Narrow iron- and nickel-K absorption lines from the eclipsing low-mass X-ray binary AX~J1745.6$-$2901

TL;DR

This paper reports the detection of narrow, highly ionized iron and nickel absorption lines in the X-ray spectrum of the eclipsing low-mass X-ray binary AX J1745.6-2901, revealing details about the absorber's location, velocity, and ionization state.

Contribution

The study provides the first high-resolution spectral analysis of the ionized absorber in AX J1745.6-2901, constraining its distance, velocity, and physical properties using XRISM data.

Findings

Absorber shows narrow Fe XXVI and Ni XXVIII lines with velocity widths ~110 km/s.

Absorber is located beyond 10^9 cm from the neutron star.

Absorber's outward velocity is much less than the escape velocity.

Abstract

We report the presence of a highly ionized absorber in the transient, eclipsing low-mass X-ray binary AX J1745.6-2901, observed from Feb. 26 to 29, 2024 with XRISM's Resolve and Xtend instruments. During a soft/high state without dips, Resolve's high spectral resolution (E/dE ~ 1000, full width at half maximum) revealed narrow velocity widths (sigma ~ 110 km/s) for Fe XXVI and Ni XXVIII lines, even with low photon statistics. These widths are consistent with binary orbital motion. The observed modest blueshift velocity (~160 km/s) indicates that the absorber is located sufficiently far from the neutron star (> 10^9 cm), so that gravitational redshift effects are not dominant. On the other hand, broad-band spectral analysis using a photoionized plasma model applied to the Xtend data constrains the absorber to lie within a radius of < 10^9.5 cm, as inferred from the upper limits of the best-fit ionization parameter (log xi ~ 4.4) and the large column density (~ 1.6 x 10^24 cm^-2). At this distance, the observed outward velocity of the absorber is about an order of magnitude smaller than the escape velocity from the neutron star.