Return of 4U~1730--22 after 49 years silence: the peculiar burst properties of the 2021/2022 outbursts observed by Insight-HXMT

TL;DR

This paper reports the reactivation of the neutron star 4U~1730--22 after 49 years, analyzing 10 X-ray bursts with Insight-HXMT, revealing unique burst profiles, spectral features, and implications for burst emission geometry and neutron star distance estimation.

Contribution

It presents the first detailed analysis of the peculiar burst properties of 4U~1730--22's 2021/2022 outbursts, including double-peaked and PRE bursts, and explores their physical implications.

Findings

First detection of double-peaked non-PRE burst in this source.

Residuals in soft and hard X-ray bands suggest enhanced emission or Comptonization.

Burst emission rise phase indicates possible anisotropic emission or surface exposure.

Abstract

After in quiescence for 49 years, 4U~1730--22 became active and had two outbursts in 2021 \& 2022; ten thermonuclear X-ray bursts were detected with Insight-HXMT. Among them, the faintest burst showed a double-peaked profile, placing the source as the 5th accreting neutron star (NS) exhibiting double/triple-peaked type-I X-ray bursts; the other bursts showed photospheric radius expansion (PRE). The properties of double-peaked non-PRE burst indicate that it could be related to a stalled burning front. For the five bright PRE bursts, apart from the emission from the neutron star (NS) surface, we find the residuals both in the soft ($<$3 keV) and hard ($>$10 keV) X-ray band. Time-resolved spectroscopy reveals that the excess can be attributed to an enhanced pre-burst/persistent emission or the Comptonization of the burst emission by the corona/boundary-layer. We find, the burst emission shows a rise until the photosphere touches down to the NS surface rather than the theoretical predicted constant Eddington luminosity. The shortage of the burst emission in the early rising phase is beyond the occlusion by the disk. We speculate that the findings above correspond to that the obscured part (not only the lower part) of the NS surface is exposed to the line of sight due to the evaporation of the obscured material by the burst emission, or the burst emission is anisotropic ($\xi>1$) in the burst early phase. In addition, based on the average flux of PRE bursts at their touch-down time, we derive a distance estimation as 10.4 kpc.