NuSTAR observation of increased activity in 4U 1630-472
ATel #16390; O. K. Adegoke (Caltech), J. A. Garcia (NASA/GSFC & Caltech), J. A. Tomsick (UC Berkeley, SSL), G. Mastroserio (INAF, Cagliari), R. M. T. Connors (Villanova), K. Foster (Caltech), F. Harrison (Caltech)
on 24 Dec 2023; 08:41 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Oluwashina Adegoke (oadegoke@caltech.edu)
Subjects: X-ray, Black Hole, Transient
Referred to by ATel #: 16391
4U 1630-472 is a transient black hole candidate (BHC) that has been known to go into regular outbursts since its discovery in 1962. No optical counterparts have been identified for the source as it is in a crowded region, close to the Galactic plane and behind a large neutral column (N_H ~1e+23 cm^{-2}). Unlike most BHCs, 4U 1630-472 spends very little time in the canonical hard state. In the past, the source has also shown flux dips in its light curves and spectral absorption lines implying a viewing angle close to the disk plane and moderate to extreme outflowing disk winds. Since its last major outburst from around July 2022, which included a re-brightening in August of 2023, 4U 1630-472 dimmed significantly in flux for a few weeks. However, for the past several weeks, it has been showing signs of increased activity again, exceeding 100 mCrab in MAXI, mostly below ~15 keV as it is not detected by Swift/BAT.
With the advantage of being able to observe sources relatively close to the Sun, NuSTAR observed 4U 1630-472 for ~25ks on 2023 December 16-17. The time-averaged spectrum of the source is predominantly soft and hardly detected beyond 20 keV. When fitted with an absorbed Comptonized-disk blackbody model, it shows relativistic reflection features including the broad iron K alpha line and a Compton hump. Super-imposed on the broad iron K alpha line is a prominent absorption line possibly from iron centered at ~6.8 keV. If associated with He-like iron XXV, this will correspond to a blueshift of ~5,000 km/s, indicative of very fast wind outflow. Evidence for likely extreme outflows has been reported for 4U 1630-472 before now. Basic model fit to the entire spectrum in the 3-18 keV range gives diskbb T_in=1.19+/-0.02 keV, photon index Gamma=3.4^{+0.9}_{-0.8} and tbabs absorption column N_H=9.7^{+0.3}_{-0.2}e+22 cm^{-2}, using wilm abundances. The unabsorbed 3-20 keV flux of the source during the observation is 4.07+/-0.04e-09 ergs/cm^2/s (about 170 mCrab).
The source is still showing signs of increased activity as at present. Further observations are encouraged.
