NuSTAR Hard X-ray Survey of the Galactic Center Region. I. Hard X-ray Morphology and Spectroscopy of the Diffuse Emission

We present the first sub-arcminute images of the Galactic Center above 10 keV, obtained with NuSTAR. 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 power-law spectra with Gamma similar to 1.3-2.3 up to similar 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 K alpha fluorescence line emission associated with two Sgr A molecular clouds: MC1 and the Bridge.

Broadband X-ray spectral analysis with a Monte-Carlo based X-ray reflection model self-consistently determined their intrinsic column density (similar to 10(23) cm(-2)), primary X-ray spectra (power-laws with Gamma similar to 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 similar to 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-WD similar to 0.9M(circle dot). 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.