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arXiv · 2603.21727

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

Kojiro Tanaka·Yoshitomo Maeda·Ryota Tomaru·Lia Corrales·María Díaz Trigo·Chris Done·Tadayasu Dotani·Manabu Ishida·Satoru Katsuda·Yoshiaki Kanemaru·Richard Kelley·Aya Kubota·Hironori Matsumoto·Masayoshi Nobukawa·Megumi Shidatsu·Randall Smith·Hiromasa Suzuki·Hiromitsu Takahashi·Yohko Tsuboi·Hideki Uchiyama·Shigeo Yamauchi·Anje Yoshimoto·Q. Daniel Wang·Jon M. Miller·Frederick S. Porter·Shinya Yamada
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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.

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Kojiro Tanaka, Yoshitomo Maeda, Ryota Tomaru, Lia Corrales, María Díaz Trigo, Chris Done, Tadayasu Dotani, Manabu Ishida, Satoru Katsuda, Yoshiaki Kanemaru, Richard Kelley, Aya Kubota, Hironori Matsumoto, Masayoshi Nobukawa, Megumi Shidatsu, Randall Smith, Hiromasa Suzuki, Hiromitsu Takahashi, Yohko Tsuboi, Hideki Uchiyama, Shigeo Yamauchi, Anje Yoshimoto, Q. Daniel Wang, Jon M. Miller, Frederick S. Porter, Shinya Yamada. 2026-03-23. Narrow iron- and nickel-K absorption lines from the eclipsing low-mass X-ray binary AX~J1745.6$-$2901. https://doi.org/10.1093/pasj%2Fpsag027

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