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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

Historically, the classification of CVs was based on
the optical outburst properties, by which one may
distinguish four groups of CVs: (i) classical novae;
(ii) recurrent novae; (iii) dwarf novae; (iv) nova-like
objects (e.g., Giovannelli & Martinez-Pais, 1991 and
references therein; Ritter, 1992; Giovannelli, 2008).
This classification, however, is neither self-consistent
nor adequate, and it is much better to consider
primarily the observed accretion behaviour (Smak,
1985). One obvious advantage of such an approach
is connected with the time scales of various accretion
phenomena, which are sufficiently short to avoid any
major observational bias. The mass accretion rates
in CVs usually range from 10−11 to 10−8 M⊙ yr

−1

(Patterson, 1984); the time scales are from tens of
seconds (oscillations in dwarf novae at outbursts) to
years (super-outbursts of SU UMa stars or long term
variations in VY Scl stars). However, in the class of
nova-like objects there are two sub-classes: DQ Her
stars and AM Her stars. In these sub-classes of CVs,
white dwarfs possess magnetic fields with intensity
high enough to dominate the accretion disk and all
the phenomena related to it. These classes of mag-
netic CVs, whose names come from the prototypes
DQ Her and AM Her, later took the names of In-
termediate Polars and Polars, respectively. A short
history of their discovery was discussed by Warner
(1995). Fundamental papers about these sub-classes
are those by Patterson (1994), Warner (1996a,b).
The class of IPs has been split into two subclasses
with relatively a large magnetic field anf with a rela-
tively weak magnetic field (Norton et al., 1999). One

example of a system belonging to the latter subclass
is DO Dra (previously registered as YY Dra) (An-
dronov et al., 2008).

Depending on the magnetic field intensity at the
white dwarf, the accretion of matter from the sec-
ondary star onto the primary can occur either via an
accretion disc (in the so-called Non-Magnetic CVs:
NMCVs) or via channelling through the magnetic
poles (in the case of Polars: PCVs), or in an in-
termediate way (in the case of Intermediate Polars:
IPCVs).

SS Cyg is the most observed and most intriguing
CV. For reviews, see the papers by Giovannelli &
Martinez-Pais (1991), Giovannelli (1996), Giovan-
nelli & Sabau-Graziati (1998). The most exten-
sive review about SS Cyg before the advent of the
space era is that by Zuckerman (1961). The light
curves of SS Cyg have been produced continuously
by the AAVSO observations since 1896 (Mattei, et
al., 1985; Mattei, Waagen & Foster, 1991, 1996;
Mattei, Menali & Waagen, 2002; AAVSO web page
(http://www.aavso.org/)).

The optical outbursts of SS Cyg are not always
the same. Howarth (1978) discussed three possible
kind of outbursts, long, short and anomalous with
average periodicity of 50.21 days. Giovannelli et al.
(1985), Lombardi, Giovannelli & Gaudenzi (1987),
Gaudenzi et al. (1990; 2011) and Giovannelli &
Martinez-Pais (1991) discussed outbursts that origi-
nate different optical, UV and X-ray behaviour of the
system.

On the basis of its optical light curves, SS
Cyg was classified as a dwarf nova (Bath & van
Paradijs, 1983), with white dwarf mass equal to

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Acta Polytechnica Vol. 52 No. 1/2012

Chandrasekhar’s limit (Patterson, 1981). However,
we will discuss its intermediate polar nature, analyz-
ing its multifrequency behaviour and different inter-
pretations of the data from the literature. Moreover,
on the basis of more realistic values for its orbital
parameters, we will try to reconcile all experimental
multifrequency data with the magnetic nature of SS
Cyg.

2 On the controversial nature

of SS Cyg

With the historical classification of CVs based on
the optical outburst properties, SS Cyg (α2000 =
21h42m48s.2; δ2000 = +43

◦ 35′09”.88 with the galac-
tic coordinates l2000 = 090.5592, b2000 = −07.1106),
whose distance is 166.2 ± 12.7 pc (Harrison et al.,
1999), is the brightest of the dwarf novae. Its op-
tical magnitude ranges from ∼ 12 to ∼ 8.5 during
quiescent and outburst phases, respectively.

Because of these characteristics, it is the most ob-
served CV, not only in the optical wavelength region,
where measurements are available from the end of
the 19th century to the present, but also in other
wavelength regions.

