Legacy of starburst is a nest of X-ray pulsars

Millions of years after the star-birth rate in our close galactic neighbour, the Small Magellanic Cloud (SMC) went through a boom, astronomers are witnessing the consequences in the form of powerful blasts of X-rays from evolved stellar binaries sytems undergoing mass transfer from one to the other. Dr Malcolm Coe of the University of Southampton will tell the UK/Ireland National Astronomy Meeting in Dublin that the SMC has become a nest of X-ray binary pulsars as the result of the creative burst that spawned thousands of massive new stars 100 million years ago. The catalogue of objects currently stands at 36 systems, but is rising fast.

The Small Magellanic Cloud is an irregular companion galaxy to the Milky Way at a distance of approximately 200,000 light years. Some 100 million years ago its orbit led to a close encounter with another of the Milky Way’s satellite galaxies, the Large Magellanic Cloud. Astronomers believe that a major period of starbirth occurred as a result of the interaction, in which many thousands of young, massive stars were created. These stars quickly evolved ( at a rate many times faster than our own Sun evolves) and the resulting flurry of supernovae gave rise to many neutron stars, some of which are in binary systems with one of the original massive stars that has not become a supernova. The presence of a huge star ejecting material right next to a neutron star sets the scene for an exchange of material between the couple and the consequent production a intense X-rays.

Dr Coe, and colleagues at the NASA Goddard Space Flight Center, have been monitoring the SMC on a weekly basis for the last 3 years with the Rossi X-ray Timing Explorer, an orbiting X-ray observatory. They have been detecting an increasing number of these binary systems when they undergo powerful X-ray outbursts. Though the RXTE has poor imaging quality, it has superb timing capabilities and each system can be identified by its characteristic signature – its pulse period. This pulse period represents the spin period of the neutron star and ranges from a few seconds up to a thousand seconds. By watching for these different periods Dr Coe and colleagues have been building up a behavioural history of these distinct representatives of a generation of stars.

“We are finding two types of X-ray outbursts,” explains Dr Coe. “There are regular outbursts, apparently taking place every time the neutron star passes through the disk of material that has developed around the middle of its massive companion. And then there are irregular, larger outbursts when the massive star suddenly ejects material that swamps the orbital path of the neutron star. Most systems are identified for the first time while undergoing one of the larger outbursts.”

“The presence of these X-ray binary pulsars gives us a unique opportunity to learn something about a cosmic event 100 million years ago and every new object we discover helps us build up a better picture” comments Dr Coe.
 
Notes for editor

1. All presentations at the UK/Ireland National Astronomy Meeting in Dublin, 8-11 April, are open to the media, who are welcome to attend free of charge. There will be a staffed press room. Full details at http://star.arm.ac.uk/nam2003/ or contact the RAS press officer Jacqueline Mitton on (+44) (0) 1223 564914.

2. Images relating to this story are available at:

http://www.astro.soton.ac.uk/~mjc/pr