Bremsstrahlung gamma rays from light dark matter

Abstract

We discuss the often-neglected role of bremsstrahlung processes on the interstellar gas in computing indirect signatures of Dark Matter (DM) annihilation in the Galaxy, particularly for light DM candidates in the phenomenologically interesting O(10) GeV mass range. Especially from directions close to the Galactic Plane, the γ-ray spectrum is altered via two effects: directly, by the photons emitted in the bremsstrahlung process by energetic electrons which are among the DM annihilation byproducts; indirectly, by the modification of the same electron spectrum, due to the additional energy loss process in the diffusion-loss equation (e.g. the resulting inverse Compton emission is altered). We quantify the importance of the bremsstrahlung emission in the GeV energy range, showing that it is sometimes the dominant component of the γ-ray spectrum. We also find that, in regions in which bremsstrahlung dominates energy losses, the related γ-ray emission is only moderately sensitive to possible large variations in the gas density. Still, we stress that, for computing precise spectra in the (sub-)GeV range, it is important to obtain a reliable description of the Galaxy gas distribution as well as to compute self-consistently the γ-ray emission and the solution to the diffusion-loss equation. For example, these are crucial issues to quantify and interpret meaningfully γ-ray map 'residuals' in the inner Galaxy.

Galactic Center gamma-ray ``excess'' from an active past of the Galactic Centre?

“…Lowering n ISRF would enhance the electron flux, but deplete the emissivity (and vice versa), with the two effects mostly compensating. This was explicitly seen with the analogous case for bremsstrahlung in [15]. Additionally, we shall illustrate the consequences of variation in the parameters on the different spectra.…”

“…We also calculated the bremsstrahlung gamma-ray emission from this population of electrons, following [15] and adopting gas densities as implemented in the GALPROP code (see Fig. 2 of [15]).…”

“…2 ISRF , our signal will be dominated by the electron population with energies ≥ 15 GeV (assuming ISRF photon energies of ISRF ∼ 1 eV) and therefore this regime should be valid for our purposes (see however [15] for some limitation to this picture, especially relevant at sub-GeV energies). We checked explicitly that this expectation is indeed verified, using the equations in [20] in their general form and accounting for bremsstrahlung energy losses in obtaining the electron spectra.…”

Dark matter interpretation of the Fermi -LAT observation toward the Galactic Center

“…4 For annihilations into leptons, secondary γ-ray emission via IC processes is neglected. Note that for DM masses 10 GeV, IC photons are mainly produced at energies < 1 GeV [60,61].…”

Gamma-ray signatures of Dark Matter

“…Our constraints can therefore be applied to generic DM candidates with a mass in the multi-TeV range which annihilate into gauge bosons. This section is based on the results in [4]. We stress the often-neglected role of bremsstrahlung processes on the interstellar gas in computing indirect signatures of DM annihilation in the Galaxy,…”