SS Cyg shows oscillations of ∼ 10 s in both the
optical and the X-ray ranges, orbital modulations
(Porb ≃ 6.6 h) of the intensities of Balmer and UV
emission lines and of the continuum, and almost peri-
odic outbursts (Poutb ∼ 50 days, Howarth, 1978). All
these characteristics, together with relatively high lu-
minosity both in outburst and in quiescence, render
SS Cyg the most appropriate laboratory for studying
the physical processes occurring in dwarf novae and
in CVs in general.

The orbital parameters of the binary system
were derived by Giovannelli et al. (1983) with
the use of theoretical and experimental constraints
from measurements obtained in different energy re-
gions. They are i = 40◦+1−2, M1 = 0.97

+0.14
−0.05 M⊙,

M2 = 0.56
+0.08
−0.03 M⊙, R2 = 0.68

+0.03
−0.01 R⊙, Rod =

2.9 × 1010 cm, Rid = 3.6 × 10
9 cm, where 1 and 2

refers to the primary and secondary star, respectively.
Rod and Rid are the outer and inner accretion disk
radius. These parameters have been confirmed by di-
rect measurements of radial velocities (Martinez-Pais
et al., 1994). Martinez-Pais et al. also determined
that the optical companion of SS Cyg system is a
K2–K3 late-type star.

The mass of the white dwarf of the binary system
SS Cyg was considered for long time as high as Chan-
drasekhar’s limit — because of a paper that appeared
in the Astrophysical Journal (Patterson, 1981) — a
value completely unuseful for any sort of serious in-
terpretation of the many multifrequency experimen-
tal data and for modeling.

In a study of the matter flow structure in SS Cyg
using the Doppler tomography technique, Bisikalo et
al. (2008) found that Rid = (2.6−3.3)×10

9 cm, which
is another important confirmation of the goodness of
Giovannelli et al.’s parameters.

Despite the enormous amount of multifrequency
experimental data spread over many years, the mor-
phology and the nature of SS Cyg are still unset-
tled questions. Indeed, SS Cyg was classified as a
non-magnetic CV (NMCV) by Bath & van Paradijs
(1983). Ricketts, King & Raine (1979) explained
the X-ray emission from SS Cyg as owing to the ra-
dial inflow of matter onto a magnetized white dwarf
(B ∼ 106 G) from a disrupted accretion disk. Fab-
biano et al. (1981), using coordinated optical-UV
and X-ray measurements of SS Cyg, noted that its
behavior is not compatible with a viscous disk model
and confirmed the magnetic nature of the white dwarf
with B ≤ 1.9 × 106 G. SS Cyg at quiescence is quite
similar to AM Her and its behaviour is consistent
with a picture of polar magnetic accretion. Fur-
ther multifrequency data of SS Cygni showed incom-
patibility of its behavior with that of NMCV, and
strongly favored its classification as an intermediate
polar (see, e.g. Giovannelli et al., 1985; Giovan-
nelli & Martinez-Pais, 1991; Kjurkchieva, Marchev &
Og loza, 1999; Marchev, Kjurkchieva & Og loza, 1999;
Gaudenzi et al., 2002; Long et al., 2005; Schreiber,
Hameury & Lasota, 2003; Schreiber, & Lasota, 2007).

Moreover, in SS Cyg, Lhard−X < LUV+soft−X.
This is compatible with thermonuclear burning onto
the WD surface. Thermonuclear burning was first
suggested by Igor Mitrofanov (1978). Gaudenzi et al.
(2002) found that thermonuclear burning can occur
in ∼ 24 % of the WD surface. Kording et al. (2008)
detected a radio jet from SS Cyg. The hardness in-
tensity diagram shows an analogy between X-ray bi-
naries (XRBs) and SS Cyg. This result supports the
presence of a rather strong magnetic field at the sur-
face of the white dwarf. Upper limits to linear and
circular polarizations have been found as 3.2 ± 2.7 %
and −3.2 ± 2.7 %, respectively.

INTEGRAL/IBIS and SWIFT/XRT observa-
tions have shown that a conspicuous number of CVs
have a strong hard X-ray emission (Landi et al., 2009;
Scaringi et al., 2010). In their published sample of
22 CVs, 21 are classified as magnetic CVs (MCVs)
(intermediate polar: IP) and only one (SS Cyg) as
NMCV, meanwhile all its characteristics are practi-
cally equal to those of the other 21 objects. This is
one more strong circumstantial proof in favor of the
magnetic nature of SS Cyg. Scaringi et al. (2010) re-
ported the detection of one more IP: AO Psc, which
is added to the former sample. The experimental ev-
idence that SS Cyg emits in the hard X-ray energy
range is, in our opinion, conclusive evidence about its
magnetic nature.

