Acta Polytechnica Vol. 52 No. 1/2012 The Intriguing Nature of the Cataclysmic Variable SS Cygni F. Giovannelli, L. Sabau-Graziati Abstract The classification of SS Cyg as a dwarf nova (DN), a subclass of the non–magnetic (NM) cataclysmic variable (CV) has been considered by most of the community as well established because of a paper appeared in Nature (Bath & van Paradijs, 1983), which was a bandwagon for all the papers discussing SS Cyg behaviour both from experimental and theoretical points of view. This classification has been widely accepted until nowadays, in spite of the many arguments and circumstantial proofs about its possible intermediate polar nature, as claimed by Franco Giovannelli’s group for more than 25 years. The goal of this paper is to present an objective discussion of the problems connected with the controversial nature of SS Cyg, using all the different interpretations of its multifrequency data in order to demonstrate beyond doubt its intermediate polar nature. Keywords: cataclysmic variables, dwarf novae, intermediate polars, optical, spectroscopy, photometry, sub-mm, IR, radio, UV, X-rays, individual: SS Cyg ≡ BD+42◦ 4189a ≡ AAVSO 2138+43 ≡ 3A 2140+433 ≡ 1H 2140+433 ≡ INTEGRAL1 121 ≡ 1RXS J214242.6+433506 ≡ EUVE J2142+43.6 ≡ SWIFT J2142.7+4337. 1 Introduction example of a system belonging to the latter subclass is DO Dra (previously registered as YY Dra) (An- Historically, the classification of CVs was based on dronov et al., 2008). the optical outburst properties, by which one may Depending on the magnetic field intensity at the distinguish four groups of CVs: (i) classical novae; white dwarf, the accretion of matter from the sec- (ii) recurrent novae; (iii) dwarf novae; (iv) nova-like ondary star onto the primary can occur either via an objects (e.g., Giovannelli & Martinez-Pais, 1991 and accretion disc (in the so-called Non-Magnetic CVs: references therein; Ritter, 1992; Giovannelli, 2008). NMCVs) or via channelling through the magnetic This classification, however, is neither self-consistent poles (in the case of Polars: PCVs), or in an in- nor adequate, and it is much better to consider termediate way (in the case of Intermediate Polars: primarily the observed accretion behaviour (Smak, IPCVs). 1985). One obvious advantage of such an approach SS Cyg is the most observed and most intriguing is connected with the time scales of various accretion CV. For reviews, see the papers by Giovannelli & phenomena, which are sufficiently short to avoid any Martinez-Pais (1991), Giovannelli (1996), Giovan- major observational bias. The mass accretion rates nelli & Sabau-Graziati (1998). The most exten- −11 −8 −1 in CVs usually range from 10 to 10 M⊙ yr sive review about SS Cyg before the advent of the (Patterson, 1984); the time scales are from tens of space era is that by Zuckerman (1961). The light seconds (oscillations in dwarf novae at outbursts) to curves of SS Cyg have been produced continuously years (super-outbursts of SU UMa stars or long term by the AAVSO observations since 1896 (Mattei, et variations in VY Scl stars). However, in the class of al., 1985; Mattei, Waagen & Foster, 1991, 1996; nova-like objects there are two sub-classes: DQ Her Mattei, Menali & Waagen, 2002; AAVSO web page stars and AM Her stars. In these sub-classes of CVs, (http://www.aavso.org/)). white dwarfs possess magnetic fields with intensity The optical outbursts of SS Cyg are not always high enough to dominate the accretion disk and all the same. Howarth (1978) discussed three possible the phenomena related to it. These classes of mag- kind of outbursts, long, short and anomalous with netic CVs, whose names come from the prototypes average periodicity of 50.21 days. Giovannelli et al. DQ Her and AM Her, later took the names of In- (1985), Lombardi, Giovannelli & Gaudenzi (1987), termediate Polars and Polars, respectively. A short Gaudenzi et al. (1990; 2011) and Giovannelli & history of their discovery was discussed by Warner Martinez-Pais (1991) discussed outbursts that origi- (1995). Fundamental papers about these sub-classes nate different optical, UV and X-ray behaviour of the are those by Patterson (1994), Warner (1996a,b). system. The class of IPs has been split into two subclasses On the basis of its optical light curves, SS with relatively a large magnetic field anf with a rela- Cyg was classified as a dwarf nova (Bath & van tively weak magnetic field (Norton et al., 1999). One Paradijs, 1983), with white dwarf mass equal to 11 Acta Polytechnica Vol. 52 No. 1/2012 Chandrasekhar’s limit (Patterson, 1981). However, In a study of the matter flow structure in SS Cyg we will discuss its intermediate polar nature, analyz- using the Doppler tomography