PoS(Golden2015)018 http://pos.sissa.it/ ∗ [email protected] [email protected] [email protected] [email protected] Speaker. We present the first results fromoscillations a in recent Polars. campaign to A searchspectra for brief and and review time characterise of frequency quasi-periodic representations the of current thethe rapid theoretical prototypical variability Polars, model in V834 the is optical Cen, given. light areplanned from future presented Power one work and of density of discussed. this campaign We towards conclude advancing by the discussing understanding the of QPOs in mCVs. ∗ Copyright owned by the author(s) under the terms of the Creative Commons c Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). The Golden Age of Cataclysmic Variables7-12 and September Related 2015 Objects - III, Golden2015 Palermo, Italy Martine Mouchet LUTH-Observatoire de Paris, UMR 8102-CNRS, UniversitéFrance Paris-Diderot, 92190 Meudon, E-mail: David A. H. Buckley Southern African Large Telescope Foundation, PO BoxSouth 9, African Observatory, Astronomical 7935, Observatory, South PO Africa BoxE-mail: 9, Observatory, 7935, South Africa Jean-Marc Bonnet-Bidaud Service d’Astrophysique-Laboratoire AIM, CEA/DSM/Irfu, 91191 Gif-sur-Yvette,E-mail: France Hannes Breytenbach Astrophysics, Cosmology and Gravity Centre, DepartmentTown, of South Astronomy, Africa University of Cape South African Astronomical Observatory, PO BoxE-mail: 9, Observatory, 7935, South Africa Quasi-periodic oscillations in magnetic Cataclysmic Variables: Results for V834 Cen PoS(Golden2015)018 ]. ]. 13 6 , 5 , 4 Hannes Breytenbach ]) - a fact which has 8 follow a ballistic trajec- 1 of the WD primary. Once the 2 ]. The presence of polarized emission is ). Material on the surface of the companion 9 1 accretion hot-spots on the surface of the WD [ 2 two ]. Subsequent X-ray surveys by HEAO-1, EXOSAT, ROSAT, 11 , ]. 10 12 - the radius at which the magnetic energy density matches the kinetic energy density. ]. A preview of the system’s geometry may be sketched by considering 2 , 1 ] for a recent review of magnetic WDs). Formally, we can take the size of the magnetosphere to 3 Alfvén radius G (see [ 9 10 ∼ to Here we say “may”, since the ballistic inflow might not be present,The or size might of be the very magnetosphere brief, depends if primarily the on primary the magneto- magnetic field strength of the WD, which can range from In general, the accretion flow may be far more complex than the idealised (synchronous, uni- A defining feature of the highly magnetic mCVs is that they emit light that is polarized, both Magnetic Cataclysmic Variables (mCVs) are an intriguing class of interacting binaries con- 7 1 2 10 sphere extends as far as L1. ∼ be specified by the led to the sobriquet ‘Polar’ being adopted for them [ RXTE, INTEGRAL, Swift, inter alia, have30 now more brought suspected the as number such) of [ known Polars to 116 (with The detailed trajectory of the accretion stream is largely determined by the parameters of the WD polar) scenario described above. Inphotometry with the SALTICAM case on of the South FL Africaneclipse Ceti, Large mapping, for Telescope unambiguous (SALT) example, evidence has for recent revealed, through high time resolution linearly and circularly, on the order of several percent (see for example [ is lost through the system’s innertory Lagrange in (L1) the point whereafter orbital it(ionized) plane may gaseous before material encountering enters the theof magnetosphere primary’s the magnetosphere, flow becomes the comparable Lorentzing to force the - on trajectory and an to eventually element divert strongerfield out than lines of - of the the the orbital gravitational primary. planethe force, By magnetic and, caus- poles, the roughly the time speaking, flow the attach hasenough infalling on become to material highly the produce supersonic, encounters magnetic and the the the surface majoritycollimated resulting of of portion collision the the is of WD energetic total the at luminous flowsition near radiation from the emanating a WD from relatively surface the cool,hydrodynamic is system. shock supersonic known region as The flow that sharply the to decelerates accretion a thefield column. material, extremely and thereby hot converting The randomizing bulk subsonic the tran- kinetic velocity flow energyaccretion occurs to column temperature. through (PSAC)), a that It dominates isof the the wavelengths. post-shock radiative region output This (or of includes post-shock the X-rayoptical bremsstrahlung system cyclotron from across emission from the a fast-moving hot wide electrons (10-100keV) range spirallingas gas, along UV magnetic infra-red and field and soft lines, (20-50eV) as X-ray well emission from the accretion-heated surface of the WD [ a consequence of thehigh fact magnetic that field most where of chargedemission the particles is radiation radiate thus in via produced the theinfrared. by cyclotron system free mechanism. is The electrons produced The prototype, in in polarized an AM the a X-ray Herculis PSAC, emitter region - and by with the the is UHURUaccretor first most X-ray from satellite, identified prominent its and polarization polar in subsequently [ - optical identified as was and compact indeed magnetic discovered to be the flow of material between the two stars (cf. Figure sisting of a highlyred-dwarf magnetized companion White [ Dwarf (WD) accreting material from a Roche-lobe filling QPOs in mCVs: V834 Cen 1. Introduction PoS(Golden2015)018 ], it has 16 ]. This dis- 19 8 Hz (1.25-2.5 . 0 Hannes Breytenbach − 4 . 0 ∼ 5h. The WD radius was calculated . 1 (1981), based on a numerical study = ]. Recent numerical work has taken the orb 17 ]. P et al. , 18

M 3 . 3 0 = ]. The magnetic field lines displayed are two shells of 2 7 M ,

M 8 . 0 = from the orbital axis of the binary. If the binary is synchronised, as is WD ]. During the intervening 3 decades, numerous studies have expanded ◦ M 20 = 1 M ]). Tantalising evidence for multi-polar accretion geometries has also emerged from ]). Such varied scenarios can readily be explored using numerical techniques (see for A model representation of a Polar showing: The White Dwarf in cyan with its magnetic 15 14 The first detection of quasi-periodic intensity variation in the optical light of polars was ac- complished by Middleditch (1982), who described a “noise” feature at on this originative analysis, leadingthe to stability the of accretion emergence flows of onto a compact coherent magnetic theoretical objects, model and to hence, explain the presence of QPOs been shown that, at leastsignificantly 3 more polars complex (EF than Eri, simple BL offset Hyi dipoles and [ CP Tuc) possess magnetic field structures 1.1 Quasi-Periodic Oscillations in the Accretion columns of Polars s) with r.m.s. amplitude ofcovery a came few not percent long inof after the the the systems time-dependent AN prediction hydrodynamical UMa by and equationsbe Langer V834 of thermally Cen a unstable [ 1 [ dimensional flow, that the PSAC should example [ spectropolarimetric studies: Employing the Zeeman tomography technique developed in [ cue from such observational findings, and studiescomplex that magnetic explore geometries the have implications begun of to accretion appear in more [ using the Nauenberg mass radius relation [ Figure 1: field lines in magenta; thethe two Roche in lobe black. of For the theproduce secondary this purpose figure: in of illustration, blue, the and following the system parameters particle where stream chosen between to the case for most ofremains the static polars, (in the the orientation configuration of shown) the throughout magnetic the orbit. field with respect tomagnetic the field: secondary geometry (dipolemay or not coincide multi-pole), with magnetic the WD axisWith centre); variation tilt, but of field also these by centre parameters, thefor position accretion a e.g. (which rate, great WD [ variety mass of - accretion and spin geometries period. become possible (see QPOs in mCVs: V834 Cen a dipole field, tilted by 20 PoS(Golden2015)018 Hannes Breytenbach ]. The stability of the PSAC is determined, 21 4 We have seen that, under certain conditions, the PSAC is 3 Geometries of the PSAC models. Dashed lines show the dipolar geometry. [Reproduced ]]. 6 Although the model just described provides a plausible mechanism for generating QPOs, there In the preceding discussion, we have strategically avoided a potentially protracted discussion involving a number Intuitively, we can understand how oscillations are generated as follows: Suppose a pertur- 3 are a number of discrepancies, most conspicuously, the absenceof factors, of such as X-ray gravity, radiative QPOs. forces, opacity, turbulence, etc., An all accretion of which may play an important role. Figure 2: from [ thermally unstable, giving rise to oscillationsis characterised in by the the height cooling of timescale the of shock the front, post-shock the gas. period of which bation in the accretion flownon-zero provides velocity. a force The whichadiabatic overall pushes expansion, volume the its of temperature shock gas will frontframe in tend upward of to the the with shock, decrease. a PSAC infalling small has On materialing is the