A rapid decline of the black hole transient 4U 1630-472 in soft X-rays: approaching the hard state?

The 1-day averaged 2-4 keV intensity of the black hole low mass x-ray binary 4U 1630-472 as seen with the MAXI dropped by more than 50% within a single day, from 0.481 +/- 0.013 crab on September 17 (MJD 56187) to 0.223 +/- 0.010 crab on September 18 (MJD 56188). In comparison, the averaged intensities in the 4-10 keV and 10-20 keV increased from 0.417  +/- 0.011 crab to 0.523 +/- 0.013 crab and from 0.188 +/- 0.019 crab to 0.236 +/- 0.023 crab, respectively, while the averaged 15-50 keV intensity increased from 0.176 +/- 0.008 to 0.190 +/- 0.007 crab as seen with the Swift/BAT. This sharp flux decline in 2-4 keV and increase in harder bands show that 4U 1630-472 is approaching the hard state. This is also seen in the hardness ratios between BAT and MAXI (2-4 keV) or (4-10 keV) (see the plots at http://www.shao.ac.cn/wenfei/Too/ ). 

Interestingly, during the soft-to-hard transition in the past month, the intensity initially increased in both soft and hard bands until the sharp drop of the 2-4 keV flux around September 17-18. This is in contrast to those transitions seen in most transient low mass x-ray binaries. However, this recalls the previous transition of the same source from April 28 (MJD 56045) (Atel#4075, Atel#4077) to May 3 (MJD 56050) occurred early this year during which the source acted similarly: the intensities in the soft and the hard bands increased simultaneously from April 28 to May 3 and reached a very bright hard state (>0.4 crab), which was followed by a period of large amplitude flux variation in both soft and hard X-rays (> 50%, see the plots at http://www.shao.ac.cn/wenfei/Too). Considering the peculiar soft X-ray drop, the evidence for a transition back to the hard state, and expected strong broadband X-ray variability on a daily basis, multi-wavelength observations as well as X-ray observations are encouraged.

The daily X-ray long-term monitoring of the black hole transient 4U 1630-47 with the MAXI and the Swift/BAT of the recent outburst is shown here.  Notice the soft X-ray and the hard X-ray flux rose in the past month while the source turned to hard as seen in the hardness ratio evolution.

The daily X-ray monitoring of 4U 1630-47 in the past 30 days is shown here. It shows clearly the drop in the 2-4 keV flux and the simultaneous increase in the 4-10 keV and 15-50 keV.