X-Raying an Accretion Disk in Realtime: the Evolution of Ionized Reflection during a Superburst from 4U 1636-536

TL;DR

This study uses time-resolved spectroscopy to analyze the evolution of ionized reflection in the accretion disk of neutron star 4U 1636-536 during a superburst, revealing changes in ionization and reflection regions.

Contribution

It provides the first detailed, time-resolved analysis of ionized reflection during a superburst, demonstrating the evolution of reflection regions and ionization states in the accretion disk.

Findings

Ionization state decreases over time during the superburst.

Reflection initially from outer disk regions shifts inward.

A strong local absorber develops during the superburst.

Abstract

When a thermonuclear X-ray burst ignites on an accreting neutron star, the accretion disk undergoes sudden strong X-ray illumination, which can drive a range of processes in the disk. Observations of superbursts, with durations of several hours, provide the best opportunity to study these processes and to probe accretion physics. Using detailed models of ionized reflection, we perform time resolved spectroscopy of the superburst observed from 4U 1636-536 in 2001 with RXTE. The spectra are consistent with a blackbody reflecting off a photoionized accretion disk, with the ionization state dropping with time. The evolution of the reflection fraction indicates that the initial reflection occurs from a part of the disk at larger radius, subsequently transitioning to reflection from an inner region of the disk. Even though this superburst did not reach the Eddington limit, we find that a strong local absorber develops during the superburst. Including this event, only two superbursts have been observed by an instrument with sufficient collecting area to allow for this analysis. It highlights the exciting opportunity for future X-ray observatories to investigate the processes in accretion disks when illuminated by superbursts.