NuSTAR Hard X-ray Survey of the Galactic Center Region II: X-ray Point Sources

Kavli Affiliate: Melania Nynka

| First 5 Authors: JaeSub Hong, Kaya Mori, Charles J. Hailey, Melania Nynka, Shuo Zhang

| Summary:

We present the first survey results of hard X-ray point sources in the
Galactic Center (GC) region by NuSTAR. We have discovered 70 hard (3-79 keV)
X-ray point sources in a 0.6 deg^2 region around Sgr A* with a total exposure
of 1.7 Ms, and 7 sources in the Sgr B2 field with 300 ks. We identify clear
Chandra counterparts for 58 NuSTAR sources and assign candidate counterparts
for the remaining 19. The NuSTAR survey reaches X-ray luminosities of ~4 x and
~8 x 10^32 erg s^-1 at the GC (8 kpc) in the 3-10 and 10-40 keV bands,
respectively. The source list includes three persistent luminous X-ray binaries
and the likely run-away pulsar called the Cannonball. New source-detection
significance maps reveal a cluster of hard (>10 keV) X-ray sources near the Sgr
A diffuse complex with no clear soft X-ray counterparts. The severe extinction
observed in the Chandra spectra indicates that all the NuSTAR sources are in
the central bulge or are of extragalactic origin. Spectral analysis of
relatively bright NuSTAR sources suggests that magnetic cataclysmic variables
constitute a large fraction (>40-60%). Both spectral analysis and logN-logS
distributions of the NuSTAR sources indicate that the X-ray spectra of the
NuSTAR sources should have kT > 20 keV on average for a single temperature
thermal plasma model or an average photon index of Gamma = 1.5 – 2 for a
power-law model. These findings suggest that the GC X-ray source population may
contain a larger fraction of X-ray binaries with high plasma temperatures than
the field population.

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