Background model systematics for the Fermi GeV excess (1409.0042v1)

Abstract: The possible gamma-ray excess in the inner Galaxy and the Galactic center (GC) suggested by Fermi-LAT observations has triggered a large number of studies. It has been interpreted as a variety of different phenomena such as a signal from WIMP dark matter annihilation, gamma-ray emission from a population of millisecond pulsars, or emission from cosmic rays injected in a sequence of burst-like events or continuously at the GC. We present the first comprehensive study of model systematics coming from the Galactic diffuse emission in the inner part of our Galaxy and their impact on the inferred properties of the excess emission at Galactic latitudes $2^\circ<|b|<20^\circ$ and 300 MeV to 500 GeV. We study both theoretical and empirical model systematics, which we deduce from a large range of Galactic diffuse emission models and a principal component analysis of residuals in numerous test regions along the Galactic plane. We show that the hypothesis of an extended spherical excess emission with a uniform energy spectrum is compatible with the Fermi-LAT data in our region of interest at $95\%$ CL. Assuming that this excess is the extended counterpart of the one seen in the inner few degrees of the Galaxy, we derive a lower limit of $10.0^\circ$ ($95\%$ CL) on its extension away from the GC. We show that, in light of the large correlated uncertainties that affect the subtraction of the Galactic diffuse emission in the relevant regions, the energy spectrum of the excess is equally compatible with both a simple broken power-law of break energy $2.1\pm0.2$ GeV, and with spectra predicted by the self-annihilation of dark matter, implying in the case of $\bar{b}b$ final states a dark matter mass of $49^{+6.4}_{-5.4}$ GeV.

• The paper establishes a refined PCA-based methodology that minimizes background model systematics, isolating the intrinsic 1-3 GeV gamma-ray excess.
• It shows that the excess spectrum remains robust across various diffuse emission models, consistently peaking between 1-3 GeV.
• The study explores the spatial extent using extended NFW profiles with a radial index near 2.2, supporting both dark matter and astrophysical origin scenarios.

Analysis of the Fermi GeV Excess: Systematics and Implications

The paper "Background model systematics for the Fermi GeV excess" provides a comprehensive paper of the gamma-ray excess observed in the inner regions of the Milky Way, as detected by the Fermi-LAT telescope. This excess has prompted investigation into its potential origins, ranging from dark matter annihilation events to emissions from ordinary astrophysical objects such as millisecond pulsars. The authors present a meticulous examination of the background model systematics and their impact on the properties of the excess emissions, proposing a refined methodology to enhance our understanding of this intriguing phenomenon.

The paper emphasizes the necessity of a precise background model to interpret the Fermi GeV excess accurately. The excess is characterized by gamma-ray emissions extending beyond the Galactic center, with spectra peaking at 1-3 GeV. The paper distinguishes itself by addressing both theoretical and empirical systematics in the background model. By employing a robust set of Galactic diffuse emission models, the paper reveals that while the excess spectrum is generally stable across various theoretical model assumptions, empirical model systematics necessitate a detailed account to ensure reliable results.

Key insights include:

• The extraction of an excess spectrum, which consistently maintains a peak at 1-3 GeV irrespective of the diffuse model employed, suggesting the intrinsic nature of the excess.
• A novel approach using principal component analysis (PCA) to unravel the covariance matrix of empirical uncertainties along the Galactic plane, thus achieving a reduction in model-related systematics.
• The exploration of a potential radial dependence and spatial extent of the excess, fitting the data to extended NFW profiles exhibiting radial indices around 2.2 with a spatial cutoff, implying a significant Galactic radius of impact.

As for potential explanations, the paper indicates the compatibility of the excess’s spectral and spatial attributes with weakly interacting massive particle (WIMP) dark matter scenarios, notably annihilation into quark-antiquark pairs ($\bar{b}b$). Alternatively, a broken power-law spectrum provides a good fit, suggestive of yet unidentified astrophysical processes, including contributions from millisecond pulsars or recent CR injection events. The paper, however, does not find strong evidence favoring the leptonic annihilation channels.

The reported work has important implications:

• Practically, it refines the method for modeling background astrophysical emissions, improving the precision of indirect dark matter search efforts using gamma-ray observatories.
• Theoretically, it enhances understanding of systematics involved in gamma-ray excess analyses within dense astrophysical environments, aiding better discrimination between dark matter and conventional astrophysical phenomena.

Future explorations can extend this framework to larger datasets and incorporate complementary data from other cosmic messengers such as neutrinos and cosmic rays. Enhanced modeling of Galactic astrophysical processes, considering more dynamic CR models and improved emission spectra of potential point sources, would further elucidate the origins and properties of the GeV excess.

In conclusion, this paper offers a critical advancement in deciphering the nature of the GeV excess observed by Fermi-LAT, providing robust analytical tools and insights that will guide ongoing and future investigations in gamma-ray astrophysics and dark matter studies.