NuSTARHARD X-RAY SURVEY OF THE GALACTIC CENTER REGION. I. HARD X-RAY MORPHOLOGY AND SPECTROSCOPY OF THE DIFFUSE EMISSION

Abstract

We present the first sub-arcminute images of the Galactic Center above 10 keV, obtained with NuS-TAR. NuSTAR resolves the hard X-ray source IGR J17456−2901 into non-thermal X-ray filaments, molecular clouds, point sources and a previously unknown central component of hard X-ray emission (CHXE). NuSTAR detects four non-thermal X-ray filaments, extending the detection of their powerlaw spectra with Γ ∼ 1.3-2.3 up to ∼ 50 keV. A morphological and spectral study of the filaments suggests that their origin may be heterogeneous, where previous studies suggested a common origin in young pulsar wind nebulae (PWNe). NuSTAR detects non-thermal X-ray continuum emission spatially correlated with the 6.4 keV Fe Kα fluorescence line emission associated with two Sgr A molecular clouds: MC1 and the Bridge. Broad-band X-ray spectral analysis with a Monte-Carlo based X-ray reflection model self-consistently determined their intrinsic column density (∼ 10 23 cm −2 ), primary X-ray spectra (power-laws with Γ ∼ 2) and set a lower limit of the X-ray luminosity of Sgr A* flare illuminating the Sgr A clouds to L X 10 38 erg s −1 . Above ∼ 20 keV, hard X-ray emission in the central 10 pc region around Sgr A* consists of the candidate PWN G359.95−0.04 and the CHXE, possibly resulting from an unresolved population of massive CVs with white dwarf masses M W D ∼ 0.9M . Spectral energy distribution analysis suggests that G359.95−0.04 is likely the hard X-ray counterpart of the ultra-high gamma-ray source HESS J1745−290, strongly favoring a leptonic origin of the GC TeV emission.

NuSTAR HARD X-RAY SURVEY OF THE GALACTIC CENTER REGION. II. X-RAY POINT SOURCES

“…Mori et al (2015) explored these in the 40-79 keV trial maps generated from three observations of the SgrA * field. One of the objects coincides with the head of G359.95-0.04, a PWN, and the other, detected at ∼4σ and a bit elongated in shape, does not seem to have a clear counterpart in the Chandra and XMMNewton images.…”

“…A bright GR source generates a set of radial streaks of high background in the detectors. These background components are dominant at low energies, below 10-40 keV, while the internal background becomes dominant above 40 keV; see Wik et al (2014) and Mori et al (2015) for more details.…”

“…Mori et al (2015) present the results from the NuSTAR observations of the diffuse hard X-ray emission in the central 0.2 deg 2 region around SgrA * . In this paper, we report the first survey results and catalog of hard X-ray point sources in a 0.6 deg 2 region around SgrA * and a 0.06 deg 2 region around the SgrB2 cloud.…”

The NuSTAR Serendipitous Survey: The 40-month Catalog and the Properties of the Distant High-energy X-Ray Source Population

“…We exclude from the survey NuSTAR fields with bright science targets, identified as fields with > 10 6 counts within 120 of the on-axis position. We also exclude the dedicated extragalactic (COSMOS, ECDFS, EGS, GOODS-N, UDS) and Galactic survey fields (the Galactic center survey; Mori et al 2015;Hong et al 2016; and the Norma Arm survey; Fornasini et al, in prep. ).…”

“…This can largely be explained as a result of insufficient-depth soft X-ray coverage, or zero coverage. However, for the sources with sufficient-depth soft X-ray coverage this may indicate either a spurious NuSTAR detection, a transient detection, or the detection of an unidentified contaminating feature such as stray light (e.g., Mori et al 2015). We estimate that there are 34 (out of 102) such sources, that lack a soft X-ray counterpart but have sufficiently deep soft X-ray data (from Chandra or XMM-Newton) that we would expect a detection (given the NuSTAR source flux in the overlapping 3-8 keV band).…”