thermalized therefore now other at increased arriving hand, a with and, higher relative antemperature temperature, to increased under dependence depositing velocity, the therefore of heat rest becom- the into cooling thelation processes PSAC. will Here play we the propagate. can crucial see role Ifshed how in the its the determining excess radiative if heat cooling the as is oscil- photonsIf, effective, and however, the cooling tend to is newly contract, ineffective, added the causingdriving hot the excess expansion, gas initial heat contributing perturbation dumped will to to in quickly the equilibrate. the the upward cooling post-shock perturbation rate region of of will the the continue post-shockshock front. region front matches We will the can halt, rate argue momently of heat beginningcools that, deposition to and once from move contracts. shocked in The gas, the inverse the oppositeward, of direction infalling the as gas above the is mechanism post-shock now now arriving gas applies:and relatively not as slower, becoming contributing the more thermalized shock at heat moves a down- thanstroke, lower is the temperature being gas dissipated has through cooling. oncecompletes one more At cycle the gained of bottom enough the end thermal oscillation. of energy the to halt the contraction as the front QPOs in mCVs: V834 Cen (see review by Wu (2000)primarily, and by references the therein)[ cooling processesto involved. be thermally Flows unstable. dominated However, bydependence the (such presence bremsstrahlung as of cooling, cyclotron cooling tend or processes Compton with cooling), a has stronger a temperature stabilising effect. PoS(Golden2015)018 8Hz) . 0 − 3 . 0 ∼ Hannes Breytenbach ]. It is worth noting, however, that the ]), as well as EM-CCD based cameras 22 13 5 show that the QPO is almost always detected (the 4 ]. Our eventual aim is to complete a systematic study ). This is encouraging in that it shows that faint, rapid ]. Here we present the first results from this campaign for 1 27 29 , ], or SALTICAM [ 28 26 ]. All previous detections were made using photomultiplier tubes (PMTs), a 25 ]) has changed this situation, and has the potential to re-enliven enquiry into the , shows the differential light curves for V834 Cen on 7 separate nights. The corre- gives an indication of the orbital phase dependence of the QPO. The power excess 27 24 3 6 , 23 Figure Figure At the time of this writing, there are 5 Polars known to have QPOs. Besides the two systems A systematic program to search for QPOs in Polars, including newly discovered systems, stretching across time. Herethere always the seems intermittent to be nature some of power the excess, the oscillation total is power contained clearly in seen: the QPO, Although as well as the sponding power density spectra in Figure due to the QPO can be seen in the main panel of the figure as the band of colour ( the Polar V834 Cen. 3. Results & Discussion single case here that failed tothe detect short the duration QPO of (Run 2014-07-06) thevariability is can run attributable reliably (see to be Table bad detected using weather CCD and cameras. already mentioned above, QPOsBL have Hyi been [ detected in optical light from EF Eri, VV Pup, and has recently started at SAAO.(SHOC) This on program the utilizes SAAO the 1.9m Sutherland reflector High-speed [ Optical Cameras number of sources in thissuch, study conclusions constitute drawn only from a studying fraction such of a the sample known may polars be (see subject to below), some and2. selection as bias. Observations behaviour of mCV accretion columns, and particularly QPOs. of the QPO phenomena inand a high variety field of systems, polarslow asynchronous with and systems, different high physical two-pole accretion attributes, accretors, ratethe for low systems. physics example underlying and low the This high QPOs, work taking massquantitative will the physical systems, hopefully study parameters. from lead purely to In phenomenologicalthe conjunction a to experimental with better one results based this understanding from on observational a of program, parallelwill program we yield of further expect simulating relevant that insights accretion [ columns in the laboratory technology which has in recentsensitivity. years lost Until favour recently, against CCD CCD cameraslacked cameras, due sufficient despite to frame having the rates a latter’s to superior far enablesituation studies superior is of quantum partly short efficiency, time-scale responsible have phenomenaThe for such recent the as appearance, QPOs. nearly however, of 2 This fastgration decade frame-transfer times CCD long (e.g. cameras dearth capable ULTRACAM [ in of sub-second QPO inte- detections in Polars. QPOs in mCVs: V834 Cen column cooling predominantly through bremsstrahlung,comprehensive is search expected for X-ray to QPOs produce in5 X-rays. that archival display XMM-Newton A optical data recent QPOs) of has 24 shown Polars this (including convincingly [ the (e.g. SHOC [ PoS(Golden2015)018 Hannes Breytenbach . The QPO is visible as an excess of 3 6 8Hz. The power specturm for each night is offset for display purposes . 