses (determining the shape) and spike and halo parameters (controlling the normalization). The solid line shows predictions for $m_{\chi }=45\mathrm{GeV}$, $m_s=10\mathrm{GeV}$, ${\gamma }_c=1.3$, and ${\gamma }_{\mathrm{sp}}=1.8$; the dashed line shows predictions for $m_{\chi }=110\mathrm{GeV}$, $m_s=50\mathrm{GeV}$, and an adiabatic spike in a halo with ${\gamma }_c=1.1$.

Figure 2

Sensitivity to HSHP DM from the Fermi telescope’s observations of the GC. Top: the minimum value of the DM annihilation cross section for which the primary photon emission exceeds observed point source spectra at 95% C.L. in at least one bin, as a function of DM mass, for adiabatic spikes with ${\gamma }_c=1.1$ and $z=0.2$. Bottom: the predicted flux in a box-shaped spectral feature from HSHP DM annihilating inside adiabatic spikes for ${\gamma }_c=1.3$ (blue, dashed), 1.2 (cyan, dotted), 1.1 (purple, dash dotted). Results are shown for $z=0.99$, where the box is sufficiently narrow to appear as a line, and compared to the Fermi-LAT limit [44] on line flux from the GC (black).

Figure 3

The region of combined $\text{halo}+\text{spike}$ parameter space where Fermi LAT observations constrain thermal $p$-wave annihilation cross sections for $m_{\chi }=110\mathrm{GeV}$. In cyan are continuum constraints for HSHP DM ($z=0.2$), from comparison with 2FGL J1745.6-2858, 3FGL J1745.6-2959c, and 3FGL J1745.3-2903.c in the dashed, dotted, and dot-dashed lines respectively; the blue solid line shows the line search limit of Ref. [44] on HSHP DM ($z=0.99$); the purple solid line shows the line search limit on HSAP DM ($z=0.99$). Dotted blue and purple lines show an estimate of the improved sensitivity to HSHP and HSAP DM, respectively, offered by a dedicated line search. Regions above the curves exceed observations. Adiabatic spikes are indicated by the dotted black line.