Gamma-ray Lines in the Fermi Data: is it a Bubble?

TL;DR

The paper questions the gamma-ray line excess at 130 GeV observed in Fermi LAT data, suggesting it may be due to astrophysical processes in the Fermi bubbles rather than dark matter annihilation.

Contribution

It demonstrates that a broken power-law spectrum can mimic the gamma-ray line feature, challenging the dark matter interpretation of the excess.

Findings

Broken power-law fits the data as well as a line feature.

The excess overlaps with the Fermi bubbles region.

Astrophysical processes likely explain the spectral break.

Abstract

Recently, tentative evidence for an excess of gamma rays at energies around 130 GeV has been reported from analyses of data from the Fermi Large Area Telescope (LAT). The excess is potentially of great interest, as it could be associated with the pair-annihilation of Galactic dark matter and the subsequent production of monochromatic or internal bremsstrahlung gamma rays. The 130 GeV excess appears when an optimized selection of the target region of interest is employed, a procedure that depends upon the assumed dark matter density profile. For the profiles producing an appreciable signal, these target regions vastly overlap with the region corresponding to the so-called "Fermi bubbles". We argue that the tentative evidence for a line feature is likely due to hard photons in the Fermi bubbles regions, where the gamma-ray spectrum contains a spectral break in the energy range of interest (100-150 GeV). Although the origin of this broken power-law is unclear, it is probably related to standard astrophysical processes and not to dark matter annihilation. A broken power-law provides as good a fit as a line "excess", even within small energy windows, and a significantly better fit for large energy windows.