Cataclysmic variables (CVs) are close binary systems consisting of a white dwarf primary and a late-type main sequence secondary star. Novae are a sub-class of cataclysmic variable characterized by the presence of a sudden increase of brightness, called outbursts, that occurs due to thermonuclear runway in the envelope of the primary. The ensuing rapid release of nuclear energy causes the photometric luminosity to increase beyond the Eddington limit. In this process, white dwarf envelope expands to red giant dimensions on a dynamical time-scale, ejecting a mass between 10^-6 to 10^-4 solar mass at a speed up to few thousands km/s, causing the system brightness to increase typically by 10-20 mag. Early hydrodynamic simulations reveal that only a fraction of the envelope is ejected in the initial explosion. The remaining envelope quickly returns to hydrostatic equilibrium and enters a steady nuclear burning phase with a constant bolometric luminosity close to Eddington limit.
The rate of flux decline in novae is tightly correlated with their absolute magnitude at maximum. The study of novae in the external galaxies is important to infer their distances, as these objects are one of the brightest standard candles up to the Virgo cluster and the tracers of differences in the stellar content among galaxies. Their study is further important to determine their contribution to the interstellar medium, elemental abundances and nature of the outburst mechanism, and the study of physical processes such as dust formation.
M31 has been a target to search novae for a long time. As a microlensing survey program is ideally suited to monitor the flux variation during transient events, we have extended our microlensing search to detect nova outbursts in M31. Since M31 novae are much brighter than a typical microlensing event during their outburst phase, they can be detected as a resolved star.
We found two novae in the Nainital Microlensing Survey in our 13'x13' field of the M31 disk. The nova NMS-1 has been tracked from the initial increase in flux while nova NMS-2 has been observed during its descending phase in brightness. The photometry of the nova NMS-1 during the outburst phase suggests its peak R and I magnitudes at the brightest phase are -7.96 and -8.02 mag respectively with a rate of decline of ~ 0.11 mag/day which suggests that it was a fast nova. The correlation of rate of decline with the observed peak flux is in good agreement with the maximum magnitude versus rate of decline(MMRD) curve for the M31 novae. NMS-1 became cooler with time about six weeks after the maximum brightness phase accompanied by a slow increase in I flux though the R flux still continues to decrease which is in contrast with the normal behaviour of the novae.
The decay rate for nova NMS-2 suggests that either it was a slow nova or we observed it at a very late stage after the maximum brightness phase. From the photometric light curve of nova NMS-2, we suspect a secondary bump, however, it is not unusual for a typical nova evolution.
(See detail in Joshi et al. (2004) and Joshi et al. (2012)
Dr. Yogesh C. Joshi
Scientist-F
Aryabhatta Research Institute of Observational Sciences (ARIES)
Ministry of Science and Technology, Govt. of India
Nainital-263001
India
Office Room: 125
New Building, First Floor
Office Phone: +91-(0)5942-270754
Office Extn.: 754
Email: yogesh@aries.res.in
Last modified Thursday, 01 March 2024