The Thermonuclear X-Ray Bursts of 4U 1730-22

Peter Bult;Giulio C. Mancuso;Tod E. Strohmayer;Arianna C. Albayati;Diego Altamirano;Douglas J. K. Buisson;J??r??me Chenevez;Sebastien Guillot;Tolga G??ver;Wataru Iwakiri;Gaurava K. Jaisawal;Mason Ng;Andrea Sanna;Jean H. Swank

2022-01-01

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Abstract

We present observations of the historic transient 4U 1730–22 as observed with the Neutron Star Interior Composition Explorer (NICER). After remaining in quiescence since its 1972 discovery, this X-ray binary showed renewed outburst activity in 2021 and 2022. We observed 4U 173–22 extensively with NICER, detecting a total of 17 thermonuclear X-ray bursts. From a spectroscopic analysis, we find that these X-ray bursts can be divided into a group of bright and weak bursts. All bright bursts showed 1–2 s rise times and a photospheric radius expansion phase, while the weak bursts showed a slower ∼5 s rise with a tendency for concave shapes. From the photospheric radius expansion flux, we estimate the source distance at 6.9 ± 0.2 kpc. We consider various interpretations for our observations and suggest that they may be explained if accreted material is burning stably at the stellar equator and unstable ignition occurs at a range of higher latitudes.