Luminosity functions consistent with a pulsar-dominated Galactic Center Excess

Abstract

A new population of millisecond pulsars is a long-standing proposed explanation for the excess of GeV-scale gamma rays emanating from the region surrounding the center of the Milky Way (the "Galactic Center excess"). We examine several simple parameterizations of possible luminosity functions for this population, as well as several benchmark luminosity functions proposed in the literature, and compare the predicted populations of resolved point sources to the Fermi 4FGL-DR2 point source catalog and a sub-population recently identified using wavelet-based methods. We provide general results that can be used to translate upper limits on the number of resolved point sources associated with the excess, and the fraction of the flux in the excess that can be attributed to resolved sources, into limits on the luminosity function parameter space. We discuss a number of important systematic uncertainties, including in the detection threshold model and the total flux attributed to the excess. We delineate regions of parameter space (containing existing benchmark models) where there is no apparent tension with current data, and the number of total pulsars needed to explain the excess is in the range of O(10 4−5 ). Forecasting the effects of lowered point source detection thresholds, we show that novel analysis methods that probe sub-threshold point source populations can hope to resolve more than 30% of the flux of the excess.

Applying simulation-based inference to spectral and spatial information from the Galactic Center gamma-ray excess

“…For this model, ⟨L⟩ ∼ 0.66L b , and the number of pulsars (resolved or unresolved) producing the GCE is A core = 637. The flux from GCE pulsars is then F ∼ 1.8 × 10 −9 erg cm −2 s −1 , which is in good agreement with the literature [24]. In this case, a relatively small number of pulsars each produce a relatively large number of gamma rays seen by Fermi, resulting in pronounced non-Poisson fluctuations in the photon count distribution.…”

“…As such, one expects the photon count distribution to be more Poisson-like. The flux from core pulsars is F ∼ 4 × 10 −10 erg cm −2 s −1 , which is near the lower end of the range of fluxes which could match the GCE [24].…”

“…In addition to specifying the spatial distributions of the pulsars, we must also specify their luminosity distributions. We take the luminosity function to be [24]:…”

“…The CPD (without energy information) has been previously used to analyze the GCE, but the results remain inconclusive, as mismodelling of the background can significantly bias the analysis [20][21][22][23]. Moroever, depending on the details of the MSP luminosity function, the MSP CPD can be very close to Poisson, and thus hard to distinguish from that of the DM [24].…”

Millisecond Pulsars in Dense Star Clusters: Evolution, Scaling Relations, and the Galactic-Center Gamma-ray Excess

“…However, it is important to point out that this is an optimistic upper limit since the 70% used is for a cluster like 47 Tucanae, which is one of the most massive and densest globular clusters in the Milky Way, and is likely to have more dynamical interactions than typical globular clusters (Ye et al 2022). Note that these numbers are consistent with the numbers predicted by gamma-ray luminosity functions (Dinsmore & Slatyer 2021).…”