The X-ray transient 4U 1630-47 transitioned from the hard to soft spectral state

ATel #13623; Riley M. T. Connors (Caltech), John A. Tomsick (SSL/UCB), Javier A. Garcia (Caltech)
on 7 Apr 2020; 21:27 UT
Credential Certification: Riley Connors (rconnors@caltech.edu)

Tweet

We report on X-ray observations of the black hole X-ray binary 4U 1630-47 made with the Neil Gehrels Swift Observatory (Swift) X-ray telescope (XRT) on MJD 58940, and MAXI (The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-ray Images, Matsuoka, M. et al., PASJ, 61, 999, 2009) on MJD 58920-58924, roughly 20 days prior, showing the source transitioned from the hard to the soft spectral state during this 20 day period.

Swift-XRT observed 4U 1630-47 on 2020 April 1, at 17:33:35 UTC, MJD 58940, with the sequence ID 00031224062. The source was observed for ~ 1 ks at a count rate of ~ 50 cts/s, yielding a total of 4.8e4 source counts in the 0.3-10 keV band. The source appears to have reached the very soft, thermally dominated state, well known for 4U 1630-47 (e.g., Tomsick et al. 2005, ApJ, 630, 413). We processed the cleaned event file from the Swift XRT quick look data. We extracted source counts from a circle with a radius of 47'', and took the background from an annular region away from the source. We then rebinned the source spectrum with S/N = 5. The 0.3-10 keV Swift XRT spectrum can be with with a highly absorbed multi-temperature disk blackbody (Tbabs*diskbb; abundances from Wilms et al. 2000, ApJ, 542, 914), with a hydrogen column density of 1.26 +/- 0.02 x 10^23 cm^-2, inner disk temperature of 1.68 +/- 0.03 keV, and disk normalization 120 +/- 10. The high disk temperature indicates the source is well into the soft state. The absorbed 2-10 keV source flux was 6.9e-9 erg/cm^2/s.

We obtained a MAXI Gas Slit Camera (GSC) spectrum for MJD 58920-58924, a 4-day period prior to the reported INTEGRAL observations made on MJD 58925 (Onori et al., ATel #13564), using the on-demand data processing (http://maxi.riken.jp/mxondem/). The average absorbed MAXI-GSC 2-10 keV flux over the 4 days was 6e-10 erg/cm^2/s, an order of magnitude lower than the 2-10 keV Swift-XRT flux obtained 15-20 days later. The spectrum can be fit with a 2-10 keV absorbed power law with photon index 1.6 (+0.6,-0.4), and hydrogen column density 4 (+8,-4) x 10^22 cm^-2. The INTEGRAL flux reported during observations made just a day later appears to show a slight increase with respect to the MAXI flux, suggesting the MAXI observations caught 4U 1630-47 in its hard state before it then brightened and transitioned into the soft state, confirmed by Swift-XRT 15 days later.

For black hole transients, plasma ejections leading to radio flares are often seen after the hard states at the beginnings of outbursts. Thus, we suggest a connection between the observed X-ray evolution of this outburst and the radio flare reported by Girard et al. (ATEL#13592). This particular source showed a similar radio flare during a 1998 outburst, but 4U 1630-47 more typically has radio-quiet X-ray outbursts.

We thank the Swift PI Brad Cenko for approving this ToO request and allowing us to complete this analysis. This research has made use of MAXI data provided by RIKEN, JAXA and the MAXI team.