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Acta Polytechnica Vol. 52 No. 1/2012

However, there are many papers in the literature
that seem to contradict the intermediate polar na-
ture of SS Cyg. Indeed, Gnedin et al. (1995) found
from observations of intrinsic circular polarization in
SS Cyg performed in the wings of the Balmer hy-
drogen lines that the true value of the magnetic field
probably lies in the range 0.03 < B < 0.3 MG.

Using Extreme Ultraviolet Explorer satellite ob-
servations, Mauche (1996) detected quasi-periodic
oscillations (QPOs) from SS Cyg with a period in
the range 7.19–9.3 s. This variation correlates with
the extreme ultra-violet (EUV) flux as P ∝ I−0.094EUV .
With a magnetospheric model to reproduce this vari-
ation, he found that a high-order, multipole field is
required, and that the field strength at the surface
of the white dwarf is 0.1 < B < 1 MG. This field
strength is at the lower extreme of those measured or
inferred for bona fide magnetic cataclysmic variables.
However, they do not exclude the possibility that at
an outburst the accretion of matter could occur onto
the magnetic poles of the white dwarf. Giovannelli
(1981) found QPOs from SS Cyg — during a rise up
to a maximum of one long optical outburst — with
periods in the range 8.96–9.91 s that show the in-
verse relationship between outburst luminosity and
oscillation period as general properties of CVs (e.g.
Nather & Robinson, 1974; Nevo & Sadeh, 1978). The
amplitude of the optical oscillations has a maximum
at the maximum of the outburst. Then the white
dwarf is heated directly by the surrounding luminous
material, as by accretion, as suggested by Hildebrand
et al. (1980) for AH Her. The variation of the rapid
oscillation periods in SS Cyg has the same trend with
respect to AH Her. The temperature is minimum
at the peak of the outburst. A possible explanation
could be that very close to the optical maximum the
inner part of the disk moves closer to the white dwarf
surface and it becomes denser and renders cooling
possible. This optical behaviour is in agreement with
the X-ray behaviour of SS Cyg. Indeed, at the opti-
cal maximum, the hard X-ray emission is lower than
during quiescence, and all the energy in the X-ray re-
gion is emitted below 2 keV (Ricketts, King & Raine,
1979).

With regard to the question of the possible mag-
netic nature of SS Cyg, Mauche et al. (1997) dis-
cussed the case of the UV line ratios of CVs, which
seem to be almost independent of the nature (mag-
netic or not) of CVs. Okada, Nakamura & Ishida
(2008) found from CHANDRA HETG observations
in SS Cyg that the spectrum in quiescence is dom-
inated by H-like Kα lines, and is dominated in out-
burst by He-like lines, which are as intense as H-like
lines. The broad line widths and line profiles indicate
that the line-emitting plasma is associated with the
Keplerian disk. In quiescence the lines are narrower
and are emitted from an ionizing plasma at the en-

trance of the boundary layer. Ishida et al. (2009)
found from SUZAKU observations of SS Cyg that
the plasma temperature in quiescence is 20.4 keV and
in outburst 6.0 keV. The 6.4 keV line is resolved in
narrow and broad components, which indicates that
both the white dwarf and the accretion disk con-
tribute to the reflection. The standard optically thin
boundary layer is the most plausible picture of the
plasma configuration in quiescence. The reflection
in outburst originates from the accretion disk and
an equatorial accretion belt. The broad 6,4 keV line
suggests that the optically thin thermal plasma is dis-
tributed on the accretion disk, in a manner similar
to that of a solar corona.

Long et al. (2005) found by fitting the double-
peaked line profile in SS Cyg that the FUV line-
forming region is concentrated closer to the white
dwarf than the region that forms the optical lines.
Their study provides no evidence of a hole in the in-
ner disk. However, they cannot fit SS Cyg data by a
simple model as white dwarf plus accretion disk.

The SS Cyg system is also important as labora-
tory for the study of circumstellar dust in CVs. In-
deed, Jameson et al. (1987) detected IR emission
from SS Cyg in outburst in IRAS Bands I and II
(11.8 µm and 24.4 µm). The most likely origin of the
IR emission is circumstellar dust heated by the en-
hanced UV flux during outburst. Dubus et al. (2004)
performed optical and mid-IR observations of several
CVs including SS Cyg in quiescence. For SS Cyg the
measurements at 11.8 µm are consistent with the up-
per limits obtained by Jameson et al. (1987) when
the source was not yet in full outburst. The observed
variability in the mid-IR flux on short time scales
is hardly reconcilable with intrinsic or reprocessed
emission from circumbinary disk material, while on
the contrary a free–free emission from a wind should
be reconcilable. If any sizeable circum-binary disk is
present in the system, it must be self-shadowed or
perhaps dust–free, with the peak thermal emission
shifted to far-IR wavelengths.

