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

**Authors:** Poshak Gandhi, Takao Nakagawa, Aurora Simionescu, Norbert Schartel,  Hitomi Collaboration, Felix Aharonian, Hiroki Akamatsu, Fumie Akimoto, Steven W. Allen, Lorella Angelini, Marc Audard, Hisamitsu Awaki, Magnus Axelsson, Aya Bamba, Marshall W. Bautz, Roger Blandford, Laura W. Brenneman, Gregory V. Brown, Esra Bulbul, Edward M. Cackett  
**arXiv:** [1711.07727](https://arxiv.org/abs/1711.07727)  
**Date:** 2017-11-21  
**Summary page:** https://www.summarizepaper.com/en/paper/1711.07727v1/

## Key points

- Hitomi Collaboration conducted an observation of high-mass X-ray binary system IGR J16318-4848
- Strong absorption of N_H~10^{24} cm^{-2) suggests dense material surrounding the system
- Utilized advanced instrumentation aboard Hitomi: X-ray microcalorimeter (SXS), soft and hard X-ray imagers (SXI and HXI)
- SXS spectrum resolved Fe K{\alpha_1} and K{\alpha_2} lines with precise measurements, revealing velocity of 160^{+300}_{-70} km s^{-1}
- Fe within IGR J16318-4848 predominantly ionized to states ranging from Fe I to Fe IV
- Estimated density and thickness of materials present in the system based on ionization parameters and distances between X-ray source and absorber
- Observations confirmed strong absorption characteristics without evidence of a Compton shoulder component
- Reprocessing materials likely distributed within a narrow solid angle or scatter with warm free electrons or neutral hydrogen

## Summary

The Hitomi Collaboration recently conducted an observation of the high-mass X-ray binary system IGR J16318-4848, revealing exceptionally strong absorption of N_H~10^{24} cm^{-2}. This suggests the presence of dense material surrounding the system. Previous studies had identified prominent fluorescence lines of iron (Fe) in its spectrum, along with continuum emission. To gain deeper insights into the nature and geometry of the system, researchers utilized advanced instrumentation aboard Hitomi. The X-ray microcalorimeter (SXS) provided high spectroscopic resolving power while the soft and hard X-ray imagers (SXI and HXI) offered wide-band sensitivity. Despite limitations in photon counts due to unintended off-axis pointing during the observation, the SXS spectrum successfully resolved Fe K{\alpha_1} and K{\alpha_2} lines with precise measurements of their centroids and widths. Notably, these measurements revealed a velocity of 160^{+300}_{-70} km s^{-1}, one of the most accurate and smallest width measurements obtained from any X-ray binary to date. This value was significantly lower than expected Doppler broadening effects typically observed in similar systems. Combining information from SXI and HXI spectra regarding the K-shell edge energy led researchers to estimate that Fe within IGR J16318-4848 was predominantly ionized to states ranging from Fe I to Fe IV. Furthermore, based on calculations involving ionization parameters and distances between the X-ray source and absorber, researchers were able to estimate both density and thickness of materials present in this high-mass X-ray binary system. The observations confirmed extraordinarily strong absorption characteristics without any evidence of a Compton shoulder component. These findings suggest that reprocessing materials within IGR J16318-4848 are likely distributed within a narrow solid angle or primarily scatter with warm free electrons or neutral hydrogen. Overall, this detailed analysis provides valuable insights into the physical properties and behavior of this intriguing astronomical object.