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The Dynamic X-ray Sky
Of the thousands of objects in the sky that emit X-rays, some are very
steady X-ray emitters. These are the supernova remnants and some
X-ray binary systems. But many more objects change their X-ray
intensity. In some of these objects, the changes are very regular and
periodic. In others, the changes are irregular and unpredictable.
And some sources are bright in X-rays only for a short time.
This video feature explores the different types of variable X-ray
objects, and the causes underlying the variations. The clips
are from a video of the X-ray sky produced by MIT using data from
the All Sky Monitor on the Rossi X-ray Timing Explorer. Note that
in all the clips, the X-ray intensity is indicated by the size
of the dot for the object.
More information about the video images
Orbital Motion in X-ray Binaries
X-ray binaries consist of a normal star and a compact object, which
may be either a neutron
star or black hole. In some X-ray binaries, the X-ray intensity
varies regularly as the normal star moves in its orbit around
the compact object. The orbits in these particular systems are not
usually circular, so as the normal star gets closer, more material
is dumped onto its compact companion, causing more X-rays. When it
is further, there is less material and fewer X-rays.
Click on the image to view the video. (4.5 MB) (Description)
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In this clip
we see two X-ray binaries which have different orbital periods.
The period of GX301-2 has a 40 day period and a highly eccentric
orbit, whereas the period of Cen X-3 is 2 days and in a nearly circular
orbit. Can you see the variation in brightness with the period
in GX301-2 ?
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Transients
There are some objects that don't emit X-rays all the time.
They usually appear suddenly, are bright for a short time, and
then fade slowly while remaining visible for a few weeks or
months. Some occur only once, while others will recur after
a span of months or years. These "transient"
objects are often X-ray binaries. There are a number of
causes for this transient behavior. Some occur because
the normal star becomes active and spews material toward
the compact companion. In others, material accumulates slowly on
the compact object and then suddenly ignites when it becomes
dense and hot enough.
Click on the image to view the video. (3.8 MB) (Description)
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Here an outburst of 17095-266 is already in
progress when the recurrent nova Aql X-1 becomes bright. Aql X-1
became bright again in Aug 1997 and March 1998.
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Black Holes and Microquasars
Black holes can be highly variable X-ray emitters. Their
X-ray intensity can vary erratically and without any pattern.
This is thought to be due to the nature of the disk of material
surrounding the black hole. Varying temperature, pressure and
magnetic fields cause instabilities in this disk which may cause
either events similar to solar flares to occur in the disk, or the
rushing of material inward to the black hole.
Click on the image to view the video. (2.4 MB) (Description)
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RXTE discovered that some black holes act
as micro-quasars, local versions of the bright but distant quasars.
Here, the micro-quasar GRS 1915 (seen at the same time as another
outburst of Aql X-1) displays very erratic brightness
variations, due to jets and a very unstable disk.
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Active Galaxies
Active Galaxies harbor supermassive black holes at their
centers. These black holes can have masses of more than a million
times that of our sun. Material near the center of the galaxy
is drawn toward the black hole and emits enormous amounts of X-rays.
The only reason these objects are faint to us is because they
are so far away. The surprising aspect is that despite their
size, they show variability in their X-ray emission.
Click on the image to view the video. (2.6 MB) (Description)
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This clip shows a number of Active Galaxies. Although they
are faint, notice that their X-ray intensity varies.
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Movie Credit: RXTE ASM Team/MIT
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