Imagine the Universe!

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)

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 ?


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)

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.

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)

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.

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)

This clip shows a number of Active Galaxies. Although they are faint, notice that their X-ray intensity varies.

Movie Credit: RXTE ASM Team/MIT

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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