Cataclysmic Variables Christian Knigge University of Southampton Christian

Cataclysmic Variables: Christian Knigge University of Southampton Christian Knigge P. Marenfeld and NOAO/AURA/NSF 10 Breakthroughs in 10 Years University of Southampton School of Physics & Astronoy

Outline • Introduction – Cataclysmic variables: a primer 987654 3 2 1 35 minutes • XX 10 breakthroughs in 10 years XXXXX (a personal and hugely biased perspective. . . ) • Evolution • Accretion • Outflows Links to Other Systems (BH/NS LMXBs) • The Role of UV Astronomy • Summary Christian Knigge University of Southampton School of Physics & Astronoy

Cataclysmic Variables: A Primer The Physical Structure of CVs • White dwarf primary – UV bright • “Main-sequence” secondary Red Dwarf White Dwarf • 75 mins < Porb < 6 hrs • Roche-lobe overflow • Accretion usually via a disk ― UV-bright • Disk accretion is unstable if below critical rate Accretion Disk • dwarf novae • Mass transfer and evolution driven by angular momentum loss Credit: Rob Hynes • Evolution is (initially) from long to short periods Christian Knigge University of Southampton School of Physics & Astronoy

Cataclysmic Variables: A Primer The Orbital Period Distribution and the Standard Model of CV Evolution Knigge 2006 Christian Knigge • University of Southampton School of Physics & Astronoy

Breakthrough I: Evolution Disrupted Angular Momentum Loss at the Period Gap • Standard model prediction Howell et al. 2001 – 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) Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough I: Evolution Disrupted Angular Momentum Loss at the Period Gap Patterson et al. (2005), Knigge (2006) M-R relation based on eclipsing and “superhumping” CVs • Donors are significantly larger than MS stars both above and below the gap • Clear discontinuity at M 2 = 0. 20 M☼, separating long- and short-period CVs! – Direct evidence for disrupted angular momentum loss! Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough II: Evolution Reconstructing CV Evolution Empirically • Knigge, (2006) Baraffe & Patterson (2011) Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough III: Evolution Period 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 Observational reality pre-2006 – 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? Christian Knigge Howell et al. 2001 • University of Southampton School of Physics & Astronoy

Breakthrough III: Evolution Period Bouncers with Brown Dwarf Secondaries • 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 Littlefair et al. 2006, Science, 314, 1578 et al. 2009; see later) • A few of these are eclipsing, allowing precise system parameter determinations • At least 3 of these have M 2 < 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… Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough III: Evolution Period Bouncers with Brown Dwarf Secondaries Littlefair Patterson et al. (2007) et al. (2008) • Littlefair et al. (2007) Stehle et al. (1999) Uthas et al. (2011) Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough IV: Evolution The 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 • Observational reality pre-2009 – No convincing spike anywhere near Pmin in the CV Porb distribution Barker & Kolb 2003 Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough IV: Evolution The 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 Previously known CVs et al. 2009) CVs do in fact “bounce” at Pmin! SDSS CVs Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough V: Evolution CVs in Globular Clusters • A typical GC should contain ~100 CVs purely based on its stellar mass content CV space density: (e. g. Pretorius & Knigge 2007, 2011) Effective volume of MW: • But bright X-ray binaries are overabundant in GCs by ~100 x (Clark 1975, Katz 1975) – New dynamical formation channels are available in GCs • tidal capture (Fabian, Pringle & Rees 1976) • • White Dwarf Expected # of CVs in MW: Fraction of MW mass in GCs: 3 - and 4 -body interactions # of GCs in MW: 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: – – – → expected # of CVs per GC: 3 -body exchange encounter 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? Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough V: Evolution CVs in Globular Clusters • Early searches typically found only a handful per GC Shara et al. (1996) (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? Shara et al. (1996) Difference of the Core of 47 Tuc 47 Imaging Tuc with Chandra 47 Tuc with the ROSAT HRI (Grindlay et al. 2001; et. Heinke et al. 2005) (Hasinger al. 1994) Pooley & Hut (2006) • 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! Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough V: Evolution CVs in Globular Clusters • UV astronomy has also played a key role FUV (~1500 A) The core of 47 Tuc: U-band – 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? Knigge et al (2002, 2003, 2008) Christian Knigge University of Southampton School of Physics & Astronoy

Breakthroughs VI and VII: Accretion / Outflows Outburst Hysteresis and Jets • 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, GX 339: Gallo et SS Cyg: Koerding etal. (2004) 2008, Science Dwarf nova eruption (optical): SS Cyg Wheatley et al (2003) 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)? – • Gallo et al. 2004 No constraints on outburst hysteresis X-ray transient outburst (X-ray): GX 339 Gallo et al (2004) 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 • Adapted from Fender, Belloni & Gallo 2004 Discovery of the first CV radio jet! Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough VIII: Accretion Periodic Variability: Oscillations • 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): – • Psaltis, Belloni & van der Klis 1999 NS & BH LMXBs strong correlations between different types of oscillations, especially LKO and HBO CVs also exhibit two types of oscillations – • Warner & Woudt 2004 26 CVs Is there a direct connection to LMXBs? s Yes! (Warner & Woudt [2002. . . 2010], Mauche [2003]) – DNOs : QPOs in CVs ↔ – Universality of accretion physics extends to periodic variability – Models relying on ultra-strong gravity or B-fields are ruled out Christian Knigge LKOs : HBOs in LMXBs DNOs in VW Hyi Woudt & Warner (2002) University of Southampton School of Physics & Astronoy

Breakthrough IX: Accretion Non-Periodic Variability: The RMS-Flux Relation • What about non-periodic accretion-induced variability (“flickering”)? • Stochastic variability has been closely studied in XRBs A Black CV (Pretorius Knigge 2007) 2001) Hole XRB & (Uttley & Mc. Hardy MV Lyr (Scaringi et al. 2011) • Key discovery: the “rms-flux relation” (Uttley & Mc. Hardy 2001) – • Rules out “additive” models (e. g. shot-noise) What about CVs? – Non-trivial to study: variability time-scales are much longer, AGN (Vaughan et al. 2011) so need high-cadence, uninterrupted long-term light curves An XRB (Churazov et al. 2003) Neutron Star XRB (Uttley & Mc. Hardy 2001) --> Kepler! NGC 4051 (Seyfert show 1)the • CVs also • Accretion-induced variability is universal! – rms-flux relation! (Scaringi et al. 2011) Key properties shared by supermassive BHs, stellarmass BHs, NSs and WDs Christian Knigge University of Southampton School of Physics & Astronoy

Breakthrough X: Evolution / Accretion / Outflows Do all CVs go nova? • 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) • Shara et al. 2007, Nature 446, 159 But all known novae were actually discovered as such – How can we establish the general link empirically ? • r 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 prototypical 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”. . . Christian Knigge University of Southampton School of Physics & Astronoy

Summary The 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 Christian Knigge University of Southampton School of Physics & Astronoy