In this review talk I cover some recent developments in understanding the
role that magnetic fields play in cataclysmic variables. I discuss the recent
DNO--QPO unification models; the disk--magnetosphere boundary; some issues
concerning the soft blackbody component and the nature of the X-ray spectra in
MCVs; whether the SW Sex stars are magnetic, and finally I mention the weird
behavior of FS Aur and HS2331+3905.Comment: To appear in ``Compact Binaries in the Galaxy and Beyond'', RevMexA

Hellier, Coel

English

arXiv

In this review talk I cover some recent developments in understanding the role that magnetic fields play in cataclysmic variables. I discuss the recent DNO--QPO unification models; the disk--magnetosphere boundary; some issues concerning the soft blackbody component and the nature of the X-ray spectra in MCVs; whether the SW Sex stars are magnetic, and finally I mention the weird behavior of FS Aur and HS2331+3905

Hellier, C

Name not available

Some updates on the role of magnetic fields in cataclysmic variables

Keele Research Repository

arXiv:astro-ph/0402103v1  4 Feb 2004To appear in “Compact Binaries in the Galaxy and Beyond (2004)” RevMexAA(SC)SOME UPDATES ON THE ROLE OF MAGNETIC FIELDS INCATACLYSMIC VARIABLESCoel Hellier1RESUMENSpanish abstract – to be supplied by the editors.ABSTRACTIn this review talk I cover some recent developments in understanding the role that magnetic fields play incataclysmic variables. I discuss the recent DNO–QPO unification models; the disk–magnetosphere boundary;some issues concerning the soft blackbody component and the nature of the X-ray spectra in MCVs; whetherthe SW Sex stars are magnetic, and finally I mention the weird behavior of FS Aur and HS 2331+3905.Key Words: ACCRETION, ACCRETION DISKS — NOVAE, CATACLYSMIC VARIABLES1. DNO–QPO UNIFICATIONA major advance of recent years is a new under-standing of the dwarf-nova oscillations (DNOs) andquasi-periodic oscillations (QPOs) seen in dwarf no-vae in outburst. Such things have been recorded andstudied since the 1970s, but only recently, with a se-ries of papers by Warner and Woudt, do we have acompelling account of their origin (Woudt & Warner2002; Warner & Woudt 2002; Warner, Woudt & Pre-torius 2003).The heart of the model is a suggestion that, dur-ing DN outbursts, an equatorial belt of the whitedwarf is spun up by enhanced accretion. The beltsliding over the white-dwarf core results in a dy-namo, amplifying a seed field to the point where thefield controls the accretion flow near the white dwarfby carving out a magnetosphere (Figure 1).The standard DNOs are simply pulsations at therotation period of this magnetosphere. The tran-sience of the magnetosphere explains why the DNOsaren’t seen in quiescence, while the low moment ofinertia of the belt explains the low coherence of theoscillations — both of which had previously beenproblems for a magnetic explanation of DNOs.The second element of the model is the idea thatthe magnetic field, playing on the inner edge of thedisc, excites slow-moving waves which run prograderound the disk with a period ≈ 15 times the mag-netospheric spin period. These bulges modulate thelight by simple obscuration, resulting in QPOs witha characteristic PQPO ≈ 15PDNO.A second type of DNO then results from repro-cessing of the first DNO off the QPO bulges, givingthe beat relation 1/PDNO2 = 1/PDNO1 − 1/PQPO.1Keele University, UK.Further, Warner et al. (2003) claim a third type ofDNO, which they suggest results from the rotationof a field attached to the body of the white dwarf,not the spun-up belt. This is rotating more slowlyand so produces ‘long period DNOs’. All of this in-terpretation is supported by an impressive amountof observational documentation.The above theory raises the question of whetherthe same QPO bulges are excited at the inner edge ofdisks in intermediate polars. Certainly, such bulgesare the most plausible explanation for the 5000-sQPOs seen in GK Per during outburst (e.g. Hellier,Harmer & Beardmore 2004), but such things are notgenerally reported in IPs in quiescence. One reasonmight be observational: with a typical 1000-s spinperiod, the QPO period would be ∼ 4 hrs, and itis hard to observe for the dozen cycles that wouldbe needed to prove the presence of a