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

TL;DR

This paper reports the first observed dramatic change in the boundary layer of the symbiotic recurrent nova T CrB, characterized by significant X-ray and UV flux variations during an optical brightening event, likely caused by a disk instability.

Contribution

It presents the first observation of a boundary layer change in T CrB linked to an optical brightening, providing new insights into accretion disk behavior in recurrent novae.

Findings

Hard X-ray emission nearly vanished during brightening

UV flux increased by at least 40 times

X-ray spectrum became softer with a new blackbody component

Abstract

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 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 American Association of Variable Stars Observers (AAVSO) V and B-band data indicates that during an optical brightening event that started in early 2014 ($\Delta$ V$\approx$1.5): (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 (iv) 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