Optical identification of the transient X-ray source A1524 - 61

Since the decay of the highly successful X-ray observatory on board the "Einstein" satellite, things have been relatively quiet concerning discoveries on the X-ray sky. For some time, the scene was dominated by the tiny Japanese satellite Hakucho. This spacecraft carried no imaging detectors, but turned out to be very useful for the study of already known, relatively bright sources. At ESO, we used much observing time simultaneously with Hakucho in an attempt to learn more about the so-calied X-ray burst sources (1). Recently, two more X-ray satellites have been launched: the second Japanese satellite, TENMA, and the European EXOSAT. Most of the observations with these satellites are planned in advance, but, as is the case with ground-based observations, the temporary appearance of a new celestial object may justify a change of schedule. This is what happened on August 18, 1983, when the transient source monitor on board TENMA detected a bright, new X-ray source near the border between the southern constellations Lupus and Norma. Three days later the source had reached half the brightness of Sco X-1 , wh ich may be called the "Sirius" of the X-ray sky. At La Silla, we were immediately alerted by telex in order that we could try to identify the source optically, but during just those days, odds were against any optical work. Full moon was approaching, and the Schmidt telescope was out of operation because of technical Work. Or. Tanaka, from the TENMA team, had, however, remarked that the new source appeared very close to the Position of a bright X-ray transient which had been seen from July 1971 till September 1972 by the Uhuru satellite. This Source, called 4U1543-47 (2), had never been identified optically. We immediately checked our CCO data bank for pictures of the former transient position. Indeed, CCO pictures had been taken in July 1981 during the installation period ofthe CCO camera at the 1.5 m Oanish telescope. Prior analysis of these images (which were taken in three colours) had shown no object of conspicuous colours. The images were checked once more, but with the same result. The location of the old transient was, however, not known to great accuracy, and it could, indeed, be outside the area covered by the CCO.