In 1999 two new X-ray satellites were launched - Chandra and
XMM-Newton. Here we mark a few of their discoveries and
show some of the X-ray images that are now available to
astronomers. For the first time there is comparable resolution
between X-ray and radio observations.
X-ray equipment
X-ray astronomy can only be carried out in space because X-rays are absorbed by the Earth's atmosphere. The telescopes differ from optical ones: owing to their high energy, X-rays penetrate into a mirror in much the same way as bullets slam into a wall; likewise, just as bullets ricochet when they hit a wall at a grazing angle, so too will X-rays ricochet off mirrors, allowing them to be focused.
Observations from Chandra and XMM-Newton are complementary. The latter has a larger collecting area than Chandra - better for observing faint sources - but lower resolution.
New sources shed more light on active galaxy behaviour
With the improved resolution now available to X-ray astronomers, many new sources are being discovered. This wealth of new data gives a boost to understanding the energetic processes that are at work in the heart of active galaxies. Many of the new extragalactic X-ray sources are bright and point-like, and a significant number are probably caused by material accreting round black holes.
In the starburst galaxy NGC253, four black holes seem to be gravitating towards the centre of the galaxy, where they could coalesce to form a single supermassive black hole. The observers even suggest that NGC253 is undergoing a transition between starburst activity and a quasar-like galaxy. Quasars release huge amounts of energy, outshining galaxies of hundreds of billions of stars, from a space the size of our solar system. Each black hole would have a mass ten to thousands of times that of the Sun. This interpretation is a subject of great interest.
Another team surveyed 37 different galaxies suspected of containing a supermassive black hole in their centre. Some 25% contained extremely luminous sources.
For the first time the periodic variation of an X-ray source outside the Local Group of galaxies was clearly determined - proof that it originates from a binary system likely to contain a black hole of stellar mass.
Rapid X-ray flaring has been detected from the compact radio source in the Milky Way. The faintness of the associated X-ray radiation has in the past cast doubt on whether the emission really comes from matter accreting onto a supermassive black hole. Now, the rapid flaring means that the model is back on track. A factor of five change in the luminosity was detected in just 600 s. Given the finite speed of light, the emitting region could be no bigger than 20 times the black hole's event horizon.
Chandra witnesses the death of a star
The death throes of a star are shown up in X-rays by the Chandra Observatory. The young, oxygen-rich supernova remnant is lit up by a central radiating pulsar. Reading the pattern of the elements that make up the remnant helps to unravel the connection between the pulsar and the star from which it formed. The expanding shell is 36 light-years across. The blue knots of emission contain material that is enriched in oxygen, neon and magnesium.