technique, Bisikalo et 9 ing its multifrequency behaviour and different inter- al. (2008) found that Rid = (2.6−3.3)×10 cm, which pretations of the data from the literature. Moreover, is another important confirmation of the goodness of on the basis of more realistic values for its orbital Giovannelli et al.’s parameters. parameters, we will try to reconcile all experimental Despite the enormous amount of multifrequency multifrequency data with the magnetic nature of SS experimental data spread over many years, the mor- Cyg. phology and the nature of SS Cyg are still unset- tled questions. Indeed, SS Cyg was classified as a 2 On the controversial nature non-magnetic CV (NMCV) by Bath & van Paradijs (1983). Ricketts, King & Raine (1979) explained of SS Cyg the X-ray emission from SS Cyg as owing to the ra- dial inflow of matter onto a magnetized white dwarf With the historical classification of CVs based on (B ∼ 106 G) from a disrupted accretion disk. Fab- the optical outburst properties, SS Cyg (α2000 = biano et al. (1981), using coordinated optical-UV h m s ◦ ′ 21 42 48 .2; δ2000 = +43 35 09”.88 with the galac- and X-ray measurements of SS Cyg, noted that its tic coordinates l2000 = 090.5592, b2000 = −07.1106), behavior is not compatible with a viscous disk model whose distance is 166.2 ± 12.7 pc (Harrison et al., and confirmed the magnetic nature of the white dwarf 1999), is the brightest of the dwarf novae. Its op- with B ≤ 1.9 × 106 G. SS Cyg at quiescence is quite tical magnitude ranges from ∼ 12 to ∼ 8.5 during similar to AM Her and its behaviour is consistent quiescent and outburst phases, respectively. with a picture of polar magnetic accretion. Fur- Because of these characteristics, it is the most ob- ther multifrequency data of SS Cygni showed incom- served CV, not only in the optical wavelength region, patibility of its behavior with that of NMCV, and where measurements are available from the end of strongly favored its classification as an intermediate the 19th century to the present, but also in other polar (see, e.g. Giovannelli et al., 1985; Giovan- wavelength regions. nelli & Martinez-Pais, 1991; Kjurkchieva, Marchev & SS Cyg shows oscillations of ∼ 10 s in both the Og loza, 1999; Marchev, Kjurkchieva & Og loza, 1999; optical and the X-ray ranges, orbital modulations Gaudenzi et al., 2002; Long et al., 2005; Schreiber, (Porb ≃ 6.6 h) of the intensities of Balmer and UV Hameury & Lasota, 2003; Schreiber, & Lasota, 2007). emission lines and of the continuum, and almost peri- Moreover, in SS Cyg, Lhard−X < LUV+soft−X. odic outbursts (Poutb ∼ 50 days, Howarth, 1978). All This is compatible with thermonuclear burning onto these characteristics, together with relatively high lu- the WD surface. Thermonuclear burning was first minosity both in outburst and in quiescence, render suggested by Igor Mitrofanov (1978). Gaudenzi et al. SS Cyg the most appropriate laboratory for studying (2002) found that thermonuclear burning can occur the physical processes occurring in dwarf novae and in ∼ 24% of the WD surface. Kording et al. (2008) in CVs in general. detected a radio jet from SS Cyg. The hardness in- The orbital parameters of the binary system tensity diagram shows an analogy between X-ray bi- were derived by Giovannelli et al. (1983) with naries (XRBs) and SS Cyg. This result supports the the use of theoretical and experimental constraints presence of a rather strong magnetic field at the sur- from measurements obtained in different energy re- face of the white dwarf. Upper limits to linear and ◦+1 +0.14 gions. They are i = 40 −2, M1 = 0.97−0.05 M⊙, circular polarizations have been found as 3.2 ± 2.7 % +0.08 +0.03 M2 = 0.56−0.03 M⊙, R2 = 0.68−0.01 R⊙, Rod = and −3.2 ± 2.7 %, respectively. 10 9 2.9 × 10 cm, Rid = 3.6 × 10 cm, where 1 and 2 INTEGRAL/IBIS and SWIFT/XRT observa- refers to the primary and secondary star, respectively. tions have shown that a conspicuous number of CVs Rod and Rid are the outer and inner accretion disk have a strong hard X-ray emission (Landi et al., 2009; radius. These parameters have been confirmed by di- Scaringi et al., 2010). In their published sample of rect measurements of radial velocities (Martinez-Pais 22 CVs, 21 are classified as magnetic CVs (MCVs) et al., 1994). Martinez-Pais et al. also determined (intermediate polar: IP) and only one (SS Cyg) as that the optical companion of SS Cyg system is a NMCV, meanwhile all its characteristics are practi- K2–K3 late-type star. cally equal to those of the other 21 objects. This is The mass of the white dwarf of the binary system one more strong circumstantial proof in favor of the SS Cyg was considered for long time as high as Chan- magnetic nature of SS Cyg.