Initial Swift observations of the Aug 2016 outburst of the Black Hole Candidate 4U 1630-47

ATel #9462; Aru Beri, Diego Altamirano, Poshak Gandhi (U. Southampton)
on 6 Sep 2016; 13:53 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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4U 1630-472 is one of the most active black hole candidates (BHCs) known, with a recurrence time between X-ray outbursts of about 600-700 days. Following the detection of an X-ray outburst in late Aug 2016 (ATEL #9427), we obtained Swift observations of the source.

We report analysis of two of the initial Swift-XRT observations, performed on 2016-09-04 and 2016-09-05, with a net exposure time of about 500 and 1500 seconds respectively. (There is one additional Swift observation on 2016-08-30 which we do not analyse here). To avoid the issue of pile-up, the XRT data were taken in Window Timing (WT) mode during both the observations. Temporal analysis shows an average XRT count rate of ~40 counts/s over ~0.3-10 keV during the first observation and ~50 counts/s during the second. Spectral analysis of the WT data reveals that the source is well described by an absorbed disk blackbody, during both the observations. The fits did not require an additional hard component such as a power law.

The best fit parameters obtained from the analysis of these observations are as follows:

Observation ID-00031224022
Date and Start Time of Observation : 2016-09-04 (06 h 51 min 58s UT)
N_H = (12.1 +/- 0.5) x 10^{22} cm^{-2}
kT_inner = 1.18 +/- 0.04 keV
Reduced Chi^2 = 0.99 (677 dof)
Absorbed flux = 3.2e-09 ergs/s/cm^2 (0.7-10 keV)

Observation ID-00031224023
Date and Start Time of Observation : 2016-09-05 (00h 25 min 57s UT)
N_H = (8.71 +/- 0.07) x 10^{22} cm^{-2}
kT_inner = 1.49 +/- 0.01 keV
Reduced Chi^2 = 1.2 (907 dof)
Absorbed flux = 4.7e-09 ergs/s/cm^2 (0.7-10 keV)

(All uncertainties are stated at a 1-sigma confidence level).

The observations suggest that the source is in a thermal-dominant state (see, e.g., Tomsick et al. 2005, ApJ, 630, 413). The high values of absorption we observe are similar to previous measurements (see, e.g., Wang et al. 2016, MNRAS, 456, 1579 and Capitanio et al. 2015, MNRAS, 450, 3840).

We will continue monitoring the outburst over the next 13 days. We thank Neil Gehrels and the Swift staff for rapid scheduling of these observations.