Cataclysmic Variables: 10 Breakthroughs in 10 Years

Christian Knigge University of SouthamptonSchool of Physics & Astronoy

Cataclysmic Variables:10 Breakthroughs in 10 Years

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Christian KniggeUniversity of Southampton


Outline• Introduction

– Cataclysmic variables: a primer

• 10 breakthroughs in 10 years (a personal and hugely biased perspective...)

• Evolution • Accretion• Outflows

• The Role of UV Astronomy

• Summary

35 minutes 9 8 7 6 5 4 3 2 1

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Links to Other Systems(BH/NS LMXBs)


Cataclysmic Variables: A PrimerThe Physical Structure of CVs

White Dwarf

Accretion Disk

Red Dwarf

• White dwarf primary– UV bright

• “Main-sequence” secondary

• 75 mins < Porb < 6 hrs

• Roche-lobe overflow

• Accretion usually via a disk― UV-bright

• Disk accretion is unstable if below critical rate• dwarf novae

• Mass transfer and evolution driven by angular momentum loss

• Evolution is (initially) from long to short periods

Credit: Rob Hynes


Cataclysmic Variables: A PrimerThe Orbital Period Distribution and the Standard Model of CV Evolution

• Clear “Period Gap” between 2-3 hrs

• Suggests a change in the dominant angular momentum loss mechanism:

– Above the gap: • Magnetic Braking • Fast AML High

– Below the gap: • Gravitational Radiation• Slow AML Low

• Minimum period at Pmin ≈ 80 min

– donor transitions from MS BD

– beyond this, Porb increases again

• This disrupted magnetic braking scenario is the standard model for CV evolution

Knigge 2006

Christian Knigge

Breakthrough I: EvolutionDisrupted Angular Momentum Loss at the Period Gap

• Standard model prediction

– The period gap is caused by a disruption in AML when the donor becomes fully convective

• Magnetic braking drives high above the gap• Donor is slightly out of TE and thus oversized• At , donor becomes fully convective• MB ceases (or is severely reduced)• drops --> donor relaxes (shrinks) to TE

radius• Donor loses contact with RL• CV evolves through gap as detached binary• Residual AML (e.g. GR) shrinks orbit (and RL) • Contact with donor re-established at

• Observational reality pre-2005

– No direct empirical support for this picture (other than the existence of the gap itself)

University of SouthamptonSchool of Physics & Astronoy

Howell et al. 2001

M

M

3orb sP hr

2orb sP hr


Patterson et al. (2005), Knigge (2006)

• Donors are significantly larger than MS stars both above and below the gap

• Clear discontinuity at M2 = 0.20 M☼, separating long- and short-period CVs!

– Direct evidence for disrupted angular momentum loss!

0.2gapM M

M-R relation based on eclipsing and

“superhumping” CVs

Breakthrough I: EvolutionDisrupted Angular Momentum Loss at the Period Gap


• We can even use the donor relation to quantitatively reconstruct CV evolution• CV Donors are significantly larger than MS stars because they are bloated by mass loss

– Higher Larger

• So we can use the degree of donor bloating at given to infer

• Above the gap: slightly reduced “standard” MB recipes work well• Below the gap: need enhanced AML,

significant revision of the standard model!

Breakthrough II: EvolutionReconstructing CV Evolution Empirically

Knigge (2006)Knigge, Baraffe & Patterson (2011)

Christian Knigge

Breakthrough III: EvolutionPeriod Bouncers with Brown Dwarf Secondaries

• Standard model predictions

– 99% of CVs should be found below the period gap

– A full 70% should be “period bouncers” with brown dwarf secondaries


– Not a single definitive period bouncer• Only ~10 candidates out of ~1000 CVs

– No secondary with a well-established mass below the H-burning limit

– Is this a selection effect or model failure?University of Southampton

School of Physics & Astronoy

How

ell e

t al.

2001

Christian Knigge


• SDSS has yielded a deep new sample of ~200 CVs (Szkody et al. 2002-9)...

• ...including a sub-set of faint, WD-dominated systems near Pmin (Gaensicke et al. 2009; see later)

• A few of these are eclipsing, allowing precise system parameter determinations

• At least 3 of these have M2 < 0.072 M☼ (Littlefair et al. 2006, 2008)

At least some post-period-minimum systems with brown dwarf donors do

exist!

But one of them is very strange…


Littlefair et al. 2006, Science, 314, 1578

Christian Knigge

Stehle et al. (1999)


• SDSS J1507 is one of the three eclipsing CVs with sub-stellar donors…

• … but its for other CVs

• Two ideas:

– J1507 is young -- born with a sub-stellar donor (Littlefair et al. 2007)

– J1507 is a low metallicity halo CV (Patterson et el. 2008)

How can we test which is correct?

