NuSTAR Hard X-Ray Survey of the Galactic Center Region I: Hard X-Ray Morphology and Spectroscopy of the Diffuse Emission
Hailey, Charles J.; Mori, Kaya; Giommi, Paolo; Zhang, Shuo; Bauer, Franz; Stern, Daniel; Kitaguchi, Takao; Boggs, Steven E.; Harrison, Fiona A.; Christensen, Finn E.; Craig, William W.; Forster, Karl; Grefenstette, Brian W.; Zhang, William W.; Tomsick, John A.; Krivonos, Roman; Barrière, Nicolas; Hong, Jaesub; Baganoff, Frederick K.; Nynka, Melania; Ponti, Gabriele; Alexander, David M.; Barret, Didier; Grindlay, Jonathan E.; Zoglauer, Andreas; Perri, Matteo; Perez, Kerstin; Puccetti, Simonetta; Canipe, Alicia M.; Rana, Vikram; Miyasaka, Hiromasa; Pivovaroff, Michael J.; Hornstrup, Allan; Koglin, Jason E.; Madsen, Kristen K.; Mao, Peter H.; Westergaard, Niels J.; Luu, Vy
United States, Russia, Germany, Chile, United Kingdom, France, Denmark, Italy, Japan
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 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 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 LX ≳ 1038 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 M⊙. 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.