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

Abstract

We report on a Hitomi observation of IGR J16318−4848, a high-mass X-ray binary system with an extremely strong absorption of NH ∼ 10²⁴ cm⁻². Previous X-ray studies revealed that its spectrum is dominated by strong fluorescence lines of Fe as well as continuum emission lines. For physical and geometrical insight into the nature of the reprocessing material, we utilized 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 imagers (SXI and HXI) aboard Hitomi. Even though the photon counts are limited due to unintended off-axis pointing, the SXS spectrum resolves Fe Kα1 and Kα2 lines and puts strong constraints on the line centroid and line width. The line width corresponds to a velocity of 160⁺³⁰⁰₋₇₀ km s⁻¹. This represents the most accurate, and smallest, width measurement of this line made so far from the any X-ray binary, much less than the Doppler broadening and Doppler shift expected from speeds that 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 have been confirmed. These characteristics suggest reprocessing materials that are distributed in a narrow solid angle or scattering, primarily by warm free electrons or neutral hydrogen. This measurement was achieved using the SXS detection of 19 photons. It provides strong motivation for follow-up observations of this and other X-ray binaries using the X-ray Astrophysics Recovery Mission and other comparable future instruments.