Dissecting a Disk-instability Outburst in a Symbiotic Star: NuSTAR and Swift Observations of T Coronae Borealis during the Rise to the “Superactive” State

The current superactive state of the recurrent nova T Coronae Borealis (T CrB) has been observed with unprecedented detail. Previously published observations provide strong evidence that this state is due to an enhancement of the flow of material through the accretion disk, which increased the optical depth of its most internal region, the boundary layer. NuSTAR and Swift observed T CrB in 2015 September, roughly halfway through the rise to optical maximum. In our analysis of these data, we have found that: (i) the UV emission, as observed with Swift/UVOT in 2015, was already as bright as it became in 2017, after the optical peak; (ii) the soft X-ray emission (E ≲ 0.6 keV) observed in 2017 after the optical peak, on the other hand, had not yet developed during the rising phase in 2015; (iii) the hard X-ray emitting plasma (E ≳ 2 keV) had the same temperature and about half the flux of that observed during quiescence in 2006. This phenomenology is akin to that observed during dwarf novae in outburst, but with the changes in the spectral energy distribution happening on a far longer timescale.