The thermonuclear X-ray bursts of 4U 1730-22

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 1730-22 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 $1\sim 2$ second rise times and a photospheric radius expansion phase, while the weak bursts showed a slower $\sim 5$ second rise with a tendency for concave shapes. From the photospheric radius expansion flux, we estimate the source distance at $6.9\pm 0.2$ 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.

arXiv comment13 pages, 6 figures, 2 tables. Accepted for publication in ApJ

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• Type I X-ray bursts' spectra and fuel composition from the atoll and transient source 4U 1730$-$22
• Catalog of outbursts of neutron star LMXBs
• Detection of millihertz quasi-periodic oscillations in the low-mass X-ray binary 4U 1730--22 with NICER
• NICER observations of type-I X-ray bursts from the ultra-compact X-ray binary M15 X-2