Comment on “Characterizing the population of pulsars in the Galactic bulge with the Fermi large area telescope” [arXiv:1705.00009v1]

Abstract

The Fermi-LAT Collaboration recently presented a new catalog of gamma-ray sources located within the 40 • × 40 • region around the Galactic Center (Ajello et al. 2017) -the Second Fermi Inner Galaxy (2FIG) catalog. Utilizing this catalog, they analyzed models for the spatial distribution and luminosity function of sources with a pulsar-like gamma-ray spectrum. Ajello et al. (2017) v1 also claimed to detect, in addition to a disklike population of pulsar-like sources, an approximately 7σ preference for an additional centrally concentrated population of pulsar-like sources, which they referred to as a "Galactic Bulge" population. Such a population would be of great interest, as it would support a pulsar interpretation of the gamma-ray excess that has long been observed in this region. In an effort to further explore the implications of this new source catalog, we attempted to reproduce the results presented by the Fermi-LAT Collaboration, but failed to do so. Mimicking as closely as possible the analysis techniques undertaken in Ajello et al. (2017), we instead find that our likelihood analysis favors a very different spatial distribution and luminosity function for these sources. Most notably, our results do not exhibit a strong preference for a "Galactic Bulge" population of pulsars. Furthermore, we find that masking the regions immediately surrounding each of the 2FIG pulsar candidates does not significantly impact the spectrum or intensity of the Galactic Center gamma-ray excess. Although these results refute the claim of strong evidence for a centrally concentrated pulsar population presented in Ajello et al. (2017), they neither rule out nor provide support for the possibility that the Galactic Center excess is generated by a population of low-luminosity and currently largely unobserved pulsars. In a spirit of maximal openness and transparency, we have made our analysis code available at https://github.com/bsafdi/GCE-2FIG.

Bayesian model comparison and analysis of the Galactic disc population of gamma-ray millisecond pulsars

“…Here and in Ploeg et al (2017) both K th and σ th are left free in the fit. However, Hooper 3 Also see Bartels et al (2018). & Mohlabeng (2016) fix σ th = 0.9, which is larger than our best-fit value.…”

Analyzing γ rays of the Galactic Center with deep learning

“…More recent lists of point sources, such as the 2FIG, already contain twice as many objects in the same region. Also, to extend the range of the α parameter from ±1.05 to at least ±1.7 to account for best-fit values found by other studies [32,55].…”

Foreground mismodeling and the point source explanation of the Fermi Galactic Center excess

“…Doing this exercise in a principled way would require modeling the dependence of the 4FGL detection sensitivity on the spatial location of the sources and the source spectral properties, as was performed in e.g., Refs. [68,69], which is beyond the scope of this paper. Still, as a rough estimate, we may do the following.…”