SAX/Aql X-1
Credit: S. Campana, Brera Observatory, Milano-Merate

The Fall of Aquila X-1

Objects called "Soft X-ray Transients" are composed of some type of compact object (probably a neutron star) and some type of "normal", low mass star (i.e. a star with a mass of some fraction of the Sun's mass). These objects show changing levels of low-energy, or "soft", X-ray emission, probably produced somehow by variable transfer of mass from the normal star to the compact object. In effect the compact object "gobbles up" the normal star, and the X-ray emission can provide the best view of how this process occurs. But because these objects vary it's difficult to catch them at crucial times. In February 1997 a soft X-ray transient in the constellation of Aquila known as Aql X-1 was detected in outburst by the Rossi X-ray Timing Explorer. RXTE followed the slow decline from X-ray maximum; but when the RXTE observations ended the story was still not complete, since the star had not reached its normal "quiescent" level. This important gap was filled in by the BeppoSAX X-ray satellite, which started to observe Aql X-1 at the end of the RXTE observations. The image on the right shows the decline from X-ray maximum as seen by RXTE (green points) and BeppoSAX (red points). RXTE clearly shows a rather smooth decline though the last two data points do not follow the smooth decline but seem fainter than astronomers would have predicted. The BeppoSAX observations confirm the RXTE results and show that the decline to quiescence occurs much more rapidly than expected. The image on the left shows BeppoSAX images detailing the change in brightness of Aql X-1 through March 1997.

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Page Author: Dr. Michael F. Corcoran
Last modified October 8, 2001