Indirect imaging of the accretion stream in eclipsing polars -- II. HU Aquarii

Abstract

A B S T R A C TWe apply our technique for indirect imaging of the accretion stream to the polar HU Aquarii, using eclipse profiles observed when the system was in a high accretion state. The accretion stream is relatively luminous, contributing as much as the accretion region on the white dwarf, or more, to the overall system brightness. We model the eclipse profiles using a model stream consisting of a ballistic trajectory from the L1 point followed by a magnetically channelled trajectory that follows a dipole field line out of the orbital plane. We perform model fits using two geometries: a stream that accretes on to both footpoints of the field line, and a stream that accretes only on to the footpoint of the field line above the orbital plane. The stream images indicate that the distribution of emission along the stream is not a simple function of the radial distance from the white dwarf. The stream is redirected by the magnetic field of the white dwarf at a distance 1X0±1X3 Â 10 10 cm from the white dwarf; this implies a mass transfer rate in the range 8±76 Â 10 16 g s 21 . The absorption dips in the light curve indicate that the magnetically entrained part of the stream moves from 428 to 488 from the line of centres over the three nights of observation. This is in close agreement with the results of the one-footpoint models, suggesting that this is the more appropriate geometry for these data. The stream images show that, in almost all sections of the stream, the flux peaks in B and is successively fainter in U, V and R.

The soft X-ray eclipses of HU Aqr

“…This value is in agreement with earlier measurements based on optical photometry (e.g. Schwope et al 1993;Glenn et al 1994;Harrop-Allin et al 1999), but here determined with much higher accuracy due to the absence of contaminating radiation from elsewhere in the system.…”

“…and photometry and noticed the high degree of variability of eclipse light curves indicating changes of the accretion geometry on time scales as short as hours. Hakala (1995), Harrop-Allin et al (1999), and Vrielmann & Schwope (2001) made use of the detailed shape of optical emission line and continuum eclipse light curves in order to calculate brightness maps along the different parts of the accretion stream. Using high-resolution optical spectrophotometry analysed by Doppler tomography, Schwope et al (1997) uncovered the existence of an extended ballistic accretion stream in HU Aqr.…”

Temperature determination via STJ optical spectroscopy

“…Clearly there is no evidence for a gradient in temperature along the stream. Using U BV R photometry, Harrop-Allin et al (1999) estimated the stream temperature in HU Aqr to lie in the range 6500-7400 K, with 6500 K being their preferred estimate, in satisfactory agreement with the results reported here. We thus have confidence in extending the technique to other systems.…”

Hunting for eclipses: high-speed observations of cataclysmic variables

“…It has also been studied photometrically (Southworth et al 2015). We observed a single eclipse with ULTRACAM, finding a typical polar light curve similar to that seen in HU Aqr (Harrop-Allin et al 1999). Szkody et al 2007), this system has already been studied in detail.…”