Welcome to the World of X-ray Transients!
Introduction to Transient X-ray Sources
The definition of "transient" found in the dictionary reads: passing away
with time; not permanent; temporary. Certain cosmic X-ray sources behave in
just such a manner. They seem to appear in the sky for a short time and then
disappear. Some of them eventually reappear - after years or decades. The
class of sources called "transients" was created to distinguish such sources
as different from the more permanent, stable sources observed in the X-ray
sky. It is important to remember that the object in the sky does not really
disappear, just the X-ray emission we see coming from it.
There are many reasons that certain sources have this kind of "now you
see me, now you don't" behavior. It is believed that sudden large changes
in the accretion rate of the compact object in a binary system lie at the
heart of the transient behavior. The reasons why the accretion rate may
suddenly change in a big way are clear for some systems (it can be a natural
cycle in the behavior of the normal star), but not clear for others. Finding
a consistent explanation for the behavior of all the kinds of transient
behavior observed is an active area of astronomical research.
Is a Transient Different from a Highly Variable Source?
All stars change their brightness on some timescale - it might be the
nuclear evolution timescale (millions to billions of years!) as the star traces
out a characteristic path on the H-R diagram, or it might be on a much shorter
timescale (less than centuries). For the class of stars which change
their brightness on short timescales, there are two subdivisions:
- "extrinsic" variables - which include binary systems in which the
apparent brightness is modulated by eclipses due to the binary companion
intervening or some intervening nebulosity. Such systems have helped us gain
knowledge about fundamental astrophysical parameters such as stellar sizes and
masses.
- "intrinsic" variables - which include stars which have flares,
pulsations,
quasi-periodic expansion/contraction of their stellar envelopes, mass
accretion
or ejection, explosive thermonuclear burning (novae), and cataclysmic
gravitational collapse (supernovae). Such objects have provided us with
knowledge of stellar structure, dynamics, and evolution.
Since most of the observed X-rays are generated from the
accretion process, the very nature of accretion produces a large degree of
variability in the amount of X-rays produced by a given object as a
function of time. The timescales can range from milliseconds to years.
Some of the variations are regularly spaced in time, such as those due
to the rotation period of the compact object in a binary or the revolution
period of one star around another. Other modulations might be
quasi-periodic, that is they occur "almost" on a
regular basis, and can be the result of complicated interactions between
the stars and the accretion disk surrounding the compact object. Some
variations are completely aperiodic, that is they recur at random times.
This might be due to a sudden variation in the stellar wind from the normal
star or a sudden mass ejection from the normal star.
The differentiation of objects into separate categories called "highly
variable" versus "transient" is somewhat artificial and not always clear cut,
although in general, it is believed that distinctly different physical
phenomena are responsible for what astronomers have classified as "highly
variable" versus "transient" behavior. A good rule of thumb is to refer to
sources as "transient" which behave in the following way:
- They have a fairly rapid rise in intensity followed by a gradual
decline. Typically, transients reach their maximum intensity in about a
week and decline back to undetectability in a month or two.
- The change in intensity is typically a factor of 1000 or more.
- In quiescence, the source in undetectable to a level of 10-12
erg/s/cm2 in the 2-6 keV range.
- The recurrence timescale is years.
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