Studying the Reflection Spectra of the New Black Hole X-Ray Binary Candidate MAXI J1631−479 Observed by NuSTAR: A Variable Broad Iron Line Profile

Abstract

We present results from the Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the new black hole X-ray binary candidate MAXI J1631-479 at two epochs during its 2018-2019 outburst, which caught the source in a disk dominant state and a power-law dominant state. Strong relativistic disk reflection features are clearly detected, displaying significant variations in the shape and strength of the broad iron emission line between the two states. Spectral modeling of the reflection spectra reveals that the inner radius of the opticallythick accretion disk evolves from < 1.9 r g to 12 ± 1 r g (statistical errors at 90% confidence level) from the disk dominant to the power-law dominant state. Assuming in the former case that the inner disk radius is consistent with being at the ISCO, we estimate a black hole spin of a * > 0.94. Given that the bolometric luminosity is similar in the two states, our results indicate that the disk truncation observed in MAXI J1631-479 in the power-law dominant state is unlikely to be driven by a global variation in the accretion rate. We propose that it may instead arise from local instabilities in the inner edge of the accretion disk at high accretion rates. In addition, we find an absorption feature in the spectra centered at 7.33 ± 0.03 keV during the disk dominant state, which is evidence for a rare case that an extremely fast disk wind (v out = 0.067 +0.001 −0.004 c) is observed in a low-inclination black hole binary, with the viewing angle of 29 ± 1 • as determined by the reflection modeling.

Spectral and timing evolution of MAXI J1631–479 during the 2018–19 outburst with NICER

“…The best fitting line energy was ∼ 7.4 keV with an FWHM of ∼ 0.1 keV. These features are an indication of absorption by blue-shifted H-or He-like Fe ions originating from an equatorial disk wind and have also been detected by Xu et al (2020).…”

“…They also suggest that Type-B QPOs likely have a different origin than Type-C. Ingram et al (2016Ingram et al ( , 2017 went on to carry out phase-resolved spectroscopy of QPOs to verify that the reflection spectrum varies with different phases of the QPO providing strong evidence for geometric origin. MAXI J1631-479 (hereafter J1631) is a newly discovered galactic X-ray transient which hosts a rapidly spinning massive black hole at its center (Xu et al 2020, Rout & Vadawale 2021). During the HSS, J1631 exhibits outflow of matter in the form of ultra-fast disk winds.…”

Evidence for an expanding corona based on spectral-timing modelling of multiple black hole X-ray binaries

“…Therefore, we do not include a few more recent BHXB sources in our search (e.g. Uttley et al 2018;Armas Padilla et al 2019;Shang et al 2019;Xu et al 2020). Calibrating the power colours across different instruments is an interesting problem to be considered in future works.…”

Broad-band spectral study of LMXB black hole candidate 4U 1957+11 with NuSTAR

“…The spectral properties and X-ray variability of 4U 1954+11 find similarities with other BHCs in the soft spectral state (Swift J1753.5-0127 (Yoshikawa et al 2015), MAXI J1634-479 (Xu et al 2020) and MAXI J1535-571 (Cúneo et al 2020)). The evolution of the hard component to dominate the emission at the relatively high-flux states as observed during the epochs 6 and 7 provides strong evidence that it does not harbour a NS (Di Salvo et al 2000;Wijnands et al 2002).…”