Thermonuclear X–ray bursts observed by the RXTE satellite

Exploding stars are by now familiar to both astronomy enthusiasts and science fiction fans. But a group at Monash University is studying a kind of star that explodes again and again, sometimes as regular as clockwork. The exploding stars are neutron stars, which are themselves produced in one of nature's largest explosions, the supernovae which mark the end of the life of medium–sized stars. Neutron stars explode when accumulated fuel – in this case hydrogen and helium, stripped from an orbiting companion star – ignites explosively, engulfing the star within a matter of seconds. Once the burning ends the fuel begins to accumulate once more, and the star is ready to explode again as soon as a few hours later.

A team led by Dr. Duncan Galloway has recently completed one of the largest studies of these explosions, also called X–ray bursts, to be published in the Astrophysical Journal Supplements. The team, including researchers at MIT, Caltech, and the University of Arizona in addition to Monash, analysed a sample of almost 1200 bursts detected by NASA's Rossi X–ray Timing Explorer satellite. The bursts were observed over more than 10 years from 48 known bursters, largely located towards our Galactic center.

The study reveals for the first time detailed similarities and differences between the burst sources, and confirms a key factor as the nature of the companion in determining the burst properties. Some companions donate both hydrogen and helium, but others consist mainly of helium, which significantly affects the bursting behaviour. A few of these systems behave exactly as we expect theoretically, which is a reassuring verification of our physical understanding. But the majority of systems behave in quite unexpected ways, and with the new catalog we have for the first time a large, homogeneous sample with which to test various additional effects which may be required to explain the observed behaviour.

The sample may lead to much more detailed understanding of the neutron stars themselves. The X–rays emitted during the bursts provide clues as to the physical dimensions – mass and radius – of the neutron stars, presently a high priority for measurement by astronomers. Future studies of bursts enabled by the data in the sample may even provide evidence for novel new states of matter, suggested by some theorists to perhaps make up some or all of the neutron star interior.

Links

• Preprint of accepted article
• Duncan Galloway's home page