The unusual spectrum of the X-ray transient source XRISM J174610.8-290021 near the Galactic center

TL;DR

This paper reports the discovery and analysis of a peculiar X-ray transient near the Galactic center, exhibiting unusual spectral features and variability, likely being a neutron star low-mass X-ray binary with complex ionization conditions.

Contribution

It presents the first detailed spectral and timing analysis of XRISM J174610.8-290021, revealing its unique iron line ratio and proposing a new interpretation involving scattered emission from a high-inclination neutron star binary.

Findings

Detected a transient source with unusual iron line ratios.

Identified a potential 1537 s periodicity in the source.

Spectral modeling suggests a neutron star low-mass X-ray binary with scattered emission.

Abstract

The Galactic center region was observed with the XRISM X-ray observatory during the performance verification phase in 2024 and a point-like X-ray source was detected with the X-ray imager Xtend at a position of (RA, Dec)=(17h46m10.8s, -29{\deg}00'21''), which is thus named XRISM J174610.8-290021. This source was bright in February to March and showed time variations in count rate by more than one order of magnitude in one week. The 2-10 keV X-ray luminosity was ~$10^{35}$ erg/s for the assumed distance of 8 kpc. However, after six months, it was below the detection limit. We found a hint of periodicity of 1537 s from timing analysis. The XRISM/Xtend spectrum has emission lines from helium-like iron (Fe He$\alpha$) at 6.7 keV and hydrogen-like iron (Fe Ly$\alpha$) at 6.97 keV; their intensity ratio is unusual with the latter being four times stronger than the former. If the emission is of thermal origin, the ionization temperature estimated from the iron-line intensity ratio is ~30 keV, which is inconsistent with the electron temperature estimated from the thermal bremsstrahlung, ~7 keV. Spectral models of magnetic cataclysmic variables, which are often seen in the Galactic center in this luminosity range, are found to fail to reproduce the obtained spectrum. By contrast, we found that the spectrum is well reproduced with the models of low-mass X-ray binaries containing a neutron star plus two narrow Gaussian lines. We consider that the source is intrinsically bright reaching $10^{37}$ erg/s, but is blocked from direct view due to a high inclination and only the scattered emission is visible. The photoionized plasma above the accretion disk with an ionization parameter of ~$10^{5}$ may explain the unusual iron line ratio. We further discuss the potential contribution of point sources of the type of XRISM J174610.8-290021 to the diffuse Galactic center X-ray emission.