Mon. Not. R. Astron. Soc., 470, 4298-4306 (2017/October-1)
Detection of X-ray spectral state transitions in mini-outbursts of black hole transient GRS 1739-278.
YAN Z. and YU W.
Abstract (from CDS):
We report the detection of the state transitions and hysteresis effect in the two mini-outbursts of the black hole (BH) transient GRS 1739-278 following its 2014 major outburst. The X-ray spectral evolutions in these two mini-outbursts are similar to the major outburst in spite of their peak luminosities and the outburst durations are one order of magnitude lower. We found L_hard -to -soft_ and Lpeak,soft of the mini-outbursts also follow the correlation previously found in other X-ray binaries. L_hard -to -soft_ of the mini-outbursts is still higher than that of the persistent BH binary Cyg X-1, which supports that there is a link between the maximum luminosity a source can reach in the hard state and the corresponding non-stationary accretion represented by substantial rate of change in the mass accretion rate during flares/outbursts. The detected luminosity range of these two mini-outbursts is roughly in 3.5 x 10–5 to 0.015 (D/7.5 kpc)2(M/8M☉) LEdd. The X-ray spectra of other BH transients at such low luminosities are usually dominated by a power-law component, and an anti-correlation is observed between the photon index and the X-ray luminosity below 1 per cent LEdd. So, the detection of X-ray spectral state transitions indicates that the accretion flow evolution in these two mini-outbursts of GRS 1739-278 are different from other BH systems at such low-luminosity regime.