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

¹ Key Laboratory of Stars and Interstellar Medium, Xiangtan University, Xiangtan, 411105 Hunan, PR China
e-mail: lizhaosheng@xtu.edu.cn, panyy@xtu.edu.cn
² Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049, PR China
³ School of Physics and Astronomy, Sun Yat-Sen University, Zhuhai 519082, PR China
⁴ International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
⁵ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

Received: 16 September 2022
Accepted: 13 December 2022

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 ∼10 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 ± 0.46(X = 0) kpc for a canonical neutron star with M_NS = 1.4 M_⊙ and R_NS = 10 km.