Comparing the galactic bulge and galactic disk millisecond pulsars

Abstract

The Galactic Center Excess (GCE) is an extended gamma-ray source in the central region of the Galaxy found in Fermi Large Area Telescope (Fermi-LAT) data. One of the leading explanations for the GCE is an unresolved population of millisecond pulsars (MSPs) in the Galactic bulge. Due to differing star formation histories it is expected that the MSPs in the Galactic bulge are older and therefore dimmer than those in the Galactic disk. Additionally, correlations between the spectral parameters of the MSPs and the spin-down rate of the corresponding neutron stars have been observed. This implies that the bulge MSPs may be spectrally different from the disk MSPs. We perform detailed modelling of the MSPs from formation until observation. Although we confirm the correlations, we do not find they are sufficiently large to significantly differentiate the spectra of the bulge MSPs and disk MSPs when the uncertainties are accounted for. Our results demonstrate that the population of MSPs that can explain the gamma-ray signal from the resolved MSPs in the Galactic disk and the unresolved MSPs in the boxy bulge and nuclear bulge can consistently be described as arising from a common evolutionary trajectory for some subset of astrophysical sources common to all these different environments. We do not require that there is anything unusual about inner Galaxy MSPs to explain the GCE. Additionally, we use a more accurate geometry for the distribution of bulge MSPs and incorporate dispersion measure estimates of the MSPs' distances. We find that the elongated boxy bulge morphology means that some the bulge MSPs are closer to us and so easier to resolve. We identify three resolved MSPs that have significant probabilities of belonging to the bulge population.

Luminosity functions consistent with a pulsar-dominated Galactic Center Excess

“…Log normal fits to two numerical luminosity functions: the boxy bulge luminosity function from Ref. [39] and the luminosity function from [42]. Uncertainties are assumed to be uncorrelated and represent one standard deviation from the mean.…”

“…In the first row we hold L min fixed at L min = 10 29 erg s −1 , and vary L max and α. In the second row we instead hold α fixed at the value corresponding [21], GLC [34], GCE [39], AIC [42], NPTF [23], and Disk [41].…”

“…In both cases, the numerical luminosity functions were extracted from the papers where they were presented: Refs. [39] and [42] respectively. The functions are not normalized, which is why the vertical axes are marked with an arbitrary additional term +C.…”

“…[42] argues that the Galactic Bulge could plausibly host O(10 5 ) MSPs formed by accretion-induced collapse (AIC), which would bypass the limit from non-observation of LMXBs, and also predicts a total flux broadly consistent with the GCE, using a luminosity function model based on the results of Ref. [39].…”

Evidence for inverse Compton emission from globular clusters

“…The emission from a putative population of about 40, 000 (Ploeg et al 2020) unresolved MSPs in the Galactic Center region is currently the preferred explanation for the Fermi GeV excess (Macias et al 2018;Bartels et al 2018;Macias et al 2019;Abazajian et al 2020). Since GCs also contain large numbers of unresolved MSPs, it is useful to compare the light-to-mass ratios for these two systems so as to obtain additional clues for the physical processes causing the observed high-energy 𝛾-ray emissions in their directions.…”