• UV astronomy to the rescue!– FUV spectroscopy shows that [Fe/H] = -1.2

• SDSS J1507 is an eclipsing period bouncer in the Galactic halo!

– Rosetta stone for studying effects of metallicity on accretion and evolution?


Littlefair et al. (2007)Patterson et al. (2008)

Littlefair et al. (2007)

Uthas et al. (2011)

Christian Knigge

Breakthrough IV: EvolutionThe Period Spike at Pmin

• Standard model prediction

– The number of CVs found in a particular Porb range is inversely proportional to the speed with which they evolve through it

– So there should be a spike at Pmin, in the period distribution since


– No convincing spike anywhere near Pmin in the CV Porb distribution


Barker & Kolb 2003

1) |( |CV orb orbN PP

( ) 0orb minPP

Christian Knigge

Previously known CVs

SDSS CVs

Breakthrough IV: EvolutionThe Period Spike at Pmin

• Boris Gaensicke and collaborators have obtained orbital periods for most of the new SDSS CVs

• The resulting period distribution does show a spike at Pmin for the first time (Gaensicke et al. 2009)

CVs do in fact “bounce” at Pmin!


Gaensicke et al. 2009

Christian Knigge

Breakthrough V: EvolutionCVs in Globular Clusters

• A typical GC should contain ~100 CVs purely based on its stellar mass content

• But bright X-ray binaries are overabundant in GCs by ~100x (Clark 1975, Katz 1975)

– New dynamical formation channels are available in GCs• tidal capture (Fabian, Pringle & Rees 1976)• 3- and 4-body interactions

• Could CV numbers also be enhanced?– Theory says yes, but “only” by a factor of ~2

(di Stefano & Rappaport 1994, Davies 1995/7, Ivanova et al. 2006)

• There should be hundreds of accreting WDs in GCs!

• Important and useful:– Large samples of CVs at known distances– Drivers and tracers of GC dynamical evolution – Are GCs SN Ia factories? (Shara & Hurley 2006)

So where are they?


CV space density: (e.g. Pretorius & Knigge 2007,

2011)

Effective volume of MW:

Expected # of CVs in MW:

Fraction of MW mass in GCs:

# of GCs in MW:

→ expected # of CVs per GC:3-body exchange encounter

White Dwarf

Christian Knigge



Shara et al. (1996)Difference Imaging of the Core of 47 Tuc

• Early searches typically found only a handful per GC (e.g. Shara et al. 1996, Bailyn et al. 1996, Cool et al. 1998)

– Are CVs not formed or maybe even destroyed in CVs?• Significant implications for GC dynamics!

– Selection effects?• Survey depth?• Dwarf nova duty cycle?

• X-rays would be a great way to find CVs in GCs– But this used to be really hard!

• Chandra has revolutionized the field– Deep X-ray surveys typically find tens per cluster– Numbers scale with collision rate

dynamical formation matters!

GCs do harbour significant populations of dynamically-formed CVs!

47 Tuc with the ROSAT HRI(Hasinger et al. 1994)

Shara et al. (1996)

Pooley & Hut (2006)

47 Tuc with Chandra (Grindlay et al. 2001; Heinke et al. 2005)

47 Tuc with Chandra (Grindlay et al. 2001; Heinke et al. 2005)

Christian Knigge

• UV astronomy has also played a key role

– Efficient way of finding new CVs and confirming X-ray-selected candidates (Knigge et al. 2002, Dieball et al. 2005, 2009, 2010, Thomson et al. 2012)

– Even slitless multi-object spectroscopic identification/confirmation is possible!

• Still many key unsolved questions!

– Are there enough CVs in GCs?

– Are they different from field CVs?

– Where are the double WDs?

– Are there SN Ia progenitors?

The core of 47 Tuc: U-bandThe core of 47 Tuc: FUV (~1500A)


Knigge et al (2002, 2003, 2008)


Christian Knigge

Dwarf nova eruption (optical): SS CygWheatley et al (2003)

X-ray transient outburst (X-ray): GX 339Gallo et al (2004)

Adapted from Fender, Belloni & Gallo 2004

Breakthroughs VI and VII: Accretion / Outflows

Outburst Hysteresis and Jets


Gallo et al. 2004

GX339: Gallo et al. (2004)SS Cyg: Koerding et al. 2008, Science• Both CVs (dwarf novae) and XRBs (X-ray transients) exhibit outbursts– Thermal/viscous disk instability

• XRBs– Outbursts trace a q-shape in the X-ray hardness vs intensity plane

(Fender, Belloni & Gallo 2004) hysteresis

– Collimated (radio) jets are seen (almost only) in the hard state

– Hard-soft transition produces a powerful jet ejection episode

• CVs (pre-2008)– No evidence for collimated jets in any CV

• Constraint on theories of jet formation (e.g. Livio 1999)?

