NuSTAR HARD 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 NuSTAR. NuSTAR resolves the hard X-ray source IGRJ17456-2901 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 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. Broadband X-ray spectral analysis with a Monte-Carlo based X-ray reflection model self-consistently determined their intrinsic column density (∼1023 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 LX1038 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 MWD∼0.9 Mo. Spectral energy distribution analysis suggests that G359.95-0.04 is likely the hard X-ray counterpart of the ultra-high gamma-ray source HESSJ1745-290, strongly favoring a leptonic origin of the GC TeV emission.