Historical Introduction

Cataclysmic variables (CVs) are mass-transfer binaries, in which the accretor is a white dwarf. The secondary can be a normal star with a hydrogen-rich envelope or a second white dwarf. Energy released in the gravitational field of the accreting white dwarf powers the phenomena observed in cataclysmic variables, while subsequent nuclear processing of the accreted matter leads to various types of nova events and supersoft X-ray sources (SSS). Much of the physics of CVs and the related objects is basically similar to that of X-ray binaries, in which the accretor is a neutron star or a black hole. When Sco X-1 was discovered in 1961, CVs were already known [11,73,93], and after its optical identification with a faint blue star, Shklovsky suggested that in Sco X-1 "we observe a binary system similar to WZ Sagittae in which one of the components is a neutron star" [79]. The nature of the bright galactic X-ray sources was finally settled by the exciting results obtained with Uhuru (1970-1973) as reminisced by Giacconi [19]. At least three CVs, 4U1228-29 (EX Hya) [94], 4U1809+50 (AM Her), and 4U 1849-31 (V1223 Sgr), were found among the fainter sources in the 4U-catalog [14]. The optical identification of the faint X-ray sources from Uhuru and other early missions was a tedious process, however, because of the comparatively low angular resolution and sensitivity of the early X-ray missions and the visual faintness of the CVs. A breakthrough was the identification of 4U 1809+50 with the long-known 12-15 mag variable star AM Her and Tapia's [84] discovery that AM Her was circularly polarized. The object was immediately interpreted as containing a strongly magnetic accreting white dwarf and became the prototype of the subclass of polars, a term coined by Krzeminski [41].

Nova events are regular cataclysms experienced by CVs during their long life. Close-binary supersoft X-ray sources (CBSS) were predicted in 1978 as "noneject-ing novae" [78, see also [64]], discovered in 1981 with Einstein [49], but undis-putedly accepted as such only after detailed studies with ROSAT [21,87,88]. Soft X-ray emission from a nova outburst was first seen in GQ Mus (N Mus 1983) [40]. Because of absorption in the Galactic plane, few SSS were found in the Galaxy, but some were subsequently identified among known CVs [21]. More recently, population studies of SSS have been conducted in external galaxies with ROSAT, Chandra, and XMM-Newton [24,87].

12.2 The Zoo of CVs

CVsz can be classified according to the nature of the mass accretor and the mass donor. The accretor is, by definition, a white dwarf, the donor can be a normal nuclear burning star, a substellar object (brown dwarf), or a white dwarf. In many systems, the nuclear burning stars are only moderately evolved and I refer to them loosely as main sequence stars. The physics of the accreting white dwarf is similar in symbiotic binaries, in which the secondary is an evolved star on the first giant branch or on the asymptotic giant branch (AGB). The phenomenology of these systems, however, is dominated by the emissions of the secondary or the wind and X-rays are often severely quenched. I refer to symbiotic binaries only in passing.

Table 12.1 lists the number of known members of a CV subclass, the fraction discovered by their X-ray emission, the name of the subclass, and the observed range of the X-ray vs. optical flux ratio FX/FV. The present census contains about 400 nonmagnetic and 130 magnetic CVs with known orbital period. Nonmagnetic CVs include all systems in which the white dwarf is not specifically recognized as magnetic. They accrete via an accretion disk and the observed soft and hard X-rays originate from the boundary layer (BL) between disk and white dwarf [43,65,93, and references therein]. Polars (AM Her stars) and intermediate polars (IPs/DQ Her stars) contain magnetic white dwarfs. In polars, the rotation period of the white dwarf is locked to the orbital motion of the binary and the formation of an accretion disk is prevented by the field. In intermediate polars, the white dwarf rotates freely and an accretion disk may or may not exist. The terms intermediate polar and DQ Her star are used synonymously by some authors, while others distinguish them by the white dwarf spin period. Only a handful of double degenerates/AM CVn stars is known and none of them harbors a magnetic white dwarf, although such systems may exist.

The majority of nonmagnetic CVs have been discovered in optical surveys, of which the Sloan Digital Sky Survey (SDSS) is a major source for the more recent new discoveries. Quite differently, 75% of all polars known at the end of the ROSAT era were discovered by their X-ray and XUV emission, most of them in the

Table 12.1 Subdivision of CVs according to the nature of donor and accretor

Accretor

Synchro-

Total

X-ray

Donor Star

nized

number

discov.

MS-Star/Brown Dwarf

White Dwarf

(%)

Subclass

Fx/FV

Subclass FX/FV

Non-magn.

No

400

~10

non-mag CV

0.01-1

AM CVn 0.1-5

Magnetic

No

35

65

IP/DQ Her

0.1-10

?

Magnetic

Yes

80

75

polar/AM Her

1-100

?

FX/FV is the ratio of the X-ray flux in the ROSAT PSPC band vs. the visual flux in a 1 000 A wide band [3]

"?" denotes that no example of this subclass is known

FX/FV is the ratio of the X-ray flux in the ROSAT PSPC band vs. the visual flux in a 1 000 A wide band [3]

"?" denotes that no example of this subclass is known

ROSAT-All-Sky-Survey (RASS). Although all CV subtypes have been detected as X-ray sources, they are characterized by vastly different X-ray vs. optical flux ratios FX/FV. In disk accretors, half of the gravitational energy is released in the accretion disk and the remainder appears either as emission from the BL or is added to the rotational energy of the white dwarf. The escape of X-ray photons from the BL may be severely impeded by absorption within the BL, in the accretion disk or in a wind emanating from the disk, all to the effect to lower FX/FV. In IPs/DQ Her stars, the inner accretion disk is disrupted by the magnetic field, but absorption may still occur in the magnetically guided accretion flow. In polars, on the other hand, a disk is missing, the soft X-ray flux escapes more or less uninhibited by internal absorption, and FX/FV assumes large values.

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