1. Origin of outburst X-ray emission Peter Wheatley, University of Warwick

2. Dwarf nova X-ray emission Optical EUV X-ray Wheatley, Mauche & Mattei 2003

3. outburst quiescence Patterson & Raymond 1985

4. X-ray eclipse in quiescence HT Cas with ASCA Mukai et al., 1997

5. Z Cha in quiescence with XMM-Newton Hickman et al., in prep B band 0.2-12 keV

6. Z Cha optical eclipse Hickman et al., in prep WD eclipse B band

7. Z Cha X-ray eclipse Hickman et al., in prep 1.2-10 keV 0.2-1.2 keV

8. outburst quiescence Patterson & Raymond 1985 X-ray spectra Confirmed

9. Reis et al., in prep Reis et al., in prep X-ray luminosities of white-dwarf dominated CVs 1029 erg s-1 ~ 1012 g s-1 ~ 10-14 MSun yr-1

10. outburst quiescence Patterson & Raymond 1985 X-ray spectra Confirmed

11. OY Car in outburst with EXOSAT LE Naylor et al. 1988 7-250 Å

12. EUV spectra of dwarf novae in outburst Lbl~1034 erg s-1 NH~3x1019 cm-2 V~2500 km s-1 Mauche 2004 WZ Sgehigh inclination SS Cyglow inclination

13. outburst quiescence Patterson & Raymond 1985 X-ray spectra NH~3x1019 cm-2 Confirmed ? Confirmed

14. OY Car in outburst with ROSAT - no X-ray eclipse Pratt et al., 1999

15. Z Cha in outburst with XMM-Newton quiescence outburst

16. XMM-Newton spectra of Z Cha quiescence outburst

17. X-ray scattering?

18. UX UMa with XMM-Newton Pratt et al., 2004 • Nova-like used as proxy for outburst state

19. Not X-ray scattering UX UMa Pratt et al., 2004 Wheatley & Kallman 2006 NH=5x1020 cm-2

20. Nature of outburst X-ray emission • Properties • Spectrum is unabsorbed optically-thin thermal emission • weighted to low temperatures compared with quiescence • Not eclipsed • Not scattered • Only present in outburst • Lx ~ 1031 erg s-1 ~ 10-3 Ldisc • Origin • Boundary layer - No • Not eclipsed, not scattered • Secondary star - No • Too luminous, turns off in quiescence • Disc hotspot - No • Not eclipsed, turns off in quiescence • Accretion disc corona - Maybe • Shocks in radiatively-driven wind - Maybe

21. WZ Sge with Chandra LETG Outburst Outburst kT <= 1 keV v=840 km/s RKep~2x1010cm Quiescence Wheatley & Mauche 2005

22. Lx ~ 1031 erg s-1 ~ 10-3 Ldisc • Lx <= 10-3 Lstar

23. Z Cha in outburst with XMM-Newton quiescence outburst

24. Conclusions • Eclipses confirm that all quiescent X-ray emission from BL • Useful for estimating instantaneous accretion rates • Outburst EUV spectra and lack of EUV eclipse consistent with BL dominating outburst emission as well • However, outburst X-ray spectra and lack of X-ray eclipses are NOT consistent with origin in BL • Properties of outburst X-ray emission only consistent with a disc corona • Disc corona only only in outburst points to magneto-rotational instability • Outburst X-rays may provide the best observational handle on the MRI

26. Magneto-rotational instability (MRI) Balbus & Hawley 1991 Adapted from Balbus & Hawley 1998