Gaudenzi et al. (2011) discussed the reasons for
the variable reddening in SS Cyg and demonstrated
that this reddening consists of two components: the
first is interstellar in origin, and the second (intrinsic
to the system itself) is variable and changes during
the evolution of a quiescent phase. Moreover, an or-
bital modulation also exists. The physical and chem-
ical parameters of the system are consistent with the
possibility of the formation of fullerenes.

The SPITZER space telescope detected an excess
(3–8) µm emission from Magnetic CVs, due to dust
(Howell et al., 2006; Brinkworth et al., 2007). This
is a strong push for observing SS Cyg carefully with
SPITZER. However, a weak IR excess was discov-
ered in SS Cyg looking at the SPITZER data, but
no conclusions were given since the data at different

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Acta Polytechnica Vol. 52 No. 1/2012

Table 1: Comparison of the characteristic parameters of EI UMa — a well established IP (Reimer et al., 2008) — and
SS Cyg (Giovannelli et al., 1983; Lombardi et al., 1987; Gaudenzi et al., 1990, 2002; Giovannelli & Martinez-Pais, 1991;
Schreiber & Gänsicke, 2002)

EI UMa SS Cyg

Porb = 6.434 h Porb = 6.603 h

Popt = 745 s (∼ PXMM) Popt = 745 s (∼ PXMM UV)

0.81 M⊙ < MWD < 1.2 M⊙ MWD = 0.97 M⊙

RWD = 7 × 10
8 cm RWD = 5 × 10

8 cm

MR = 0.81 M⊙ MR = 0.56 M⊙

RR = 0.76 R⊙ RR = 0.68 R⊙

LX ∼ 10 × 10
32 erg s−1 LX ∼ (6.6 − 9.8) × 10

32 erg s−1

Ṁ = 3.6 × 1017 g s−1 Ṁ ≃ (1 − 4) × 1017 g s−1

MV = 5.4 MV = 5.9

f = Rd/a = 0.2 – 0.3 f = Rd/a = 0.2

UBV Orbital modulations of UBV & UV Orbital modulations of

continuum continuum & emission lines EWs

wavelengths were not simultaneous (Harrison et al.,
2010).

3 The intermediate polar
nature of SS Cyg

In our opinion, there are several incontestable argu-
ments in favour of the IP nature of SS Cyg, namely:

i) the strong analogy of SS Cyg with the well es-
tablished IP EI UMa (Reimer et al., 2008). Table 1
shows the parameters of EI UMa and SS Cyg;

ii) in the diagram log LX vs log Ṁ for the IPs
(Warner, 1996a), SS Cyg lies just in the place of
IPs (Figure 1, left panel). The X-ray luminosity
of SS Cyg — LX ∼ (6.6–9.8) × 10

32 erg s−1 — has
been derived by the values of the distance of SS
Cyg: 166 pc (Harrison et al., 1999) and its X-ray

flux: FX ∼ (2-3)×10
−10 erg cm−2 s−1 (Giovannelli &

Martinez-Pais, 1991; McGowan, Priedhorsky & Tru-
dolyubov, 2004). The average mass accretion rate is
Ṁ ∼ 2 × 1017 g s−1 (Gaudenzi et al., 1990, and the
references therein; Schreiber & Gänsicke, 2002);

iii) in the diagram log Ṁ vs log Porb (Warner,
1996a) SS Cyg lies just in the place of IPs (Fig.
1, right panel). The orbital period of SS Cyg is
Porb = 6.6 h (Martinez-Pais et al. 1994);

iv) SS Cyg has been detected in the hard X-ray
range, together with other tens of very well known
IPs (Landi et al., 2009; Scaringi et al., 2010). Such
an emission cannot be justified without the presence
of an intense white dwarf magnetic field. Moreover, if
SS Cyg should be a dwarf nova (non-magnetic CV) —
as reported in the table of detected CVs in the former
papers — why are other dwarf novae not detected
with the same instruments?

