A NICER Look at the State Transitions of the Black Hole Candidate MAXI J1535-571 During its Reflares

The black hole candidate and X-ray binary MAXI J1535−571 was discovered in 2017
September. During the decay of its discovery outburst, and before returning to quiescence,
the source underwent at least four reflaring events, with peak luminosities of ∼1035–36 erg s−1
(d/4.1 kpc)2. To investigate the nature of these flares, we analysed a sample of NICER (Neutron
star Interior Composition Explorer) observations taken with almost daily cadence. In this work,
we present the detailed spectral and timing analysis of the evolution of the four reflares. The
higher sensitivity of NICER at lower energies, in comparison with other X-ray detectors,
allowed us to constrain the disc component of the spectrum at ∼0.5 keV. We found that during
each reflare the source appears to trace out a q-shaped track in the hardness–intensity diagram
similar to those observed in black hole binaries during full outbursts. MAXI J1535−571 transits
between the hard state (valleys) and softer states (peaks) during these flares. Moreover, the
Comptonized component is undetected at the peak of the first reflare, while the disc component
is undetected during the valleys. Assuming the most likely distance of 4.1 kpc, we find that
the hard-to-soft transitions take place at the lowest luminosities ever observed in a black
hole transient, while the soft-to-hard transitions occur at some of the lowest luminosities ever
reported for such systems.