Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

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HST Spectral Maps of Accretion Disks in Cataclysmic Binaries

Raymundo Baptista and Keith Horne
University of St.Andrews, North Haugh, St.Andrews, Fife KY16 9SS, Scotland

Abstract:

Keywords: Accretion disks, Cataclysmic variables

Introduction

The Faint Object Spectrograph on HST is being used to monitor eclipses of Cataclysmic Variables. The basic aim of the project is to use eclipse mapping techniques to derive the structure of accretion disks at different mass accretion rates and viscosity states and to test our models of disk atmospheres and chromospheres by comparison with spatially-resolved disk spectra. In this paper we report on the HST spectral mapping of the nova-like UX UMa.

Observations and Data Analysis

Time-resolved spectroscopy centered on eclipses of UXUMa was obtained with HST/FOS on 1994 August (G160L, 1100--2500Å) and 1994 November (PRISM, 1600--8500Å) at a time resolution of 5.3s, in a total of 4 data sets and 3122 spectra.

The out-of-eclipse UV spectrum (Fig.1a) shows prominent emission lines as well as many absorption features and possibly broad absorption bands, particularly near 1900Å and 2400Å. UXUMa was brighter (by up to 50% at 2000--3000Å) on 1994 Nov than on 1994 Aug. UV continuum light curves (Fig.1b) show significant flickering activity outside of eclipse, relatively much stronger than observed in the optical range. Quasi-periodic oscillations (QPO's) of period 20--30s are easily seen in the 1994 Nov light curves.

 
Figure: (a) Average out-of-eclipse (dark) and mid-eclipse (light) spectra of UX UMa for the G160L (dashed) and PRISM (solid) data. (b) Light curves of the four HST runs at the same wavelength region. Vertical dotted lines mark ingress/egress phases of the white dwarf.

The calibrated spectra were divided into 59 (G160L) and 83 (PRISM) passbands (15--40Å wide in the continuum and in the emission lines) and light curves were constructed for each one. Maximum-entropy eclipse mapping techniques (Horne 1985, Baptista & Steiner 1993) were used to solve for a map of the disk brightness distribution and for the flux of an additional uneclipsed component in each band.

Results

 
Figure: Spatially resolved spectra of UXUMa for the Aug (light) and Nov (dark) data. (a) The spectra of the accretion disk, computed for a set of concentric annular sections excluding the gas stream region. The lower panel shows the spectrum of the uneclipsed light. Error bars were derived via Monte Carlo simulations with the eclipse light curves. (b) Spectra for annular sections containing the gas stream. The disk spectra at same radius are shown as dashed lines. The lower panel shows the spectrum of the uneclipsed light as the fractional contribution with respect to the average out-of-eclipse level at each spectral bin.

 
Figure: Radial temperature profiles of the eclipse map at for the Aug (open squares) and Nov (solid circles) data. Abscissas are in units of the distance from the disk center to the inner Lagrangian point (R). The plotted points are average brightness temperatures for radial bins of width 0.05 R. Steady-state disk models for mass accretion rates of (dashed) and (dotted) M are plotted for comparison. The lowest curve is in its true temperature scale; the upper curve was vertically displaced by 0.3 dex.

Spatially resolved disk spectra (Fig.2a) show that the inner disk in the UV is characterized by a blue continuum filled with absorption lines and bands, which cross over to emission with increasing disk radius. The Balmer continuum (seen neither in emission nor in absorption at inner disk) appear in absorption at intermediate and large disk radii. The comparison of the 1994 Aug and Nov eclipse maps shows a significant (%) increase in brightness in the inner disk regions at the later epoch.

The spectrum of the disk region containing the gas stream (Fig.2b) is noticeably different from the disk spectrum at the same radius for a range of radii. This suggest that gas overflows through the impact point at disk rim and continues along the stream trajectory, producing distinct emission down to . The gas stream gives a non-negligible contribution to the CIV emission, particularly at low (Doppler) velocities, which confirms a previous suggestion by Baptista et al. (1995).

The spectrum of the uneclipsed component shows strong emission lines of Ly, NV , SiIV , and CIV (of --50% of the total out-of-eclipse flux), noticeable HeII and MgII 2800 emission, a UV continuum rising towards longer wavelengths, and a Balmer continuum in emission.

The radial temperature profiles (Fig.3) of the continuum maps are well described by a steady-state disk model. There is marginal evidence of an increase in the mass accretion rate from Aug to Nov (M), in accordance with the observed increase in brightness. Since the UXUMa disk seems to be in a high-viscosity state in both cases, this result suggests variations in the mass transfer rate from the companion star of considerable magnitude (%).

Future Work

Further analysis will extend the eclipse mapping study up to 8500Å, study and map the flickering and QPO's, and model the spatially-resolved disk spectra with state-of-the-art disk atmosphere models.

Acknowledgments:

This work was partially supported by NASA grant GO-5488 from the STScI (which is operated by AURA Inc. under NASA contract NAS5-26555).

References:

Baptista, R., Horne, K., Hilditch, R., Mason, K.O., & Drew J.E. 1995, ApJ, 448, 395

Baptista, R. & Steiner, J.E. 1993, A&A, 277, 331

Horne, K. 1985, MNRAS, 213, 129