The Galactic Center GeV excess from a series of leptonic cosmic-ray outbursts

Abstract

Abstract. It has been proposed that a recent outburst of cosmic-ray electrons could account for the excess of GeV-scale gamma rays observed from the region surrounding the Galactic Center. After studying this possibility in some detail, we identify scenarios in which a series of leptonic cosmic-ray outbursts could plausibly generate the observed excess. The morphology of the emission observed outside of ∼ 1 • − 2 • from the Galactic Center can be accommodated with two outbursts, one which took place approximately ∼ 10 6 years ago, and another (injecting only about 10% as much energy as the first) about ∼ 10 5 years ago. The emission observed from the innermost ∼ 1 • − 2 • requires one or more additional recent outbursts and/or a contribution from a centrally concentrated population of unresolved millisecond pulsars. In order to produce a spectrum that is compatible with the measured excess (whose shape is approximately uniform over the region of the excess), the electrons from the older outburst must be injected with significantly greater average energy than those injected more recently, enabling their spectra to be similar after ∼ 10 6 years of energy losses.

How to calculate dark matter direct detection exclusion limits that are consistent with gamma rays from annihilation in the Milky Way halo

“…In particular, the excess is roughly spherically symmetric and the morphology is consistent with a DM density profile proportional to r −γ , with γ ≈ 1.1 − 1.3 [36,37]. However, the origin of the excess is not yet established and alternative astrophysical explanations including cosmic-ray outbursts [43][44][45], a population of unresolved millisecond-pulsar-like sources [33,35,[46][47][48][49][50], or additional cosmic-ray sources [51,52] have been considered. If interpreted in terms of DM annihilations, the morphology of this excess can determine, up to a certain degree of accuracy, the inner slope of the DM profile, thereby adding a valuable piece of information to the modeling of the gravitational potential of the MW.…”

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

“…However, its spectral properties are strongly dependent on the assumed IEM, making it challenging to conclusively identify its origin. As a result, it remains unclear whether this signal arises from DM annihilation rather than from a currently unknown contribution from astrophysics such as a large population of milli-second pulsars, cosmic-ray (CR) proton or electron outbursts, additional cosmic ray sources, and/or emission from a stellar over-density in the Galactic bulge [11,16,[18][19][20][21][22][23]. An interesting development is the use of statistical tools which indicate that GeV photons from the direction of the inner galaxy region show significantly more clustering than would be expected from Poisson noise from smooth components [24][25][26][27].…”

What the Milky Way’s dwarfs tell us about the Galactic Center extended gamma-ray excess

“…Further, low mass x-ray binaries (likely progenitors of MSPs) in M31 have been observed to follow a power law radial spatial distribution, similar to the expectations of an NFW halo [5,28]. Other astrophysical explanations might include more dynamic events such as cosmic-ray injection into the Galactic Center (GC) [33][34][35][36].…”