Evidence for intermediate polars as the origin of the Galactic Center hard X-ray emission

Kavli Affiliate: Melania Nynka

| First 5 Authors: Charles J. Hailey, Kaya Mori, Kerstin Perez, Alicia M. Canipe, Jaesub Hong

| Summary:

Recently, unresolved hard (20-40 keV) X-ray emission has been discovered
within the central 10 pc of the Galaxy, possibly indicating a large population
of intermediate polars (IPs). Chandra and XMM-Newton measurements in the
surrounding ~50 pc imply a much lighter population of IPs with $langle M_{rm
WD} rangle approx 0.5 M_odot$. Here we use broad-band NuSTAR observations of
two IPs: TV Columbae, which has a fairly typical but widely varying reported
mass of $M_{rm WD} approx 0.5-1.0 M_odot$, and IGR J17303-0601, with a heavy
reported mass of $M_{rm WD} approx 1.0-1.2 M_odot$. We investigate how
varying spectral models and observed energy ranges influence estimated white
dwarf mass. Observations of the inner 10 pc can be accounted for by IPs with
$langle M_{rm WD} rangle approx 0.9 M_odot$, consistent with that of the
CV population in general, and the X-ray observed field IPs in particular. The
lower mass derived by Chandra and XMM-Newton appears to be an artifact of
narrow energy band fitting. To explain the (unresolved) CHXE by IPs requires an
X-ray (2-8 keV) luminosity function (XLF) extending down to at least
$5times10^{31}$ erg/s. The CHXE XLF, if extended to the surrounding ~50 pc
observed by Chandra and XMM-Newton, requires at least ~20-40% of the $sim$9000
point sources are IPs. If the XLF extends just a factor of a few lower in
luminosity, then the vast majority of these sources are IPs. This is in
contrast to recent observations of the Galactic ridge, where the bulk of the
2-8 keV emission is ascribed to dwarf novae.

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