Evidence for Intermediate Polars as the Origin of the Galactic Center Hard X-ray Emission
Hailey, Charles J.; Mori, Kaya; Zhang, Shuo; Stern, Daniel; Rahoui, Farid; Boggs, Steven E.; Harrison, Fiona A.; Christensen, Finn E.; Craig, William W.; Zhang, William W.; Tomsick, John A.; Hong, Jaesub; Nynka, Melania; Grindlay, Jonathan E.; Perez, Kerstin; Fornasini, Francesca; Canipe, Alicia M.
United States, Denmark, Germany
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 < {M}{{WD}}> ≈ 0.5{M}⊙ . Here we use broadband NuSTAR observations of two IPs: TV Columbae, which has a fairly typical but widely varying reported mass of {M}{{WD}}≈ 0.5-1.0{M}⊙ , and IGR J17303-0601, with a heavy reported mass of {M}{{WD}}≈ 1.0-1.2{M}⊙ . 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 < {M}{{WD}}> ≈ 0.9{M}⊙ , 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.