Rapidly Evolving Disk–Jet Coupling during Re-brightenings in the Black Hole Transient MAXI J1535−571

Abstract

The main outburst of the candidate black hole low-mass X-ray binary (BH LMXB) MAXI J1535−571 ended in 2018 May and was followed by at least five episodes of re-brightenings. We have monitored this re-brightening phenomenon at X-ray and radio wavelengths using the Neil Gehrels Swift Observatory and Australia Telescope Compact Array, respectively. The first two re-brightenings exhibited a high peak X-ray luminosity (implying a high mass-accretion rate) and were observed to transition from the hard to the soft state. However, unlike the main outburst, these re-brightenings did not exhibit clear hysteresis. During the re-brightenings, when MAXI J1535−571 was in the hard state, we observed the brightening of a compact radio jet that was subsequently quenched when the source transitioned to a similar soft state, as was observed during the main outburst. We report on the first investigation of disk−jet coupling over multiple rapidly evolving re-brightenings in a BH LMXB. We find that the accretion flow properties and the accompanying compact jet evolve on a similarly rapid timescale of ∼days, rather than the typical value of ∼weeks as observed for most other BH LMXBs during their main outburst events.