The light curve of a transient X-ray source

The Ariel-5 satellite has monitored the X-ray light curve of A1524--62 almost continuously from 40 days prior to maximum light until its disappearance below the effective experimental sensitivity. The source exhibited maximum light on approx.1974 December 4, at a level of 0.9 the apparent magnitude of the Crab Nebula in the energy band 3--6 keV. Although similar to previously reported transient source with a decay time constant of approx.2 months, the source exhibited an extended, variable preflare on-state of approx.1 month at a level of approximately-greater-than0.1 maximum light. The four bright (>0.2 of the Crab Nebula) transient sources observed during the first half-year of Ariel-5 operation are indicative of a galactic disk distribution, a luminosity at maximum in excess of 10$sup 37$ ergs s$sup -1$, a frequency of occurrence which may be as high as 100 yr$sup -1$, and a median decay time which is less than 1 month. (AIP)