Speaker
Description
Totani recently reported a spherically symmetric, halo-like component of the Galactic diffuse emission peaking near $20\,$GeV in fifteen years of $\textit{Fermi}$--LAT data, extracted at high latitude, where the diffuse backgrounds are cleanest, and compatible at the template level with dark matter annihilation in a smooth NFW halo.
We test that result with an independent pipeline. Reproducing the cellwise analysis and extending it to a global pixel-level likelihood on the native $0.125^{\circ}$ maps, including energy-dependent PSF forward folding, bright-source masking, and an exact per-pixel Poisson treatment across 17 years of Pass~8 data, we recover the same $20\,$GeV halo, about $20\%$ higher in normalisation and unchanged in shape. It is centrally concentrated rather than isotropic, and spectrally and morphologically distinct both from the Galactic Centre excess and from any millisecond-pulsar population. A full systematic budget shows the measurement is limited by the diffuse foreground model, not by statistics.
Read as dark matter, the excess implies $m_{\chi} \approx. 0.55$--$0.72\,$TeV with a dwarf-spheroidal tension $\mathcal{R} \approx. 5.1$, widened to $1.6$--$9.3$ once foreground and
$J$-factor uncertainties are included. Mass freedom does not close it; $p$- and $d$-wave annihilation are excluded by the relic abundance, and a decay interpretation by the isotropic gamma-ray background. Only low-velocity-enhanced annihilation reconciles the present-day halo rate, the dwarf limits and freeze-out simultaneously. This supplies the required $\approx. 45\times$ boost through the relic abundance rather than by weakening the dwarf constraint, and demands a sub-GeV mediator.
We close on the crossed channel of that annihilation: elastic photon--dark-matter scattering along the same line of sight, linear rather than quadratic in the dark matter density. The real-photon operator basis collapses to dipole and Rayleigh structures, anapole and charge
radius vanish identically on shell, which sorts the halo's candidate mediators before any data are used. We present the first operator-resolved photon--dark-matter scattering limits from astrophysical gamma-ray data.
