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

Abstract

Superbursts are long duration, rare and extremely energetic thermonuclear explosion 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 its consequences from Aql X-1 observed by NICER, MAXI, Swift and Insight-HXMT on MJD 59130.7. We find 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 discovery the 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 observe the transition from superburst, via marginally stable burning to unstable burning in NS LMXBs. We compare the quenching time and the recurrence time of type I X-ray bursts with simulations.

A NICER look at thermonuclear X-ray bursts from Aql X-1

“…The minimum reported value is 233 s for 4U 1705-44 in the MINBAR catalog (Galloway et al 2020;Keek et al 2010). Most recently, bursts 21 and 22 have been reported to happen 9.44 days after a superburst (Li et al 2021). We note that burst 20 reported here happened only three days before the reported superburst.…”