low-level, inco-herent modulation. Further, such signals might bemasked by orbital-cycle variations.A second explanation might be that the bulgesare only excited when there is strong slippage be-tween the magnetic field and the inner disk, and thatthis isn’t so in intermediate polars in their equilib-rium, quiescent state — a topic that seems worthpursuing observationally.An interesting point (noticed by Warner &Woudt 2002 and independently by Mauche 2002) isthe fact that the QPO/DNO ratio can be extendedover 5 orders of magnitude to cover the much fasterQPOs seen in neutron-star and black-hole binaries.Is this coincidence or does it imply a causal similar-ity? There is valid skepticism over any link, since ofcourse black-holes don’t have a surface, nor a perma-nent magnetic field, and thus are unlikely counter-12 HELLIERFig. 1. A schematic of Warner & Woudt’s DNO/QPO model, with dynamo action in a spun-up belt producing atransient magnetosphere, which excites travelling waves on the inner edge of the disk.parts of white dwarfs. However, in the model abovethe field is also a transient one created by a dynamo,and the action occurs at the interaction of the fieldwith the inner disk, with the solid surface playinglittle role. Thus the situations are not as dissimilaras they may at first appear.2. THE DISK–MAGNETOSPHERE BOUNDARYWhile on the subject of the disk–magnetosphereboundary, it bears restating that this is one of theleast understood regions of a CV. A paper on FO Aqrby Evans et al. (2004) shows that the accretion cur-tain appears to be swept back, trailing the magneticpole by a quarter of a cycle. The opposite was foundin PQ Gem (Mason 1997), where the accreting fieldlines lead the pole. One can then ask whether thesetwists are related to disk–field disequilibrium andthus to the torques on the white dwarf. At first sightit appears so, since the white dwarf in PQ Gem isspinning down whereas that in FO Aqr is currentlyspinning up. However, FO Aqr has changed froma period of spin-down to one of spin-up, withoutany obviously related change in the spin-pulse pro-file. Thus any interpretation is problematic, and thewhole issue of the disk–field interaction and the re-sulting torques is one that could do with more study.3. THE FOOTPRINTTurning now to the accretion footprint on thewhite dwarf, Ramsay & Cropper (2004) have pro-posed a major re-evaluation of the accretion processin AM Her stars. For two decades it has been conven-tional wisdom that the AM Her stars show a strong‘soft excess’ over that expected in the simplest ac-cretion model (a hard-X-ray-emitting shock whichirradiates the white-dwarf surface, resulting in softblackbody emission that amounts to half the totalflux). The excess is usually attributed to ‘blobbyaccretion’ in which blobs of material do not shock,but penetrate the white-dwarf surface and thermal-ize, greatly boosting the soft/hard ratio.Now, from a systematic analysis of XMM data,Ramsay & Cropper (2004) find that most AM Herspectra are indeed compatible with the simple model,and that previous reports of soft excesses were, to alarge extent, artefacts of calibration and band-passuncertainties (Figure 2).However, a small number of systems do show alarge soft excess, and are presumably dominated byblobby accretion. But why? Ramsay & Cropperdiscuss the obvious variables such as field strength,but find no obvious correlation with the presence ofa soft excess.A similar question arises in the intermediatepolars. Since Rosat (e.g. Haberl & Motch 1995)we’ve known of a minority of IPs that show a soft-blackbody component, but the majority do not.Again, we have no good explanation for the differ-ence, and no obvious correlation with field strengthor other variables to guide us.V405 Aur is one of the IPs with soft blackbodyemission, and is also peculiar in that it shows asingle-humped spin pulse at hard-X-ray energies buta double-humped pulse at softer energies. One ideaexplains the difference between single-humped anddouble-humped IPs as an absorption effect: IPs withshort, fat accretion columns beam X-rays upwardsand so produce double-humped pulsations, whereasIPs with tall, thin columns beam X-rays sidewaysand produce single-humped pulsations (e.g. Hellier1995; Allan et al. 1996).However, an analysis