AstroSat and Chandra View of the High Soft State of 4U 1630–47 (4U 1630–472): Evidence of the Disk Wind and a Rapidly Spinning Black Hole

Abstract

We present the X-ray spectral and timing analysis of the transient black hole X-ray binary 4U 1630-47, observed with the AstroSat, Chandra and MAXI space missions during its soft X-ray outburst in 2016. The outburst, from the rising phase until the peak, is neither detected in hard X-rays (15-50 keV) by the Swift/BAT nor in radio. Such non-detection along with the source behavior in the hardnessintensity and color-color diagrams obtained using MAXI data confirm that both Chandra and AstroSat observations were performed during the high soft spectral state. The High Energy Grating (HEG) spectrum from the Chandra high-energy transmission grating spectrometer (HETGS) shows two strong, moderately blueshifted absorption lines at 6.705 +0.002 −0.002 keV and 6.974 +0.004 −0.003 keV, which are produced by Fe XXV and Fe XXVI in a low-velocity ionized disk wind. The corresponding outflow velocity is determined to be 366±56 km/s. Separate spectral fits of Chandra/HEG, AstroSat/SXT+LAXPC and Chandra/HEG+AstroSat/SXT+LAXPC data show that the broadband continuum can be well described with a relativistic disk-blackbody model, with the disk flux fraction of ∼ 0.97. Based on the best-fit continuum spectral modeling of Chandra, AstroSat and Chandra+AstroSat joint spectra and using the Markov Chain Monte Carlo simulations, we constrain the spectral hardening factor at 1.56 +0.14 −0.06 and the dimensionless black hole spin parameter at 0.92 ± 0.04 within the 99.7% confidence interval. Our conclusion of a rapidly-spinning black hole in 4U 1630-47 using the continuum spectrum method is in agreement with a previous finding applying the reflection spectral fitting method.

AstroSat and MAXI view of the Black Hole binary 4U 1630-472 during 2016 and 2018 Outbursts

“…However, lower count rate and lack of significant power above 1 Hz could have possibly contributed to the non-detection of the QPO in this case. Subsequent observations require a prominent disc component and have lower rms (< 5%) in the 2016 outburst, similar to the rms obtained byPahari et al (2018), suggesting that the source remained in the soft state (see Figure…”

“…Reflection continuum modeling suggests that the source hosts a maximally rotating black hole (King et al 2014). Recently, Pahari et al (2018) obtained the spin of the compact object as 0.92 +0.02 −0.01 by continuum modelling of the spectra obtained using Chandra and As-troSat, when the source was in the soft state during the 2016 outburst. Till date, 4U 1630-472 has undergone more than 25 outbursts in the ∼ 50 years since its detection.…”

Science with the AstroSat Soft X-ray telescope: An overview

“…1). Using the XSPEC 2 kerrbb model for the relativistic disk blackbody to fit AstroSat SXT+LAXPC and contemporaneous Chandra High Energy Grating (HEG) spectra, the dimensionless black hole spin parameter was measured to be 0.92 ± 0.04 with 99.7% confidence (Pahari et al, 2018a). Similarly, a characterization of the thermal disk spectrum of the persistent BHXB LMC X-1 with SXT 2 https://heasarc.gsfc.nasa.gov/xanadu/xspec/ led to a measurement of the black hole spin parameter of ∼ 0.93 (Mudambi et al, 2020a).…”

“…The source was in a soft state with a disk flux fraction of ∼ 0.97. It can be seen that SXT covers the spectrum and its peak better than other instruments, which was particularly useful to measure the black hole spin (see section 2.; figure courtesy: Mayukh Pahari;Pahari et al, 2018a).…”

LAXPC instrument onboard AstroSat: Five exciting years of new scientific results specially on X-ray Binaries

“…A good example of using the different capabilities of instruments was the study of the black hole system 4U 1630-47 using the high energy grating spectra from Chandra along with AstroSat's LAXPC and SXT data (Pahari et al, 2018a). While the latter provided evidence for a highly ionized wind, relativistic disk model fit to the continuum using AstroSat data showed that the black hole is highly spinning, thus making an interesting connection between wind outflow and black hole spin.…”