Quasiperiodic Oscillation Signatures of Disk Restoration after Type-I X-Ray Bursts from 4U 1636−536

Abstract

Type-I thermonuclear bursts (TNBs) from neutron star low-mass X-ray binaries (NS LMXBs) originate on the neutron star’s surface from the unstable burning of the accreted material. On the other hand, kHz quasiperiodic oscillations (QPOs) are thought to originate in the innermost regions of the inspiralling accretion disk. Type-I TNBs are expected to impact the inner accretion flow, and consequently the kHz QPOs, due to the intense radiation pressure. In this work, we systematically study the evolution of the upper and the lower kHz QPOs immediately before and after a type-I TNB on 4U 1636−536 using AstroSat observations in the 3–20 keV band. The analysis of the power density spectra shows the presence of kHz QPOs within 200 s before the onset of the type-I burst. However, we have not detected any prominent signature of the same within 100–200 s after the burst. The kHz QPOs then reemerge after ≈200 s. The fractional rms variation in the 3–20 keV band drops by ≈5%–6%, supporting the nonexistence of kHz QPOs in the 200 s postburst zone. The timescale of 200 s coincides with the viscous timescale, highlighting a scenario where the inner disk is temporarily disrupted by the intense radiation from the type-I TNB. The kHz QPO then reestablishes as the inner disk is restored.