Recurrent Symbiotic Nova T Coronae Borealis Before Outburst (2308.10011v2)

Abstract: The results of photometric and spectral observations of T CrB obtained in a wide range of wavelengths in 2011-2023 are presented. We use the near-IR light curves to determine a new ephemeris $JD_{min} = 2455828.9 + 227.55 \times E$ for the times of light minima when the red giant is located between the observer and the hot component. The flux ratio H$\alpha$/H$\beta$ varied from $\sim 3$ to $\sim 8$ in 2020-2023, which may be due to a change in the flux ratio between the X-ray and optical ranges. It is shown that the value of H$\alpha$/H$\beta$ anticorrelates with the rate of accretion onto the hot component of the system. Based on high-speed follow-up observations obtained on June 8, 2023, we detected a variability of the HeII $\lambda 4686$ line with a characteristic time-scale of $\sim 25$ min, the amplitude of variability in the $B$-band was $\sim 0.07^m$. Simulations of the near-IR light curves accounting for the ellipsoidal effect allowed us to obtain the parameters of the binary system: the Roche lobe filling factor of the cool component $\mu=1.0$, the mass ratio $q=M_{cool}/M_{hot} \in [0.5, 0.77]$, the orbital inclination $i \in [55^\circ, 63^\circ]$. A comparison of the light curve obtained in 2005-2023 with the 1946 outburst template made it possible to predict the date of the upcoming outburst - January 2024.

• The paper presents detailed photometric and spectroscopic analyses revealing dynamic changes in T Coronae Borealis' accretion environment.
• It derives a new orbital period of 227.55 days and system parameters using near-IR light curve modeling, confirming a Roche lobe filling factor of 1.0.
• The study correlates spectral line variations with UV and X-ray flux shifts to predict an imminent nova outburst by January 2024.

Analysis of T~CrB's Photometric and Spectroscopic Behavior Prior to Outburst

This essay provides an expert evaluation of the research focused on the photometric and spectroscopic analysis of the symbiotic recurrent nova T Coronae Borealis (T~CrB) over the period from 2011 to 2023. T~CrB, characterized by its periodic eruptions, has been the subject of extensive observation due to its unique properties as a binary system consisting of a red giant and a white dwarf. This paper provides detailed insights into the system's behavior prior to an anticipated nova outburst.

Observational Techniques and Data

The foundation of the research lies in a comprehensive suite of observations obtained using various methods. Near-infrared (IR) photometric data were collected to paper the ellipsoidal variability associated with the orbital motion of the binary system. Simultaneously, optical spectra were obtained to analyze the relevant emission and absorption features. The paper also incorporated UV data from past and ongoing space observatories, namely IUE and Swift/UVOT, respectively, to investigate the historical and recent UV flux variations that are indicative of changes in other parts of the electromagnetic spectrum.

Key Findings

The near-IR observations reveal the ellipsoid effect with significant amplitude, indicating the tidally distorted shape of the cool component. Using these data, a new ephemeris was derived yielding an orbital period of 227.55 days. Modeling of the near-IR light curves allowed for accurate determination of system parameters: a Roche lobe filling factor of 1.0, suggesting that the cool component fills its Roche lobe, a mass ratio within [0.5, 0.77], and an inclination angle between 55° and 63°.

Spectroscopic analyses indicate substantial variation in spectral lines, particularly the H$\alpha$/H$\beta$ ratio, which ranged from approximately 3 to 8 throughout the observation period. The authors propose this variation correlates inversely with UV radiation as an indirect measure of accretion rate variations, aligning with reported X-ray flux shifts.

Implications and Future Perspectives

The variations in both photometric and spectroscopic signals suggest significant dynamic processes occurring in T~CrB's accretion environment. The decrease in UV and optical flux with a concurrent increase in Balmer decrement implies a shift in excitation processes, likely impacted by changes in X-ray flux from the hot component. The authors extrapolate these findings to predict an imminent nova outburst by January 2024, based on the historical light curve data of T~CrB's past outbursts.

In broader scientific terms, these findings refine our understanding of the accretion dynamics and light curve evolution in symbiotic novae, with T~CrB serving as a critical model system. Given the similarities to outbursts of classical novae, future studies might focus on the role of magnetic fields and pulsation in influencing the accretion disk's structure and stability. This research paves the way for preemptive observational campaigns aimed at capturing detailed outburst mechanisms in real-time, expanding our grasp on the life cycle of symbiotic binaries.

Overall, this paper contributes substantially to the predictive modeling of nova systems, emphasizing the importance of multi-wavelength observational strategies in astrophysical research.