The Accretion Disk and the Boundary Layer of the Symbiotic Recurrent Nova T Corona Borealis

NUÑEZ, NATALIA; NELSON, THOMAS; SOKOLOSKI, JENMIFER; MUKAI, KOJI; LUCY, ADRIAN; LUNA, GERARDO

Congreso; American Astronomical Society, HEAD meeting #16; 2017

American Astronomical Society

T Corona Borealis is one of four known Galactic recurrent symbioticnovae, red giant-white dwarf binaries from which multiple thermonuclearrunaway (TNR) events, or nova eruptions, have been observed. TNRrequires high pressure at the base of the accreted envelope, and arecurrence time of less than a century almost certainly requires bothhigh white dwarf mass and high accretion rate. The eruptions of T CrBwere observed in 1866 and 1946; if the 80 year interval is typical, thenext eruption would be expected within the next decade or two. Opticalobservations show that T CrB has entered a super-active state startingin 2015, similar to that seen in 1938, 8 years before the last eruption.In quiescence, T CrB is a known, bright hard X-ray source that has beendetected in the Swift/BAT all-sky survey. Here we present the result ofour NuSTAR observation of T CrB in 2015, when it had started to brightenbut had not yet reached the peak of the super-active state. We were ableto fit the spectrum with an absorbed cooling flow model with reflection,with a reflection amplitude of 1.0. We also present recent Swift andXMM-Newton observations during the peak of the super-active state, whenT CrB had faded dramatically in the BAT band. T CrB is found to be muchmore luminous in the UV, while the X-ray spectrum became complexincluding a soft, optically thick component. We present ourinterpretation of the overall variability as due to instability of alarge disk, and of the X-rays as due to emission from the boundarylayer. In our view, the NuSTAR observation was performed when theboundary layer was optically thin, and the reflection was only from thewhite dwarf surface that subtended 2π steradian of the sky as seenfrom the emission region. With these assumptions, we infer the whitedwarf in the T CrB system to have a mass of ~1.2 Msun. Duringthe very active state, the boundary layer had turned partially opticallythick and produced the soft X-ray component, while drastically reducingthe hard X-ray luminosity. We will discuss the implication of variableaccretion on the total mass accumulated since the last eruption.