Discovery of transition from marginally stable burning to unstable burning after a superburst in Aql X-1

TL;DR

This paper reports the first observation of the transition from superburst to unstable burning in a neutron star low-mass X-ray binary, revealing new insights into thermonuclear burning processes on neutron stars.

Contribution

It provides the first direct evidence of the transition from superburst to unstable burning, with detailed observations and comparison to simulations in Aql X-1.

Findings

Most accurate measurement of quenching time after superburst.

Detection of mHz quasi-periodic oscillations post-superburst.

Observation of transition from superburst to unstable burning.

Abstract

Superbursts are long duration, rare, and extremely energetic thermonuclear explosions of neutron star low-mass X-ray binaries (NS LMXBs), which are proposed to be due to unstable carbon ignition. We report the superburst properties and consequences from Aql X-1 observed by the Neutron Star Interior Composition Explorer (NICER), the Monitor of All-sky X-ray Image, Swift, and Insight Hard X-ray Modulation Telescope (Insight-HXMT) on the MJD 59130.7. We found two faint type I X-ray bursts 9.44 days after the superburst with a short recurrence time of 7.6 minutes, which is the most accurate measurement of the quenching time in all NS LMXBs with observed superbursts. We also discovered mHz quasi-periodic oscillations in the frequency range 2.7--11.3 mHz immediately after the superburst, before and after the resumption of the first type I X-ray burst from NICER, Swift, and Insight-HXMT observations. For the first time, we observed the transition from superburst, via marginally stable burning to unstable burning in NS LMXBs. We compared the quenching time and the recurrence time of type I X-ray bursts with simulations.