QPO signatures of disk restoration after type-I X-ray bursts from 4U~1636$-$536

TL;DR

This study investigates how type-I X-ray bursts temporarily disrupt the inner accretion disk in neutron star binary 4U 1636-536, affecting the presence of kHz QPOs, and finds a 200-second recovery timescale linked to disk restoration.

Contribution

It provides the first systematic analysis of kHz QPO evolution immediately before and after type-I X-ray bursts in 4U 1636-536, revealing the disk disruption and recovery process.

Findings

kHz QPOs are present 200 seconds before bursts

QPOs disappear within 100-200 seconds after bursts

QPOs reappear after approximately 200 seconds

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 quasi-periodic oscillations (QPOs) are thought to originate in the innermost regions of the in-spiralling 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 show the presence of kHz QPOs within 200,seconds before the onset of the Type--I burst. However, we have not detected any prominent signature of the same within 100--200,sec after the burst. The kHz QPOs then re-emerges after $\approx$\,200\,sec. The fractional rms variation in the 3--20\,keV band drops by $\approx$\,5--6\,\%, supporting the non-existence of kHz QPOs in the 200\,sec post-Burst Zone. The time scale of 200\,sec coincides with the viscous time scale, highlighting a scenario where the inner disk is temporarily disrupted by the intense radiation from the Type--I TNB. The kHz QPO then re-establishes as the inner disk is restored.