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

Kavli Affiliate: Marshall W. Bautz

| First 5 Authors: Hitomi Collaboration, Felix Aharonian, Hiroki Akamatsu, Fumie Akimoto, Steven W. Allen

| Summary:

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.

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