1104.2785 (Zhen Yan et al.)

A UV flux drop preceding the X-ray hard-to-soft state transition during the 2010 outburst of GX 339-4    [PDF]

Zhen Yan, Wenfei Yu

The black hole X-ray transient GX 339$-$4 was observed with the XRT and the UVOT onboard the {\it Swift} satellite across the hard-to-soft state transition during its 2010 outburst. The ultraviolet (UV) flux measured with the filter UVW2 started to drop nearly 10 days before the drop in the hard X-ray flux when the hard-to-soft state transition started. The UV flux $F_\mathrm{UV}$ correlated with the X-ray flux $F_\mathrm{X}$ as $F_\mathrm{UV}\propto F_\mathrm{X}^{0.50\pm0.04}$ before the drop in the UV flux. During the UV drop lasting about 16 days, the X-ray flux in 0.4--10 keV was increasing. We suggest that the UV emission during the bright hard state before the state transition was probably dominated by the self-absorbed synchrotron emission from a compact steady jet. The drop in the UV flux then indicates that the quenching of the jet occurred well before the hard-to-soft state transition seen in the X-rays. If confirmed with observations of future outbursts or in other sources, a drop in the UV flux then may be an indicator of a coming hard-to-soft state transition.

View original: http://arxiv.org/abs/1104.2785