of XMM data by Evans &Hellier (2004) shows that, at least in V405 Aur, thedouble-humped soft pulse is not the product of ab-sorption. Instead it is a modulation of the visiblearea of the blackbody component, resulting primar-ily from variable foreshortening of the heated pole-caps as the white dwarf rotates. It remains to beMAGNETIC FIELDS IN CATACLYSMIC VARIABLES 3481216202428Soft/Hard ratioNumber of systemsBefore XMMAfter XMM0 1 2 3 4 5 10 100 200531Fig. 2. Ramsay and Cropper’s reassessment of the energybalance in AM Her stars. Before XMM (bottom) it wasthought that most AM Hers showed a strong soft excess.Now (top) only a minority show a soft excess, and themajority are compatible with the standard model.seen what this implies for the double-humped opticalpulsation, and whether the findings are applicable toother double-humped IPs.4. THE X-RAY SPECTRAThe standard model for X-ray spectra in IPs in-vokes a stratified column in which material cools be-neath the accretion shock. By assuming opticallythin, collisionally ionized emission, one can use acode such as mekal and sum the emission betweenthe hot shock and the point where the column be-comes optically thick as it merges with the whitedwarf. Such a model gives an excellent represen-tation of the spectrum of EX Hya (Cropper et al.2002).However, Mukai et al. (2003) report that EX Hyais unusual. From an analysis of Chandra gratingspectra they find that most IPs are not compatiblewith the above model. Instead, they obtain a betterfit with a photoionization code. This raises the issueof where the emission arises. One possibility is thatthe X-ray lines are predominantly from photoionizedpre-shock material. However, as discussed by Hellier& Mukai (2004), Doppler shifts of the lines are oforder ∼ 100 km s−1, rather than the ∼ 1000 km s−1expected for material approaching the shock at nearthe escape velocity. Such low velocities imply anorigin near the base of the accretion column wherethe material has been vastly decelerated (which isalso expected in the standard model, since these arethe densest regions and emission scales with densitysquared). But this leaves us with no coherent modelfor the spectral characteristics of the majority of IPs.EX Hya is atypical, possibly owing to it beingbelow the period gap and so having a much lower lu-minosity, which perhaps results in it being easier tomodel. Its importance will increase further now thatwe know its distance to high precision, given Beuer-mann et al.’s (2003) report of a parallax distance of64.5± 1.2 pc.With EX Hya’s parameters now securely known,Beuermann et al. report that the secondary is un-dermassive, being 18–30% larger than a ZAMS star.Reassurringly, this corroborates the result of a largestudy of superhumps by Patterson et al. (2003),which shows that secondary stars in dwarf novae be-low the gap are 18% larger than ZAMS. From therelatively small scatter in the values of superhumpperiod excess, Patterson et al. were also able to con-clude that there is only one evolutionary track lead-ing to these stars, most likely without any nuclearevolution of the secondaries.5. ARE THE SW SEX STARS MAGNETIC?The SW Sex phenomenon appears to bewidespread in CVs (> 20 systems show at least someSW Sex characteristics) and is present in at least oneLMXB (Hynes et al. 2001). It is thus important forour understanding of accretion.Many recent authors have favored models whichinvoke magnetic fields to explain the SW Sex char-acteristics, regarding SW Sex stars as a variant ofthe IPs (e.g. Groot et al. 2001; Rodŕıguez-Gil et al.2001; Hoard et al. 2003 and references therein; butsee Hellier 2000 for a non-magnetic model). Suchmodels are supported by suggestions of observed pe-riodicities, including reports of periodic modulationsin polarization data that, if verified, would clinch themagnetic nature of these stars.However, at the risk of being thought unduelyskeptical and ultimately proved wrong, I note thatno periodicity has yet been corroborated by multipledatasets or by independent groups — something thatusually happens quickly for new IPs — and that CVsare notorious for flickering behavior that can mimicperiodicities in limited datasets.My main reason for skepticism about the mag-netic nature of SW Sex stars is their general