Nova multiwavelength light curves: predicting UV precursor flashes and pre-maximum halts

Abstract

The dramatic brightenings of classical novae have yielded rich data sets of detailed light curves. Modelling these light curves is a challenge for any theory of classical novae. We have used our extended grid of nova outburst calculations to predict the luminosities of erupting novae expected in three electromagnetic bands – the visual, the near UV and the X-ray. Our models predict and explain many features of novae before eruption, as well as detailed characterizations of nova outbursts and post-nova declines. The evolutionary time-scales of eruption features vary by orders of magnitude, and depend on the basic nova parameters: white dwarf mass, luminosity and accretion rate. However, all light curves are found to share common features. Some of these features are unique to only one electromagnetic passband, while others show up in two, or in all three of the analysed bands. One extraordinary feature, common to all of our low-mass white dwarfs (0.65 M_⊙) novae, is that all exhibit a sharp rise followed by a more gradual decline in the near-UV luminosity, prior to the eruption in the visual luminosity. This is because the expansion of the outer layers lags behind the rise in bolometric luminosity. These predicted precursor-UV-flashes last between a few hours and a few days, and the predicted luminosity increase is between ∼0.5 and ∼3 mag. These flashes should be easily observable if a nova event is detected early and its time coverage is dense. Many observed novae exhibit a pre-maximum halt, and this feature is found in all three electromagnetic bands of many, but not all, of our nova models. We explain the presence or absence of pre-maximum halts as due to changes in the convective energy transfer regime. Finally we note cases where the maximum visual magnitude reaches as high as −8.5 mag for low-mass white dwarfs. This re-emphasizes the fact that white dwarf mass is not always the determining factor in setting a nova's peak luminosity.

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