K.Byckling, J. Osborne, G. Wynn, A. Beardmore, V. Braito (UoL), K. Mukai (NASA/GSFC), R. West (UoL) March 15-19, 2009 Wild stars in the Old West Tucson, Arizona

Motivation X-ray behaviour during DNe outburst is not well- understood The two well-studied systems, SS Cyg and U Gem, behave differently Need to observe more systems

GW Lib in a nutshell! Discovered in 1983 through its first outburst (OB) as a 9 th magnitude star First CV with non-radial WD pulsations in quiescence ( Warner & van Zyl 1998, van Zyl et al )  WDs in CVs were thought to be too hot to be in the DAV instability strip Classified as a WZ Sge type DN: low mass accretion rates in quiescence & long recurrence times XMM observations of GW Lib confirmed that the max temperature and the quiescent luminosity are low kT_max = 5 keV, L = 9 x 10^28 erg/s (Hilton et al. 2007)

The Great Superoutburst in 2007 Second OB took place in April 2007 lasting ~26 days  the gap between the outbursts 24 yrs! AAVSO observers and Swift observed the second outburst in the optical, UV (UVOT) and X-rays (XRT) To date, only two other systems have simultaneous multiwavelength outburst lightcurves: SS Cyg and U Gem These have longer orbital periods than GW Lib: SS Cyg 6.6 h, U Gem 4.2 h, GW Lib 77 min Simultaneous multiwavelength data of the outbursts of SS Cyg and U Gem provide a basis to compare the outburst behaviour

The most striking difference in X-rays! Assumption: X-rays rise in the beginning  not seen in GW Lib X-rays decline in ~10 d in GW Lib (cf. SS Cyg ~less than 3 hrs) The X-ray re-brightening seen at the end in SS Cyg is not clear in GW Lib and U Gem GW Lib (Byckling et al. (in prep.)) SS Cyg (Wheatley, Mauche, & Mattei 2003) U Gem (Mattei, Mauche, & Wheatley 2000) Swift UVOT Å AAVSO Swift XRT keV Å 2-15 keV Å 26 d 10 d15 d ? ?

The X-ray spectra S1 S2 S3 S5 S4 S1 S2 S4 S5 wabs Nh (x 10^20 cm^-2) Temperatures S1: S4: kT_bb = keV S2: S5: Max. mekal T of S1 ~6 keV. S3: Abundance: S3

X-ray hardness ratio Soft keV Hard keV Hard/Soft X-ray luminosity: The black body luminosity not well-constrained In keV peak luminosity L = 2.1 x 10^32 erg/s ( Ṁ = 1.7 x 10^15 g/s) At the end of the OB L = 4.7 x 10^30 erg/s ( Ṁ = 3.9 x 10^13 g/s) No periods seen in the X-ray data

Disc models Why is the recurrence time much longer than other DNe? Why do we only see superoutbursts? Look for solution from the models for WZ Sge: Low disc viscosity in quiescence (Smak 1993) Evaporation of the inner disc combined with low viscosity (Meyer-Hofmeister et al. 1998) Truncation of the inner disc by a magnetic WD with standard viscosity (Warner, Livio, & Tout 1996; Matthews et al. 2007)

Conclusions The X-ray behaviour is not consistent with that of SS Cyg The UV and optical outburst lightcurves are more closely related in structure and shape than UV and X-rays  similar emission region? (cooler part of the disc away from the boundary layer) A black body tail seen in the first outburst spectrum  boundary layer luminosity? Long recurrence time (decades) and the lack of normal OBs suggest the disc in GW Lib is truncated or the disc viscosity is ~100 times lower than in other CVs

Credit: A. Beardmore

KB acknowledges funding from the European Commission under the Marie Curie Host Fellowship for Early Stage Research Training Spartan, University of Leicester, UK. Acknowledgments