Detecting the MRI accretion-disc instability in action

Peter Wheatleyp.j.wheatley@warwick.ac.uk

Peter Wheatley

University of Warwick

Movie: James Murray

Magneto-rotational instability (MRI)

Adapted from Balbus & Hawley 1998

Balbus & Hawley 1991

Numerical MRI simulations

• Simulations show MRI can drive angular momentum transport

• But observational handle desirable

Hawley et al.

Miller & Stone 2000

Corona should be phenomenon of the global disc

Observational consequences:

Hot disc corona

Figure: Zdziarski & Gierlinski 2004

Cataclysmic variables

• MRI on/off states in individual systems on accessible timescale

• Eclipses reveal geometry

• Global disc visible

XMM-NewtonWheatley & West 2003

Low state of Z Cha

Optical

Low state

low state

Patterson & Raymond 1985

High state

Wheatley, Mauche & Mattei 2003

Optical

SS Cyg

X-ray spectra

low state high state

Naylor et al. 1988

Wheatley & Mauche 2005

Chandra LETGHigh state of WZ Sge

NeV OVINeVI

FeVIII

X-ray spectra

low state high state

XMM-NewtonX-ray eclipse

Pratt et al. 2004High state - UX UMa

3-10 keV

0.3-2 keV

Chandra LETG

Wheatley & Mauche 2005

high state

low state

kT <= 1 keV

v=840km/s

RKep~2x1010cm

Disappears in low state

WZ Sge

Conclusions

• Probable detection of MRI-driven X-ray corona in CVs

• Evidence includes:1. Extended emission (eclipses)

2. Accretion disc dynamics (line widths)

3. Coronal temperatures (line ratios)

4. Absent in low state

• Excellent potential to follow MRI in action

EUVE spectroscopy

OY Car Mauche & Raymond 2000

SS Cyg Mauche, Raymond & Mattei 1995 U Gem Long et al 1996

• Non eclipsing in ob, eg exosat, and others?

EUVE spectroscopy

OY Car Mauche & Raymond 2000

SS Cyg Mauche, Raymond & Mattei 1995 U Gem Long et al 1996

X-ray spectra

quiescence outburst

OY Car with EUVEMauche & Raymond 2000

V~4000 km/sWZ Sge with ChandraNeVI

NeV OVINeVI

FeVIII

X-ray spectra

quiescence outburst

LETG X-ray spectra

kT <= 1 keVNH<<1.4x1021 cm-2

V=840km/sA(Fe)=0.2

kT = 6 keVNH = 1.4x1021cm-2

Peter Wheatley

XMM-Newton and Chandra

observations of dwarf novae

Overview: two topics

• XMM-Newton eclipse observations: OY Car and Z Cha

• Chandra spectroscopy of WZ Sge in superoutburst

X-ray eclipse with ROSATPratt et al 1999

XMM-Newton

Wheatley & West 2003

3 2 14

OY Car eclipse with XMM-NewtonWheatley & West, 2003

Magnetic accretion?

ROSAT light curves Van Teeseling,1997

Z ChaWheatley & West, in prep

Overview: two topics

• XMM-Newton eclipse observations, OY Car and Z Cha

• Chandra spectroscopy of WZ Sge in superoutburst

WZ Sge with Chandra

Wheatley & Mauche, in prep.

WZ Sge with Chandra

Wheatley & Mauche, in prep.

outburst orbital hump common superhump

Optical: Patterson et al. 2002

Chandra LETG

X-ray spectra

quiescence outburst

OY Car with EUVEMauche & Raymond 2000

V~4000 km/sWZ Sge with ChandraNeVI

NeV OVINeVI

FeVIII

X-ray spectra

quiescence outburst

LETG X-ray spectra

kT <= 1 keVNH<<1.4x1021 cm-2

V=840km/sA(Fe)=0.2

kT = 6 keVNH = 1.4x1021cm-2

Origin of outburst

soft X-raysFor Against

Origin of outburst

Boundary layerKnown source of X-rays (quiescent eclipses)

• Soft spectrum

• Lack of absorption in WZ Sge in outburst

• UX UMa eclipses

UX UMa eclipse

Pratt et al. 2004

outburst

Origin of outburst

• Soft spectrum

• UX UMa eclipses

Scattering in windKnown source of EUV: OY Car & WZ Sge

• Lack of strong continuum to scatter

• High-excitiation lines

• Width of emission lines

Origin of outburst

• Soft spectrum

• UX UMa eclipses

Scattering in windKnown source of EUV: OY Car & WZ Sge

• Lack of strong continuum to scatter

• High-excitiation lines

• Width of emission lines

Accretion disc

coronal emission

Explains:

• Soft spectrum

• Lack of abso in WZ Sge

• UX UMa eclipse

• Width of lines: RKep~2x1010cm

Detecting the MRI accretion-disc instability in action

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