Glimpse of the highly obscured HMXB IGR J16318-4848 with Hitomi

TL;DR

This study uses Hitomi's high-resolution X-ray spectroscopy to analyze the obscured high-mass X-ray binary IGR J16318-4848, revealing detailed properties of its reprocessing material and ionization state despite limited photon counts.

Contribution

First high-resolution spectroscopic analysis of IGR J16318-4848 with Hitomi, providing precise measurements of Fe line width and ionization state in a heavily obscured HMXB.

Findings

Fe Kα line width corresponds to 160 km/s velocity

Fe ionization state estimated as Fe I--IV

Confirmed strong absorption with no Compton shoulder

Abstract

We report a Hitomi observation of IGR J16318-4848, a high-mass X-ray binary system with an extremely strong absorption of N_H~10^{24} cm^{-2}. Previous X-ray studies revealed that its spectrum is dominated by strong fluorescence lines of Fe as well as continuum emission. For physical and geometrical insight into the nature of the reprocessing material, we utilize the high spectroscopic resolving power of the X-ray microcalorimeter (the soft X-ray spectrometer; SXS) and the wide-band sensitivity by the soft and hard X-ray imager (SXI and HXI) aboard Hitomi. Even though photon counts are limited due to unintended off-axis pointing, the SXS spectrum resolves Fe K{\alpha_1} and K{\alpha_2} lines and puts strong constraints on the line centroid and width. The line width corresponds to the velocity of 160^{+300}_{-70} km s^{-1}. This represents the most accurate, and smallest, width measurement of this line made so far from any X-ray binary, much less than the Doppler broadening and shift expected from speeds which are characteristic of similar systems. Combined with the K-shell edge energy measured by the SXI and HXI spectra, the ionization state of Fe is estimated to be in the range of Fe I--IV. Considering the estimated ionization parameter and the distance between the X-ray source and the absorber, the density and thickness of the materials are estimated. The extraordinarily strong absorption and the absence of a Compton shoulder component is confirmed. These characteristics suggest reprocessing materials which are distributed in a narrow solid angle or scattering primarily with warm free electrons or neutral hydrogen.