Comparing the Galactic Bulge and Galactic Disk Millisecond Pulsars. (arXiv:2008.10821v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Ploeg_H/0/1/0/all/0/1">Harrison Ploeg</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gordon_C/0/1/0/all/0/1">Chris Gordon</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Crocker_R/0/1/0/all/0/1">Roland Crocker</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Macias_O/0/1/0/all/0/1">Oscar Macias</a>
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.
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.
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