Cataclysmic Variables
Introduction to Cataclysmic Variables (CVs)
Cataclysmic variables (CVs) are binary star systems which have a white dwarf and a normal star companion.
They are typically small - the
entire binary system
usually has the size of the Earth-Moon system - with an orbital period in the range 1-10
hrs. The white dwarf is often referred to as the "primary" star, and
the normal star as the "companion" or the "secondary". The companion
star, a more or less normal star like our Sun, loses material onto the
white dwarf via accretion.
Since the white dwarf is very dense, the gravitational potential energy is
enormous, and some of it is converted into X-rays during the accretion
process. There are probably over a million such cataclysmic variables
in the Galaxy, but only
those close to our Sun (several hundreds) have been studied in
X-rays so far. This is because CVs are fairly faint in X-rays; they
are just above the coronal
X-ray sources and far below the X-ray binaries in terms of how
powerful their X-ray emissions are.
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A diagram of a Cataclysmic Variable, showing the normal star,
the accretion disk, and the white dwarf. The hot spot is
where matter from the normal star meets the accretion disk.
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Classical Novae and Dwarf Novae
Optical astronomers discovered CVs based on their outbursts in the
middle of the 19th century. CVs are classified
into subclasses according to the properties of
the outbursts: classical novae
and dwarf novae.
Classical novae are seen to erupt once, and the amplitude of the outburst
is the largest among CVs. Classical nova outbursts
are caused by sudden nuclear fusion of
hydrogen-rich material on the surface of the white dwarf. (Because
white dwarfs are the cinders of stars like the Sun, hydrogen fusion is
possible only when fresh fuel is accreted onto its surface.)
500 day light curve of the dwarf nova SS Aur
Dwarf novae outbursts result from temporary increases in the rate of
accretion onto the white dwarf. Dwarf novae outbursts are smaller
in amplitude and higher in
frequency than classical novae. U Gem is the prototype of dwarf
novae. The brightness in the
visible light of
U Gem increases 100-fold every 120 days or so, and returns to the original
level after a week or two.
Optical astronomers have also recognized
'recurrent novae' (eruptive behaviors in-between classical and dwarf novae),
and 'nova-like systems'(stars that have similar spectra to other types
of CVs in the visual light, but have not been seen to erupt).
X-ray Emission from CVs
Some X-ray sources detected by Uhuru turned out to be CVs; later
on, the Einstein observatory observed many CVs to be weak X-ray
sources. This is not surprising, as the accreting matter can easily
reach temperatures of 100 million degrees or so near the white dwarf surface.
Studies of CVs in X-rays therefore reveal the details of the
accretion process near the primary. In the majority of CVs, accretion
proceeds via an
accretion
disk. This occurs because the material leaving the secondary has angular
momentum from the binary motion and, therefore, accretion cannot take place in
a straight line. Rather, a disk-like structure is formed in the plane of the
binary orbit (i.e.,
an accretion disk). Friction within the disk heats
up the accreting material, and forces the material to gradually spiral down
onto the white dwarf surface. X-rays in these CVs are generally believed to
come from where the accretion disk hits the white dwarf surface (known as the
boundary layer), the details of which are currently a topic of active
research.
However, most of the strongly X-ray emitting CVs turn out to have a
magnetic white dwarf primary (some are known to have a magnetic field
over a hundred million times stronger than that of the Earth). Since
the accreting material is ionized, such a magnetic field can control the flow.
Thus the geometry of accretion is very different in these magnetic CVs;
accretion disks are truncated or absent, "column" and
"curtain" are two of the words used to describe the geometry near
the surface. In these cases, accretion is closer to vertical, along the
magnetic field lines, which results in a stronger shock and stronger X-ray
emission than when the accretion is via a disk. Magnetic CVs have been
discovered mostly through their X-ray emission over the last 30 years.
In some cases, nuclear fusion, rather than accretion, can become the
dominant energy source in a CV. The case of the classical nova outburst
has been mentioned above. In addition, X-ray astronomers have discovered
a class of objects called the "super-soft sources" (or SSS): the name
derived from the X-ray spectrum of these systems, which is dominated by
soft (lower energy) X-ray photons, typically below 0.5 keV. Detailed
studies of the spectra of these SSS have revealed that they have
the characteristic of X-rays from the hot (T ~ 200,000 - 800,000K), high
gravity (g ~ 1,000,000 m/s/s) surface of a star. Such high gravity implies a
white dwarf more massive than our Sun. Theoretical considerations indicate
that the condition that leads to continuous nuclear burning on the white
dwarf surface in these SSS may eventually lead to a supernova explosion.
Last Modified: November 2004
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