A multi wavelength study of novae in M31

Abstract

We present a comprehensive multi-wavelength study of extragalactic novae, with particular emphasis on rapidly recurring systems and their ultraviolet (UV) properties. Novae are thermonuclear runaway reactions that occur on the semi-degenerate surfaces of accreting white dwarf stars in close binary systems. These are important astrophysical laboratories for understanding binary evolution, accretion physics, shock formation, and galactic chemical enrichment. Through systematic observations using space-based UV and X-ray telescopes, along with ground-based optical observations, this work contributes to our understanding of nova physics across different evolutionary phases and recurrence timescales. We present the first systematic UV survey of novae in M31 using archival UVIT data. Through analysis of 19 different fields covering significant portions of M31, we developed background subtraction techniques to detect faint novae, particularly in the bright galactic bulge region. The resulting catalogue includes 42 novae detected in both outburst and quiescent phases. Quiescent novae showed signs of an accretion disc, whereas erupting novae showed signs of a shrinking photosphere. This work addressed the observational gap in systematic quiescent UV studies of extragalactic novae. The focus then shifts to M31N 2008-12a, a remarkable rapidly recurring nova with annual eruptions from 2008 to 2024. Through multi-wavelength observations covering eight consecutive eruptions (2017-2024), we investigated the unparalleled consistency in light curve morphology, spectroscopic properties, and supersoft Xray evolution. The analysis reveals that M31N 2008-12a hosts a white dwarf near the Chandrasekhar limit, making it a potential Type Ia supernova progenitor. We found strong evidence for jet-like bipolar structures that provide insights into nova explosion geometry. Anti-correlated UV and X-ray fluxes during the supersoft phase hinted towards disc activity resuming soon after each eruption. This leads to a study on rapidly recurring systems, focusing on LMCN 1968-12a, the first discovered extragalactic recurrent nova. Using simultaneous UV and X-ray observations of its 2024 eruption, we investigated the survival of the accretion disk following nova outbursts. The multi-wavelength light curves reveal distinct emission components: super soft X-rays from the white dwarf surface at temperatures of ∼ 106 K, and UV emission from an irradiated accretion disk at ∼ 2 × 104 K. The persistence of UV emission during the plateau phase provides strong evidence for disk survival and rapid reformation within days of the eruption. We end the study of individual events with a detailed study of the slow classical nova AT 2023tkw, discovered by the GROWTH-India Telescope in M31. The nova exhibited a complex light curve with multiple peaks and dips. Optical infrared observations reveal internal shock mechanisms responsible for the observed light curve variability, challenging traditional nova models and demonstrating the importance of shock-induced heating in nova explosions. The thesis ends with a summary of the key outputs from the thesis. We also briefly describe how synergies between upcoming and modern space-based UV and X-ray observatories and ground-based optical, infrared and radio observatories will lead to a comprehensive multi-wavelength understanding of novae populations, and will also be a key to unlocking the mysteries of outlier events. These results contribute to our understanding of nova physics, particularly the role of accretion discs in rapid recurrence, the nature of massive white dwarf systems, shock mechanisms in classical novae, and the potential connections between rapidly recurring novae and Type Ia supernova progenitors. Through this comprehensive approach, the work successfully covers a diverse class of novae ranging from slow classical systems to the fastest-recurring extragalactic novae known, providing insights into the opposite ends of the spectrum of nova behaviour and evolution.