Possible ongoing hard-to-soft transition of GX 339-4
Authors: S. E. Motta (ESA/ESAC), T. M. Belloni (INAF-OAB)
The latest MAXI observation of the black-hole candidate GX 339-4 (19 January 2015) shows that the source might be leaving the low-hard state. The BAT count rate (15-50 keV) has been dropping for the last 5 days (from ~360 mCrab to ~180mCrab), suggesting that a transition to the soft state might be ongoing. Since GX 339-4 is currently not observable by Swift/XRT nor by any other X-ray satellite because of sun constraints, we cannot exclude that the observed drop in the hard count rate is due to a transition back to the hard-state. We note, however, that even though transitions back to the hard state have been observed in the past (2004, 2009 and 2012, Motta et al. 2011, MNRAS, 418, 2292 and ATel #4247) have never been so fast. If what we are observing is a hard-to-soft transition, based on previous outbursts considerable further softening is expected in the next few days. In particular, the radio emission is expected to evolve and lead to a major ejection. Multi-wavelength observations are encouraged.
radio,x-ray,binary,black hole,the sun,transient