Burst-recurrence properties revealed with Insight-HXMT and NICER for the newly discovered accreting millisecond pulsar MAXI J1816--195

Abstract

We report the results of our analysis of 83 type-I bursts during the 2022 outburst of the newly discovered accreting millisecond pulsar MAXI J1816--195 based on Insight-HXMT and NICER observations. We focus on the burst-recurrence time and its correlation with persistent flux and outburst evolution. The extensive observations of Insight-HXMT and NICER confirm the presence of quasi-periodic thermonuclear bursts during this outburst, with a recurrence time in the range of 1.15 to 2 hours, which varies with the source persistent flux. The burst recurrence times are, in general, longer at comparable flux levels in the outburst rising phase than those in the fading phase, forming an apparent hysteresis phenomenon. These burst properties make MAXI J1816--195 a unique target for investigating the underlying burst-accretion mechanisms. We discuss the plausible explanations for the hysteresis phenomenon, which appears to be related to changes in the ignition condition or accretion geometry during the outburst.