Investigating the uniformity of the excess gamma rays towards the galactic center region

Shunsaku Horiuchi,Manoj Kaplinghat,Anna Kwa

doi:10.1088/1475-7516/2016/11/053

Shunsaku Horiuchi, Manoj Kaplinghat + Show 1 more

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https://doi.org/10.1088/1475-7516/2016/11/053

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Abstract

We perform a composite likelihood analysis of subdivided regions within the central 26° × 20° of the Milky Way, with the aim of characterizing the spectrum of the gamma-ray galactic center excess in regions of varying galactocentric distance. Outside of the innermost few degrees, we find that the radial profile of the excess is background-model dependent and poorly constrained. The spectrum of the excess emission is observed to extend upwards of 10 GeV outside ∼5° in radius, but cuts off steeply between 10–20 GeV only in the innermost few degrees. If interpreted as a real feature of the excess, this radial variation in the spectrum has important implications for both astrophysical and dark matter interpretations of the galactic center excess. Single-component dark matter annihilation models face challenges in reproducing this variation; on the other hand, a population of unresolved millisecond pulsars contributing both prompt and secondary inverse Compton emission may be able to explain the spectrum as well as its spatial dependency. We show that the expected differences in the photon-count distributions of a smooth dark matter annihilation signal and an unresolved point source population are an order of magnitude smaller than the fluctuations in residuals after fitting the data, which implies that mismodeling is an important systematic effect in point source analyses aimed at resolving the gamma-ray excess.

Highlights

IntroductionAny spectral or spatial variation (or lack thereof) in the signal would be of critical importance in discerning amongst the possible origins of the GCE
Any spectral or spatial variation in the signal would be of critical importance in discerning amongst the possible origins of the GCE
And 7 we explore the spatial dependence of the GCE spectrum and confirm previous findings by Refs. [13, 14] of GCE-associated emission upwards of ∼ 20 GeV in the inner galaxy region. We find that this high-energy tail is not present in the GCE spectrum within the galactic center regions of interest (ROIs); within approximately 5◦ it has a spectral cutoff between ∼5–10 GeV, while outside of this region the high-energy tail becomes a prominent feature for r 7◦ − 8◦

Summary

Introduction

Any spectral or spatial variation (or lack thereof) in the signal would be of critical importance in discerning amongst the possible origins of the GCE. Analyses of the excess in the inner galaxy report a power law-like high-energy tail in the GCE spectrum beyond 10 GeV, possibly extending upwards of ∼ 20 − 100 GeV [13, 14, 31]. Recent results support an unresolved point source origin for the GCE and indicate that such sources may be able to account for the entirety of the excess in the inner galaxy [34, 35]. If so, this would strongly imply that most, if not all, of the GCE signal is produced by millisecond pulsars, not dark matter annihilation.

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