Fig. 1: Left panel: relationship between X-ray luminosity and mass accretion rate for IPs that are indicated with x
(by courtesy of Warner, 1996a). Right panel: relationship between mass accretion rate and orbital period for IPs (by
courtesy of Warner, 1996a). SS Cyg positions are indicated with a red ⋆ in both panels

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Acta Polytechnica Vol. 52 No. 1/2012

Fig. 2: Observed emission fluxes converted to luminosity for magnetic CVs (after Howell et al., 1999) are indicated with
: C II in the left panel, and C IV in the right panel. SS Cyg positions are indicated with a red ⋆. Luminosity expressed
in erg s−1, and B in unit of 106 gauss

Fig. 3: Left panel: relationship between Pspin andPorb for IPs (by courtesy of Warner, 1996a). Right panel: relationship
between the magnetic moment of the white dwarfs in CVs and M⊙ (by courtesy of Warner, 1996a). Polars are indicated
with x, and IPs with their abbreviated titles. SS Cyg positions are indicated with a red ⋆. Magnetic moment µ is
expressed in G cm3, and M⊙ in unit of 10

16gs−1

Rudolf Gális et al. (2011, IBWS) when discussing
the X-ray and optical activity of INTEGRAL CVs,
explicitly mentioned SS Cyg as IP, making a compar-
ison of its X-ray behaviour with those very similar of
V 709 Cas, a well-established IP.

Then we can assume, beyond reasonable doubt,
that SS Cyg is an IP. What is the intensity of its
magnetic field? A relationship between the strength
of high-state UV emission lines and the strength of
the white dwarf magnetic field has been found by
Howell et al. (1999).

From the fluxes of UV emission lines (Gaudenzi
et al., 1986) we derived the luminosity of C II and
C IV: LCII ∼ 7.8 × 10

30 erg s−1, and LCIV ∼ 6.2 ×
1031 erg s−1, respectively.

Using these values of luminosity and the extrap-
olation of the line best fitting the emission line lumi-
nosity of C II and C IV from Howell et al. (1999), the
magnetic field intensity of SS Cyg is 2.5 and 2.0 MG,

respectively as shown in Fig. 2, left and right panels,
respectively.

Taking into account the errors in the fluxes of
the considered UV lines, we can argue a reasonable
value of the white dwarf magnetic field in SS Cyg as
B ∼ 2.2 ± 1.0 MG.
Moreover,

v) a periodicity at 12.18±0.01 m in R and I bands
was detected in SS Cyg by Bartolini et al. (1985),
probably the beat period between the spin period of
the white dwarf and the orbital period of the sys-
tem. If the rotation is direct with orbital motion
Pspin = 11.82±0.01 m, corresponding to 709.2±0.6 s.
If the rotation is inverse Pspin = 12.56 ± 0.01 m
(753.6 ± 0.6 s). Tramontana (2007) found a period-
icity of 12.175 ± 0.539 m using 504 images obtained
in the R band on October 26, 2006 with SS Cygni
in outburst at the TACOR teaching telescope of La
Sapienza University. Using 10 ks XMM-OM UV ob-

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Acta Polytechnica Vol. 52 No. 1/2012

servations of SS Cyg, Braga (2009) found a period of
709 ± 1 s that should be the rotational period of the
white dwarf, following the model of IPs developed by
Warner (1986).

Using Pspin = 709 s and the relationship Pspin vs
Porb (Warner, 1996a), SS Cyg lies in the zone of
the IPs very close to the line of 1 M⊙ (Figure 3,
left panel). The mass of the SS Cyg white dwarf is
0.97+0.14−0.05 M⊙ (Giovannelli et al., 1983).

Finally, with B ≈ 2 MG, and the radius of the
white dwarf Rwd = 5 × 10

8 cm (Martinez-Pais et al.,
1994), the magnetic moment is µ ≈ 2.5 × 1032 G cm3.
Using this value of µ and the average value of Ṁ, SS
Cyg lies just in the IPs region, as shown in the right
panel of Figure 3.

Therefore, we can definitively affirm that SS
Cyg is an intermediate polar.

Acknowledgement

We are pleased to thank the organizers of the Karlovy
Vary 8th INTEGRAL/BART Workshop for the invi-
tation. One of us (FG) wishes to thank the LOC for
logistical support.

This research has made use of NASA’s Astro-
physics Data System.

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Franco Giovannelli
INAF – Istituto di Astrofisica
Spaziale e Fisica Cosmica – Roma
Area di Ricerca di Roma-2
Via del Fosso del Cavaliere 100, I 00133 Roma, Italy

Lola Sabau-Graziati
INTA – Dpt de Programas Espaciales y Ciencias del
Espacio
Ctra de Ajalvir Km 4 – E 28850 Torrejón de Ardóz,
Spain

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