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

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 \texttt{relxillNS} model). The $f_a$ model provides a higher blackbody temperature and higher burst flux compared with the \texttt{relxillNS} model. The peak fluxes of two bursts from the $f_a$ model, $4.36\times 10^{-8}\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ and $9.10\times 10^{-8}\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, 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 $\sim 20-30\%$ of the total burst emissions.

8 pages, 7 figures. This version matches the published version

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• Discovery of a long thermonuclear X-ray burst from the ultra-compact binary 4U 1850$-$087
• Probing thermonuclear bursts and X-ray reflection features in Aql X-1 during 2024 outburst
• Radiation Driven Warping of Accretion Discs Due to X-ray Bursts
• New insight into the hard X-ray emission influenced by the type-\uppercase\expandafter{\romannumeral1} bursts observed by Insight-HXMT during outburst of 4U 1636--536
• A transition from mixed-fuel to pure-helium thermonuclear bursts in Terzan 5 X-3/Swift J174805.3-244637
• Long thermonuclear burst driven thermal-viscous instability of accretion disk: triggering an outburst-like X-ray flare