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

TL;DR

This paper reports on NICER observations of 17 thermonuclear X-ray bursts from the transient neutron star binary 4U 1730-22 during its 2021-2022 outburst, revealing distinct burst types and estimating the source distance.

Contribution

It provides detailed spectral analysis of burst types and proposes a new interpretation involving stable equatorial burning and unstable higher-latitude ignition.

Findings

Bright bursts show rapid rise and photospheric radius expansion.

Weak bursts have slower rise times and different shapes.

Estimated source distance is approximately 6.9 kpc.

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 $1\sim2$ second rise times and a photospheric radius expansion phase, while the weak bursts showed a slower $\sim5$ second rise with a tendency for concave shapes. From the photospheric radius expansion flux, we estimate the source distance at $6.9\pm0.2$ 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.