0 − 3 . 0 ∼ /Hz. 2 Light curves of V834 Cen for 7 different nights (dates indicated in legend). Displayed Frequency power spectra of light curves in Figure. Figure 4: power in the range by 0.01 rms Figure 3: are the differential flux values, binned toby 5s the intervals. value Each indicated light curve in is the offset right for margin. display purposes QPOs in mCVs: V834 Cen PoS(Golden2015)018 2 orbital . 1 ∼ . Here the QPO 4 Hannes Breytenbach 2 hours ( ∼ 7 ]. 31 100s duration), each one overlapping the previous one by 75%, de- ∼ 3 s). Red noise due to accretion-induced flickering can be seen in both the . 3 ]. The central panel of the figure (mostly blue) is a power density map over the − 2 30 . 1 ∼ Averaged spectral density estimate of all data presented in Figure Time Frequency Representation (a.k.a. Dynamic Power Spectrum) for the 2014-04-25 1024 data points ( = 10 light curve of V834 Cen. The topmostcycles). panel shows The the right light panel curve covering showsfor the the estimated entire frequency run variability [ spectrum (viafrequency Welch’s range method) 0 - 4.65 Hz.of 2 The map was producedtrending by by segmenting the a light cubic curve polynomial intoFourier (low Transform segments pass (FFT) filtering periodogram. to The suppress0.8Hz power red range excess noise) ( from and the calculating QPO the can be Fast seen in the 0.3 to Figure 6: main- and right panels as aprevious peak QPO in detection power by at Larsson lower (1992)[ frequencies. These results are consistent with a appears to be multi-modal:intervening The minima. averaging Physical exposes mechanisms a that might numberthe lead of text. to this individual kind maxima of separated behaviour are by explored in Figure 5: QPOs in mCVs: V834 Cen PoS(Golden2015)018 (s) Duration 3681 6909 7599 6432 7546 8696 2710 , deserves ADU (s) / 6 − e Hannes Breytenbach Exposure Time 0.3 0.25 0.1 0.1 0.1 0.1 0.5 ) ] for details. 27 ADU / − e ( and the right panel of Figure 5 8 EM Gain - - - - 20 12 25 ). This pattern appears to be consistent across all the runs. 6 ∗ ]. While most observations do not probe this frequency range, we cannot for 3 MHz, PreAmp @ 4.9, CON 3 MHz, PreAmp @ 4.9, CON 1 MHz, PreAmp @ 2.5, EM 1 MHz, PreAmp @ 2.5, EM 1 MHz, PreAmp @ 2.5, EM Instrument Mode 1 MHz, PreAmp @ 2.5, CON 1 MHz, PreAmp @ 2.4, CON 29 Observational setup for data presented below. In addition to the results presented here, we have obtained high time resolution white-light We have presented the first results from a dedicated campaign to search for- and characterise The multi-modal power distribution seen in Figure Instrumental Mode is specified as: CCD readout frequency (Pixels/s), pre-amplifier gain in 2014-04-28 2014-07-06 2013-02-07 2013-06-12 2013-06-16 2013-06-18 2014-04-25 Run date photometry on a large number of Polars. This data is currently being analysed and will form the the moment exclude the possibilityobservational data of does such not high agree with frequencythe this oscillations, model theoretical but is calculation. still note As lacking that such, in we the some must key current conclude aspects. that concentration with frequency tends tofaint vary phase without of the any orbit, obvious the pattern. QPOThe falls QPO beneath We the is note detection most that limit strongly during given(between the detected the 83000 uncertainties in and in the 84000 the data. s phase in where Figure the source is brightening from its faintest QPOs in Polars. Thedetection system of V834 QPOs using Cen CCD hasous studies cameras, in are and some positive. the ways On consistencyaccretion served the column of as theoretical leading the to front, a oscillations results the proof-of-concept in presented standarderating the for picture shock with and the height, of sustaining previ- while thermal QPOs, providing instability cannot a inthe mechanism account for the predicted for gen- oscillation the frequencies lack for ofrange typical X-ray WD 6-20 QPO mass detections. Hz[ and Furthermore, accretion rate lie somewhere in the Table 1: some contemplation. 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