Dramatic change in the boundary layer in the symbiotic recurrent nova T Coronae Borealis

A sudden increase in the rate at which material reaches the most internal part of an accretion disk, i.e., the boundary layer, can change its structure dramatically. We have witnessed such a change for the first time in the symbiotic recurrent nova T CrB. Our analysis of XMM-Newton , Swift Burst Alert Telescope (BAT)/X-Ray Telescope (XRT)/UltraViolet Optical Telescope (UVOT), and the American Association of Variable Stars Observers (AAVSO) V - and B -band data indicates that during an optical brightening event that started in early 2014 ( ΔV ≈ 1.5) the following occurred: ( i ) the hard X-ray emission as seen with BAT almost vanished; ( ii ) the XRT X-ray flux decreased significantly, while the optical flux remained high; ( iii ) the UV flux increased by at least a factor of 40 over the quiescent value; and ( i v) the X-ray spectrum became much softer and a bright, new blackbody-like component appeared. We suggest that the optical brightening event, which 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.