– No constraints on outburst hysteresis

• Elmar Koerding et al. (2008)

– Do dwarf novae also execute a q-shaped outburst pattern?

• Yes they do!

– Best chance to see a powerful jet is during the “hard-to-soft” transition during the rise to a dwarf nova outburst

• Discovery of the first CV radio jet!

Christian Knigge

• Both XRBs and CVs often exhibit (quasi-)periodic oscillations on short (~dynamical) time-scales

• Origin is poorly understood, but intimately connected to accretion/outflow processes in the innermost disk regions

• Key result in XRBs (accreting NSs and BHs):– strong correlations between different types of oscillations, especially LKO and

HBO

• CVs also exhibit two types of oscillations

– Is there a direct connection to LMXBs?s

• Yes! (Warner & Woudt [2002...2010], Mauche [2003])

– DNOs : QPOs in CVs ↔ LKOs : HBOs in LMXBs

– Universality of accretion physics extends to periodic variability

– Models relying on ultra-strong gravity or B-fields are ruled out


Breakthrough VIII: AccretionPeriodic Variability: Oscillations

Psaltis, Belloni & van der Klis 1999

Warner & Woudt 2004

NS & BHLMXBs

26 CVs

DNOs in VW HyiWoudt & Warner (2002)

Christian Knigge

An XRB (Churazov et al. 2003)

A CV (Pretorius & Knigge 2007)


Breakthrough IX: AccretionNon-Periodic Variability: The RMS-Flux Relation

Black Hole XRB (Uttley & McHardy 2001)

Neutron Star XRB (Uttley & McHardy 2001)AGN (Vaughan et al. 2011)

NGC 4051(Seyfert 1)

• What about non-periodic accretion-induced variability (“flickering”)?

• Stochastic variability has been closely studied in XRBs

• Key discovery: the “rms-flux relation” (Uttley & McHardy 2001)

– Rules out “additive” models (e.g. shot-noise)

• What about CVs?– Non-trivial to study: variability time-scales are much longer,

so need high-cadence, uninterrupted long-term light curves

--> Kepler!

• CVs also show the rms-flux relation! (Scaringi et al. 2011)

• Accretion-induced variability is universal!– Key properties shared by supermassive BHs, stellar-

mass BHs, NSs and WDs

MV Lyr (Scaringi et al. 2011)

MV Lyr (Scaringi et al. 2011)

Christian Knigge

Shara et al. 2007, Nature 446, 159

• We all “know” that CVs burn accreted matter explosively (Fujimoto, Iben, Starrfield, Shaviv, Shara, Townsley, Bildsten, Yaron...)

→ Nova Eruptions (typical recurrence time ~10,000 yrs)

• But all known novae were actually discovered as such

– How can we establish the general link empirically ?

• Ejected nova shells may be detectable for ~1000 yrs!

• So Shara et al. (2007) searched for resolved nebulae around ordinary CVs in the GALEX imaging archive....

• ...and disovered an ancient nova shell around the proto-typical dwarf nova Z Cam

→ ordinary CVs do undergo nova eruptions!

• Postscript: Chinese astronomers would have disagreed with the classification of Z Cam as an “ordinary CV”...


r

Breakthrough X: Evolution / Accretion / OutflowsDo all CVs go nova?


SummaryThe last decade has seen several breakthroughs in our understanding of CVs, many of which were made possible by ultraviolet observations

• Evolution

– The basic disrupted-angular-momentum-loss picture of CV evolution is correct !– We know how to reconstruct CV evolution from both primary and secondary properties – CVs do exist in significant numbers in GCs– CVs not discovered as novae can still have nova shells --> all CVs experience nova eruptions

• Accretion, Outflows and Links to Other Systems– CV outbursts exhibit hysteresis (“turtlehead” diagram) – just like XRBs and AGN

– CVs can drive radio jets – just like XRBs and AGN

– Accretion-induced oscillations in CVs are… – just like those in XRBs

– Stochastic variability in CVs follows an rms-flux relation – just like XRBs and AGN

The physics of disk accretion is universal

CVs provide excellent, nearby, bright accretion laboratories

Cataclysmic Variables: 10 Breakthroughs in 10 Years

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