Survival of the accretion disk in LMC Recurrent Nova 1968-12a: UV--X-ray case study of the 2024 eruption

TL;DR

This study investigates whether the accretion disk survives the 2024 eruption of the rapidly recurring RN LMCN 1968-12a by combining UV and X-ray data from AstroSat and Swift. The data reveal a rapid UV decline followed by a plateau modulated by the $1.26$-day orbital period, and a supersoft X-ray source that turns on around day 5 with a double-peaked light curve, indicative of external obscuration. X-ray spectroscopy finds a WD-surface temperature near $T \\approx 10^6$ K, while UV SEDs point to an irradiated disk with $T_{ m UV} \\approx 2\times 10^4$ K and $R_{ m UV} \\sim 2-3\,R_\\odot$, suggesting the disk survives eruption and re-establishes accretion within days. These results support disk survival in rapidly recurring novae and have implications for their recurrence behavior and potential Type Ia supernova progenitor paths.

Abstract

We report on UV and X-ray observations of the 2024 eruption of the recurrent nova LMCN 1968-12a, a rapidly recurring extragalactic system with a $\sim$4.3 year recurrence period and a massive white dwarf (WD). The eruption was discovered on 2024 August 1.8 by \textit{Swift}, and subsequently monitored using \textit{AstroSat}'s UVIT and SXT, along with Swift/UVOT and XRT. The multi-wavelength light curves reveal a rapid UV-optical decline, followed by a plateau phase exhibiting 1.26-day modulations consistent with the orbital period. The Supersoft (SSS) X-ray emission, that emerged by day 5, exhibited a double peak, suggesting variable obscuration that could be due to an inhomogeneous nova ejecta or due to a nova super-remnant along the line of sight. Time-resolved X-ray spectroscopy shows a blackbody component with T $\approx 10^6$ K. The SEDs obtained concurrently in the UV, peaking at T $\approx$ 20,000 K and with a source radius $\sim$2-3 R$_\odot$, are inconsistent with emission from the secondary star or nova photosphere alone. Instead, the UV emission is attributed to an irradiated accretion disk that survived the eruption. The persistent UV plateau and its temperature suggest that the accretion disk was not completely disrupted and resumed activity within days, consistent with recent findings in other rapidly recurring novae such as U~Sco and M31N~2008-12a.

Figures (4)

• Figure 1: UV and X-ray light curves of LMCN 1968-12a. Top: UV (left) and X-ray (right) light curves of the 2024 eruption of LMCN 1968-12a. Bottom left: UV and X-ray light curves of the 2024 eruption overplotted on the light curves of previous eruptions. Bottom right: Zoomed-in view of the second epoch of AstroSat observations, showing variability consistent with a 1.26-day period, though only a single cycle is covered. The 2024 eruption data are available in machine readable format.
• Figure 2: Top panel: AstroSat/SXT spectra observed at Epoch I (2024 August 15 16:19-22:35 UT; $t_{\mathrm exp} \approx 100~$min) and Epoch II (2024 August 21 14:28-14:58 UT; $t_{\mathrm exp} \approx 30$ min). Bottom panel: Swift/XRT spectra on 2024 August 16 00:23-00:45 UT ($t_{\mathrm exp} \approx 22$ mins) and August 21 14:14-14:36 UT ($t_{\mathrm exp} \approx 22$ mins), at epochs close to the AstroSat observations. Best-fit tbabs $\times$ bb models are shown in red. The fit residuals are also shown for each subplot. The best-fit parameters are given in Figure \ref{['fig:SED']}.
• Figure 3: XRT light curve of the 2024 eruption is shown in the top panel. Evolution of the supersoft black body temperature and N$_{H}$ are plotted in the middle and bottom panels, respectively. The 68% and 90% confidence intervals for temperature and N$_H$ are also shown.
• Figure 4: Left: Blackbody fits to the X-ray (SXT and XRT) and UV (UVIT and UVOT) data are overplotted on the observed spectra for epoch I (top) and epoch II (bottom); the corresponding best-fit parameters are listed in the legends. Right: The top panel shows the temporal evolution of the flux in different UV filters, while the bottom panel presents the evolution of the inferred model parameters, i.e., the radius and effective temperature of the UV-emitting source.