A NICER look at the state transitions of the black hole candidate MAXI J1535-571 during its reflares
V. A. Cúneo ... [et al.].
Serie: Trabajos publicados del IAR ; no. 1548
Resumen: The black hole candidate and X-ray binary MAXI J1535-571 was discoveredin 2017 September. During the decay of its discovery outburst, andbefore returning to quiescence, the source underwent at least fourreflaring events, with peak luminosities of ∼1035-36 ergs-1 (d/4.1 kpc)2. To investigate the nature ofthese flares, we analysed a sample of NICER (Neutron star InteriorComposition Explorer) observations taken with almost daily cadence. Inthis work, we present the detailed spectral and timing analysis of theevolution of the four reflares. The higher sensitivity of NICER at lowerenergies, in comparison with other X-ray detectors, allowed us toconstrain the disc component of the spectrum at ∼0.5 keV. We found thatduring each reflare the source appears to trace out a q-shaped track inthe hardness-intensity diagram similar to those observed in black holebinaries during full outbursts. MAXI J1535-571 transits between the hardstate (valleys) and softer states (peaks) during these flares. Moreover,the Comptonized component is undetected at the peak of the firstreflare, 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 ina black hole transient, while the soft-to-hard transitions occur at someof the lowest luminosities ever reported for such systems.
