XMM-Newton rediscovers old Exosat source
9 March 2000During operations to fine tune the XMM-Newton Optical Monitor, theobservatory was pointed close to the Large Magallenic Cloud galaxy, to a location where 20 years ago ESA's first X-ray observatory, Exosat,discovered a bright new X-ray source which astronomers named EXO 0748-676. This source consists of two stars orbiting around each other in only 3.8 hours. One of these stars is a neutron star, essentially a giant atomic nucleus 20 km in diameter. This star is so dense that a teaspoon full would weigh a hundred million tonnes! The other star is a normal star, much like the Sun, but distorted into an egg-like shape by the intense gravitational field of the neutron star. Gas is being pulled from the normal star and is falling onto the neutron star via a giant swirling accretion disk. As the gas spirals in the accretion disk and falls onto the neutron star it reaches temperatures of several million degrees - hot enough to emit X-rays.
EXO 0748-676 is a very special X-ray source because of the wealth of phenomena that it shows. For 8 minutes every 3.8 hours the X-rays almost entirely disappear as the normal star passes into our line of sight to the neutron star. The key word here is `almost'. About 5% of the X-rays remain, even though the neutron star is completely hidden from view. Astronomers believe that many of these remaining X-rays come from a giant corona - a bit like that on the Sun, but much hotter and more intense - that surrounds the tiny neutron star. The eclipses are indicated on the second image. This shows the number of X-rays detected against time on the horizontal axis. The fact that the X-rays do not fully disappear during eclipse is clearly visible in the figure.
EXO 0748-676 also shows erratic reductions in X-ray intensity which are not as deep, or regular, as the eclipses. These reductions, or dips, are caused by material in the flared outer regions of the accretion disk which blocks the passage of some of the X-rays. The fact that the amount of dipping varies around the 3.8 hour orbit, and from orbit to orbit, means that the accretion disk is highly structured and varies with time. The study of these dips has revealed much about the structure of accretion disks and the type and temperature of the gas flowing onto the neutron star. Finally, EXO 0748-676 undergoes X-ray bursts - brief intense flashes of X-rays that are due to nuclear burning of material that has accumulated on the surface of the neutron star. These bursts typically last for about a minute and cool as they decay. No one knows how fast the neutron star in EXO 0748-676 is rotating. In some other X-ray binaries variations in X-ray intensity during bursts have allowed researchers to show that these neutron stars are rotating around 300 times every second! There is no doubt that astronomers will be keenly searching for similar signals in XMM-Newton data. ESA's XMM-Newton observatory was launched by Ariane-5 on 10 December 1999. The Calibration and Performance Verification phase for XMM-Newton's science instruments began on 3 March, with routine science operations due to start in June.
ESA's Exosat satellite was operational from May 1983 until April 1986 and in that time made 1780 observations in the X-ray band of many classes of astronomical object.