A new outburst of 4U 1630-472 and its very soft X-ray spectra

ATel #11722; Dongming Mao, Jie Lin, Zhen Yan, and Wenfei Yu (Shanghai Astronomical Observatory)
on 16 Jun 2018; 09:15 UT
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Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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We report on a new X-ray outburst of the black hole transient 4U 1630-472 based on MAXI monitoring observations and a Swift ToO observation performed yesterday. 4U1630-472 is a recurrent black hole X-ray transient with repetitive outbursts (Kuulkers et al. 1997) and has turned into an outburst at least 20 times as reported in the literature (Capitanio et.al. 2015). It is very likely that it has been inactive for about 600 days since the end of the previous outburst in 2016 (e.g. Atel#9427, Atel#9462), although we can not rule out low level activities based on X-ray monitoring data of the crowded sky region.

MAXI monitoring data indicates that from 2018 June 7 (MJD 58276) to June 15 (MJD 58284), the X-ray intensity of 4U 1630-472 in the 4-10 keV band increased from 67.85 +/- 4.8 mCrab to 186.2 +/- 9.92 mCrab, and the flux in 2-4 keV band increased from 60.06 to 144.74 +/- 9.81 mCrab, respectively. However, since the source is in a crowded region, it was uncertain whether the enhance X-ray flux came from 4U 1630-472.

We requested a Swift ToO observation in the direction of 4U 1630-472, aiming at determining if 4U 1630-472 is in outburst. The Swift observation was performed on 2018 June15 from 07:48:02 to 07:52:51 and from 09:26:02 to 09:28:36 (UT). In the XRT observation performed in WT mode, we found a bright X-ray source located at RA 16:34:01.255, DEC -47:23:26.460, which is just about 9 arcseconds from the reported position of 4U 1630-472, thus the transient is likely 4U 1630-472. The X-ray energy spectrum is very soft, which can be fitted with an absorbed disk black body model with the best-fit BB temperature at the inner disk edge of about 1.0+-0.1 keV, indicating the source has already stayed in the soft spectral state. The net XRT X-ray count rate in the 0.5-10 keV band is 32.8 +/- 0.5 counts/s. From the XRT energy spectrum, we estimate its X-ray flux as (1.0+-0.11)x 10^-8 erg/s/cm^2 in the 0.5-10 keV energy range, corresponding to about 0.22 Crab. This is consistent with MAXI monitoring data. We would like to emphasize that since the beginning, the outburst has been very soft as seen with MAXI data.

We would like to thank the Neil Gehrels Swift Observatory PI Brad Cenko for scheduling the ToO observations and the MAXI team for providing monitoring data to the public.