lackof X-ray emission, and particularly pulsed X-rays.4 HELLIERIPs emit copious X-rays, with an obvious coherentpulsation. Polarization is much harder to find, andpulsed polarization has been seen in only a tenth ofthe known IPs.In contrast, if the claims for SW Sex stars aretrue, their fields are strong enough to dominatethe emission-line behavior, and to produce phase-variable polarization, but we do not see pulsed X-rays. This discrepancy, if true, would be telling ussomething fundamental about accretion.A further implication concerns VY Scl stars.There has been a long-standing problem over the lackof dwarf-nova outbursts in the low states of VY Sclstars, given that disk-instability models predict thatthey should occur. One idea is that irradiation keepsthe inner disk too hot for such outbursts (Leach et al.1999), but Hameury & Lasota (2002) prefer a modelin which a strong magnetic field evacuates the in-ner disk. They note that many SW Sex stars showVY Scl low states, and cite the magnetic models assupport of their VY Scl hypothesis.It is clear that the explanation of SW Sex behav-ior has wide implications. It would thus be good tohave corroboration of periodicities sufficient to con-vince even an ardent skeptic, or to have sufficent nullresults to settle the matter the other way.6. THE WEIRD STARS FS AUR ANDHS 2331+3905As a last topic I turn to the stars FS Aur andHS 2331+3905, although it is unclear whether theissue concerns magnetic fields or some sort of diskprecession. FS Aur has an 86-min orbital period butalso shows a large-amplitude photometric modula-tion at 3.4 hrs (Tovmassian et al. 2003). No super-hump has been seen with a period so much longerthan the orbital period, but the period is also tooshort to be disk precession. Could it be the spinperiod of a magnetic white dwarf? Well, so far weknow of no system with a spin period longer thanthe orbital period.The issue becomes even stranger with the discov-ery of HS 2331+3905 (Araujo-Betancor et al. 2004).This star has an 81-min orbital period and a 3.5-hrperiodicity, making it similar to FS Aur. However,in FS Aur the 3.4-hr periodicity appears in photom-etry only, and not in radial velocities, whereas inHS 2331+3905 the 3.5-hr periodicity is seen in ra-dial velocities, but not in photometry. Explaining3-hr periodicities in 80-min binaries is hard enoughwithout having to explain that also!REFERENCESAllan, A. et al. 1996, MNRAS, 279, 1345Araujo-Betancor, S. et al. 2004, these proceedingsBeuermann, K., Harrison, T. E., McArthur, B. E., Bene-dict, G. F., & Gaensicke, B. T. 2003, A&A, 412, 821Cropper, M., Ramsay, G., Hellier, C., Mukai, K.,Mauche, C., & Pandel, D. 2002, Roy. Soc. Phil. Tr.A, 360, 1951Evans, P.A., & Hellier, C. 2004, in preparationEvans, P.A., Hellier, C., Ramsay, G., & Cropper, M.2004, MNRAS, in pressGroot, P. J., Rutten, R.G. M., & van Paradijs, J. 2001,A&A, 368, 183Haberl, F., & Motch, C. 1995, A&A, 297, L37Hameury, J.-M., & Lasota, J.-P. 2002, A&A, 394, 231Hellier, C., 1995, in ASP Conf. Ser., 85, “Cape Work-shop on Magnetic Cataclysmic Variables”, eds. Buck-ley, D., Warner, B. (San Francisco: ASP), 185Hellier, C., 2000, New Ast. Rev., 44, 131Hellier, C., Harmer, S., & Beardmore, A. P. 2004, MN-RAS, in press.Hellier, C., & Mukai, K. 2004, MNRAS, submittedHoard, D.W., Szkody, P., Froning, C. S., Long, K. S., &Knigge, C. 2003, AJ, 126, 2473Hynes, R. I. et al. 2001, MNRAS, 324, 180Leach, R., Hessman, F.V. King, A.R., Stehle, R., & Mat-tei, J. 1999, MNRAS, 305, 225Mauche, C. W. 2002, ApJ, 580, 423Mason, K.O. 1997, MNRAS, 285, 493Mukai, K., Kinkhabwala, A., Peterson, J.R., Kahn,S.M., & Paerels, F. 2003, ApJ, 586, L77Patterson, J. et al. 2003, PASP, 115, 1308Ramsay, G., & Cropper, M. 2004, MNRAS, 347, 497Rodŕıguez-Gil, P., Casares, J., Martinez-Pais, I. G.,Hakala, P., & Steeghs, D., 2001, ApJ, 548, L49Tovmassian, G. et al. 2003, PASP, 115, 725Warner, B., & Woudt, P. 2002, MNRAS, 335, 84Warner, B., Woudt, P., & Pretorius, M. L. 2003, MNRAS,344, 1193Woudt, P., & Warner, B. 2002, MNRAS, 333, 411MAGNETIC FIELDS IN CATACLYSMIC VARIABLES 5Coel Hellier: Keele University, Staffordshire, ST5 5BG, U.K. (ch@astro.keele.ac.uk).

Some updates on the role of Magnetic Fields in Cataclysmic Variables

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