K.Byckling, J. Osborne, G. Wynn, A. Beardmore, V. Braito (UoL), K. Mukai (NASA/GSFC), R. West (UoL)

March 15-19, 2009Wild 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 9th magnitude star

First CV with non-radial WD pulsations in quiescence (Warner & van Zyl 1998, van Zyl et al. 2000) 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 2007Second 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, & Wheatley2000)

Swift UVOT2200-4000 Å

AAVSO

Swift XRT0.3-10 keV

72-130 Å

2-15 keV

2-15 keV

72-130 Å

26 d 10 d 15 d

? ?

The X-ray spectra

S1S2

S3

S5S4

S1S2

S4 S5wabs Nh (x 10^20 cm^-2) Temperatures

S1: S4: kT_bb = keV S2: S5: Max. mekal T of S1 ~6 keV. S3: Abundance:

S3

05.004.083.23

38.599.361.8

23.117.115.8

28.738.325.13

86.636.463.14

001.0001.0013.0

01.001.002.0

X-ray hardness ratio

Soft0.3-1.0 keV

Hard1.0-10 keV

Hard/Soft

X-ray luminosity:

The black body luminosity notwell-constrained

In 0.3-10 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-raydata

Disc modelsWhy 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)

ConclusionsThe 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