Constraints on dark matter annihilation from a nearby subhalo candidate

TL;DR

The work addresses indirect dark matter detection from a dynamically inferred nearby subhalo candidate using gamma-ray observations. By modeling the subhalo with an NFW profile at distance $d \approx 0.78$ kpc and virial mass $M_{\rm sub} \sim 6\times 10^7\,M_\odot$, the study leverages an exceptionally large predicted $J$-factor of order $J \sim 10^{23}\ {\rm GeV^2\,cm^{-5}}$. A three-template approach accounts for distance-related morphology changes, and a log-normal prior on $J$-factor normalization is used in a joint likelihood to set 95% CL upper limits on the DM annihilation cross section for the $b\bar{b}$ and $\tau^+\tau^-$ channels. These limits, potentially orders of magnitude stronger than those from dwarfs or blind searches if the subhalo is dynamically confirmed, illustrate the potential of gravitationally selected subhalos as powerful targets for indirect DM searches.

Abstract

A recent analysis of pulsar timing data has reported evidence for a massive ($\sim 6 \times 10^7 M_{\odot}$) dark matter subhalo located only $\sim 0.8$ kpc from Earth. This candidate implies an exceptionally large $J$-factor of $\sim 10^{23}\,{\rm GeV^2\,cm^{-5}}$, exceeding that of known classical dwarf spheroidal galaxies by orders of magnitude and rivaling the Galactic Center. In this work, we utilize more than 17 years of $\textit{Fermi}$-LAT data to search for gamma-ray emission from this subhalo. We identify a tentative excess in the region with ambiguous origin. Adopting a conservative strategy, we retain this excess without modeling additional astrophysical components, treating it instead as unmodeled background to derive upper limits on the dark matter annihilation cross-section for the $b\bar{b}$ and $τ^+τ^-$ channels. Despite this conservative treatment, the resulting limits remain stringent due to the exceptionally large $J$-factor. Subject to the dynamical confirmation of the subhalo, these constraints are potentially orders of magnitude stronger than those obtained from combined analyses of dwarf spheroidal galaxies and blind subhalo searches.

Figures (2)

• Figure 1: Residual TS map of the $15^\circ \times 15^\circ$ region centered on the nearby subhalo (marked with a green cross), the red dashed line represents the scale radius of $r_s=0.1\,{\rm kpc}$ for the best-fit distance.
• Figure 2: Constraints on the dark matter annihilation cross section $\langle \sigma v\rangle$ at 95% CL for the $b\bar{b}$ (left) and $\tau^+\tau^-$ (right) channels. The solid black curve represents the fiducial limit derived in this work, while the green shaded band indicates the uncertainty arising from the $1\sigma$ variation in the subhalo distance. For comparison, we show limits from stacked dwarf spheroidal galaxies McDaniel:2023bju (red dashed line), subhalo candidates among Fermi unidentified sources Coronado-Blazquez:2019pny (cyan dashed line), and Ursa Major III Crnogorcevic:2023ijs (grey dashed line). The canonical thermal relic cross section Steigman:2012nb is indicated by the purple dashed line.