Fast and steady re-brightening of the accretion disk of T CrB past the deep minimum of August-September 2023

ATel #16404; U. Munari (INAF Padova), P. Ochner (UNIPD), S. Dallaporta, P. Valisa, A. Vagnozzi, S. Moretti, A. Bergamini, and G. Cherini (ANS Collaboration)
on 13 Jan 2024; 17:53 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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During 2015-2023 the symbiotic recurrent nova T CrB has undergone a super-active accretion phase (SAP; Munari, U., et al. 2016, NewA, 47, 7), very similar to the one that preceded the 1946 eruption (Payne-Gaposchkin and Wright, 1946, ApJ, 104, 75), a fact that has risen the expectations for an imminent new outburst (Schaefer, B. E., 2023, MNRAS, 524, 3146; Munari, U., 2023, RNAAS, 7, 145; Zamanov, R., et al., 2023, A&A, 680, L18).

The accretion disk of T CrB is not eclipsed by the M3III giant companion during the orbital motion, owing to the i=63/67deg orbital inclination. During SAP the disk outshined the giant in U-band by nearly 4 magnitudes, so that U brightness appears as an excellent proxy for the brightness of the accretion disk of T CrB.

We have been collecting BVRI photometry of T CrB since 2006 with various telescopes operated by ANS Collaboration, and UBV photometry since 2020 with the Asiago 67/92cm Schmidt telescope. Comprehensive light- and color-curves are presented in the attached figure, where the behavior during the last 13-months is highlighted in the zooming panels on the right. In these panels, the continuous curves represent the orbitally-modulated brightness of the M3III giant which fills its Roche lobe; they are computed following the recipe in Munari (2023, RNAAS, 7, 251), and represent the overall system brightness of T CrB for a negligible contribution by the accretion disk.

The behavior exhibited by T CrB in B, (B-V), and primarily in U clearly indicates that the accretion disk reached a deep minimum in brightness around late August/early-September 2023. At that time the emission lines almost disappeared from the spectra of T CrB that we regularly obtain with the Asiago 1.82m and 1.22m, Varese 0.84m, and Stroncone 0.50m telescopes. Immediately after having reached such a minimum, T CrB bounced back and started to re-brighten at a fast and steady pace as shown by the attached figure, suggesting that the accretion disk is also quickly re-brightening. Our last photometric observation for 2024 January 12.14 UT reports T CrB at U=11.715, B=11.355, V=9.905, R=8.762, and I=7.369. The brightness increase in U is 1.4mag over the minimum of late August/early-September 2023, and correcting for the contribution of the M3III giant, this supports a 6x re-brightening of the accretion disk. This is nicely confirmed by the emission lines that are regaining intensity in our spectra.