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

ATel #4395; Wenda Zhang, Wenfei Yu, and Zhen Yan (Shanghai Astronomical Observatory)
on 19 Sep 2012; 12:14 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

Subjects: X-ray, Binary, Black Hole

Referred to by ATel #: 4406

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 http://202.127.29.4/wenfei/Too/ ).

Interestingly, during period of the gradual transition from the soft to the hard hardness ratios 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://202.127.29.4/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.