NICER observations of the evidence of Poynting-Robertson drag and disk reflection during type I X-ray bursts from 4U 1636–536

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

Received: 1 December 2021
Accepted: 14 February 2022

Abstract

Type I X-ray bursts are the result of an unstable thermonuclear burning of accreting matter on the neutron star (NS) surface. The quick release of energetic X-ray photons during such bursts interacts with the surrounding accretion disk, which raises the accretion rate due to Poynting-Robertson drag and, thus, a fraction of the burst emission is reflected. We analyzed two photospheric radius expansion bursts in the NS low-mass X-ray binary 4U 1636–536 that took place in 2017, using data from Neutron star Interior Composition Explorer. The time-resolved burst spectra showed clear deviations from a blackbody model. The spectral fitting can be significantly improved by introducing either the enhanced persistent emission (the f_a model) or the reflection from the accretion disk (the relxillNS model). The f_a model provides a higher blackbody temperature and higher burst flux compared with the relxillNS model. The peak fluxes of two bursts from the f_a model, 4.36 × 10⁻⁸ erg cm⁻² s⁻¹ and 9.10 × 10⁻⁸ erg cm⁻² s⁻¹, are slightly higher than the Eddington limits of mixed hydrogen-helium and pure helium bursts from previous observations, respectively. When the disk reflections have been taken into account simultaneously, the peak fluxes are lower to match the preferred values. We find evidence to support the finding that both the Poynting-Robertson drag and disk reflection have been appeared during these two X-ray bursts. Moreover, the disk reflection may contribute ∼20−30% of the total burst emissions.