Evolution of the recent high-accretion state of the recurrent nova T CrB: HST, Swift, NuSTAR, and XMM-Newton observations

¹ Universidad Nacional de Hurlingham (UNAHUR). Laboratorio de Investigación y Desarrollo Experimental en Computación, Av. Gdor. Vergara 2222 Villa Tesei Buenos Aires, Argentina
² Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
³ Mullard Space Science Laboratory, University College London Holmbury St Mary Dorking Surrey RH5 6NT, UK
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 University, Dept. of Astronomy 550 West 120th Street New York NY 10027, USA
⁷ School of Physics and Astronomy, University of Leicester Leicester LE1 7RH, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 September 2025
Accepted: 18 January 2026

Abstract

As the recurrent nova T Coronae Borealis (T CrB) approaches its next predicted thermonuclear eruption, it is currently exhibiting a “super-active state” (SAS) characterized by enhanced multiwavelength emission similar to the behavior recorded prior to the 1946 outburst. We present a multiwavelength analysis of the SAS and the subsequent “faint state” using observations from HST, Swift, NuSTAR, and XMM-Newton. Our results indicate that the SAS was driven by an increase in the mass accretion rate, which caused the accretion disk’s boundary layer to become optically thick. A weighted least squares regression analysis quantifies the evolution of the accretion components, displaying a highly significant (4.5σ) increase in the luminosity of the optically thin cooling flow (L_cf) and a marginal (2.58σ) decrease in the optically thick boundary layer luminosity (L_bb) as the system transitioned into the faint state. We find that this dimming is consistent with an intrinsic change in the accretion flow rather than dust obscuration, supported by the lack of infrared excess and the stability of the 2175 Å feature. Additionally, a time-series analysis using autoregressive modeling to account for correlated red noise revealed no significant periodicities, thereby disputing the previously reported ∼6000 s signal. These findings suggest that the pre-outburst evolution of T CrB is characterized by significant changes in the accretion disk structure and boundary layer, providing a self-consistent physical framework for the system’s behavior as it approaches eruption.