NICER views moderate, strong, and extreme photospheric expansion bursts from the ultracompact X-ray binary 4U 1820–30

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 International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
⁴ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

Received: 8 October 2023
Accepted: 18 December 2023

Abstract

Type I X-ray bursts in the ultracompact X-ray binary 4U 1820–30 are powered by the unstable thermonuclear burning of hydrogen-deficient material. We report the detection of 15 type I X-ray bursts from 4U 1820–30 observed by NICER between 2017 and 2023. All these bursts occurred in the low state for the persistent flux in the range of 2.5–8 × 10⁻⁹ erg s⁻¹ cm⁻² in 0.1–250 keV. The burst spectra during the tail can be nicely explained by blackbody model. However, for the first ~5 s after the burst onset, the time-resolved spectra showed strong deviations from the blackbody model. The significant improvement of the fit can be obtained by taking into account of the enhanced persistent emission due to the Poynting–Robterson drag, the extra emission modeled by another blackbody component, or by the reflection from the surrounding accretion disk. The reflection model provides a self-consistent and physically motivated explanation. We find that the accretion disk density changed with 0.5 s delay in response to the burst radiation, which indicates the distortion of the accretion disk during X-ray bursts. From the time-resolved spectroscopy, all bursts showed the characteristic of photospheric radius expansion (PRE). We find one superexpansion burst with the extreme photospheric radius r_ph > 10³ km and blackbody temperature of ~0.2 keV, 13 strong PRE bursts for r_ph > 10² km, and one moderate PRE burst for r_ph ~ 55 km.