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The current $super-active$ state of the recurrent nova T CrB has beenobserved with unprecedented detail. Previously published observations providestrong evidence that this state is due to an enhancement of the flow ofmaterial through the accretion disk, which increased the optical depth of itsmost internal region, the boundary layer. $NuSTAR$ and $Swift$ observed T CrBin 2015 September, roughly halfway through the rise to optical maximum. In ouranalysis of these data, we have found that: $i$) the UV emission, as observedwith $Swift$/UVOT in 2015, was already as bright as it became in 2017, afterthe optical peak; $ii$) the soft X-ray emission (E $\lesssim$ 0.6 keV) observedin 2017 after the optical peak, on the other hand, had not yet developed duringthe rising phase in 2015; $iii$) the hard X-ray emitting plasma (E $\gtrsim$ 2keV) had the same temperature and about half the flux of that observed duringquiescence in 2006. This phenomenology is akin to that observed during dwarfnovae in outburst, but with the changes in the spectral energy distributionhappening on a far longer time scale.

astrophysical journal/the astrophysical journal

Dissecting a disk-instability outburst in a symbiotic star: $NuSTAR$,  and $Swift$ observations of T Coronae Borealis during the rise to the  "super-active" state