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

Abstract

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 broadband 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 influences 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) central hard X-ray emission (CHXE) by IPs requires an X-ray (2–8 keV) luminosity function (XLF) extending down to at least 5 × 1031 erg s−1. The CHXE XLF, if extended to the surrounding ~50 pc observed by Chandra and XMM-Newton, requires that at least ~20%–40% of the ~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 non-magnetic CVs.