The Hα light curves of novae in M31

We present Hα and B light curves for 11 M31 novae, four of which were well observed near maximum. These data, along with the Hα light curves of two Galactic novae, demonstrate that a nova's maximum Hα flux occurs days or weeks after its continuum maximum at a monochromatic intensity 1-2 magnitudes above its peak flux in B. Moreover, after this maximum is achieved, a typical nova will radiate a third as many photons in Hα as in the entire B bandpass. The most interesting part of a nova's Hα light curve, however, is its decline. We find that, regardless of a nova's speed, its Hα decay rate after maximum is almost identical to its decay rate in B. Because the effective temperature of the central source is most likely increasing during this time, this behavior suggests that most of a nova's optical luminosity during early decline is continuum emission from the nebula, rather than direct radiation from the central source. Although the data are limited, we find no correlation of maximum Hα magnitude with nova speed, and no evidence that an Hα maximum magnitude-rate of decline relation exists. However, the Hα emission is still a useful tool for studying the underlying population of nova progenitors. To facilitate such probes, we present the mean Hα lifetimes for novae for a set of limiting magnitudes. These values will enable extragalactic nova rates to be derived from multiepoch Hα surveys.