Extreme Ultraviolet ExplorerObservations of OY Carinae in Superoutburst

Abstract

The Extreme Ultraviolet Explorer (EUVE) satellite was used for three days beginning on 1997 March 26.96 UT to obtain photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova OY Carinae in superoutburst. Because of the longer time on source (143 ks), the larger number of eclipses observed (17), and the higher count rate in the detector (0.5-2.2 counts s -1), we are able to significantly strengthen previous reports that there is little or no eclipse by the secondary of the EUV emission region of OY Car in superoutburst. The mean EUVE spectrum extends from 70 to 190 A and contains broad (FWHM ≈ 1 A) emission lines of N V, O V-O VI, Ne V-Ne VII, Mg IV-Mg VI, Fe VI-Fe VIII, and possibly Fe XXIII. Good fits of the observed spectrum are obtained with a model (similar to that of Seyfert 2 galaxies) wherein radiation from the boundary layer and accretion disk is scattered into the line of sight by the system's photoionized accretion disk wind. It is possible to trade off continuum luminosity for wind optical depth, but reasonable models have a boundary layer temperature Tbl ≈ 90-130 kK and a boundary layer and accretion disk luminosity Lbl = Ldisk 4 × 1034 ergs s-1 ≈ 10 L☉ corresponding to a mass accretion rate a 10-8 M☉ yr-1; an absorbing column density NH ≈ 1.6-3.5 × 1019 cm-2; and a wind mass-loss rate w 10-10 M☉ yr-1 ≈ 0.01a. Because radiation pressure alone falls an order of magnitude short of driving such a wind, magnetic forces must also play a role in driving the wind of OY Car in superoutburst.