Type I X-ray bursts' spectra and fuel composition from the atoll and transient source 4U 1730$-$22

TL;DR

This study analyzes NICER observations of 17 type I X-ray bursts from 4U 1730-22, revealing spectral deviations, PRE characteristics, and suggesting pure helium fuel, with implications for neutron star properties and burst mechanisms.

Contribution

It provides the first detailed spectral analysis of multiple X-ray bursts from 4U 1730-22, including deviations from blackbody models and fuel composition estimation.

Findings

78% of bursts exhibited photospheric radius expansion (PRE).

Spectral deviations from blackbody observed ~10s after burst onset.

Estimated source distance is approximately 7.54 kpc.

Abstract

NICER observed two outbursts from the neutron star low-mass X-ray binary 4U~1730$-$22 in 2021 and 2022, which showed a similar spectral evolution in the hardness-intensity diagram. Seventeen type I X-ray bursts were identified in both outbursts. The X-ray burst spectra showed clear deviations from the blackbody model, firstly $\sim10$ s after onset. Adding the enhanced persistent emission due to the Poynting-Robertson drag or the reflection from the accretion disk both significantly improved the fitting results. We found that 12 out of 17 X-ray bursts showed the photospheric radius expansion (PRE) characteristic. Considering the nine PRE bursts out of ten X-ray bursts observed by Insight-HXMT, 78\% of bursts from 4U~1730--22 exhibited PRE. According to the burst rise time, the duration, the local accretion rate, and the burst fuel composition estimated from recurrence time, we propose that these PRE bursts were powered by pure helium. From the touchdown flux of PRE bursts, we estimate the source distance of $ d=7.54\pm{0.46} (X=0)$ kpc for a canonical neutron star with $M_{\rm NS}=1.4M_\odot$ and $R_{\rm NS}=10~{\rm km}$.