On the Balmer jump in dwarf novae during the outburst

Abstract

We investigate the published observations of the size of the Balmer jump in dwarf novae during the course of an outburst. We find that there exist two kinds of behaviour even in the same object: in one type of outburst the Balmer jump is zero throughout the outburst, in the other it is very strong in absorption during the rise, gradually declines as the outburst proceeds, and becomes zero some time after outburst maximum. We investigate possibilities of modelling the observed Balmer jump and its development with the available models of spectra of accretion discs and find that atmosphere structures as provided by star-like accretion disc spectra cannot possibly account for the observations; a much flatter temperature gradient is required. We suggest that the way out is to take energy generation in the atmosphere into account in the models.