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

¹ CONICET-Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Av. Inte. Güiraldes 2620, C1428ZAA Buenos Aires, Argentina
e-mail gjmluna@iafe.uba.ar
² Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina
³ Universidad Nacional Arturo Jauretche, Av. Calchaquí 6200, F. Varela, Buenos Aires, Argentina
⁴ CRESST and X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁶ Columbia Astrophysics Lab 550 W 120th St., 1027 Pupin Hall, MC 5247 Columbia University, New York, NY 10027, USA
⁷ Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA
⁸ University College London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking RH5 6NT, UK
⁹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Via U. La Malfa 153, 90146 Palermo, Italy
¹⁰ Departamento de Física y Astronomía, Universidad de La Serena, Av. Cisternas 1200, La Serena, Chile
¹¹ Instituto de Ciencias Astronómicas, de la Tierra y del Espacio (ICATE-CONICET), Av. España Sur 1512, J5402DSP San Juan, Argentina

Received: 29 June 2018
Accepted: 23 August 2018

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 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 (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.