Accretion disks in symbiotic stars

The nature of accretion in symbiotic binaries, in which the red giant transfers material to a white dwarf (WD) companion, has been difficult to uncover. The accretion flows in a symbiotic binary are most clearly observable, however, when there is no quasi-steady shell burning on the WD to hide them. Through observations in the high energy regime, which provide a view of the innermost accretion structures, we have studied two unprecedented events in two systems, RT Cru and T CrB, which host similar massive white dwarf but have very different orbital periods with correspondingly different accretion mechanism. In the past 20 years, RT Cru has experienced two similar optical brightening events, separated by 4000 days and with amplitudes of V1.5 mag, reminiscent of dwarf-novae-type outbursts, but the hard X-ray behavior does not correspond to that observed in well-known dwarf nova. An alternative explanation for the brightening events could be that they are due to an enhancement of the accretion rate as the WD travels through the red giant wind in a wide orbit, with a period of about 4000 days. We have witnessed a change in the accretion rate for the first time in the symbiotic recurrent nova T CrB. Optical, UV and high energy data indicate that during an optical brightening event that started in early 2014 (V1.5), the hard X-ray emission has almost vanished and the X-ray spectrum became much softer and a bright, new, blackbody-like component appeared. We suggest that the optical brightening event, that could be a similar event to that observed about 8 years before the most recent thermonuclear outburst in 1946 is due to a disk instability.