On Variation Mechanisms in Recurrent Nova IM Normae

Abstract

Light curves by Woudt and Warner (WW) of recurrent nova IM Nor show eclipse-like dips that they saw as too wide for eclipses alone, and interpreted as mainly a reflection effect due to irradiation of the companion (mass donor) star with some amplitude increase due to eclipse of IM Nor’s disk. A mainly reflection interpretation cannot be made to work because reflection does not produce dips over a restricted phase range but a somewhat distorted sinusoid that extends over the entire orbital cycle. Here, the dip features are interpreted in two ways, with testing via quantitative light curve modeling that includes an equipotential disk. One way is as alternating eclipses of and by the disk that surrounds this cataclysmic variable’s accreting white dwarf, rather than purely a succession of disk-by-star eclipses. WW’s estimated period of 0.d1026 was accordingly doubled to 0.d2052, with the observed dips now half of their previous width in phase, and with the modeled eclipses matching the observed dips in width and shape. In the 2nd interpretation, a toroidal disk’s capability to produce very wide eclipses is demonstrated computationally. Furthermore, much of the perceived eclipse width can be recognized as an apparent effect due to tidal stretching of the companion star and the disk. In overview, disk eclipses and tidal variation combine with reflection to produce a light curve waveform of approximately the observed shape and duration. Eclipses, tides, and reflection all have essential roles in the 2nd interpretation and no change from WW’s period is needed. Radial velocity observations will be crucial for identification of the correct resolution of the ”excessively wide eclipse” problem.