3U 1820-30

G. Clark, Massachusetts Institute of Technology, reports that the SAS-3 Group has found that the intense x-ray bursts from the source 3U 1820-30 in NGC 6624 (cf. IAUC 2879) are recurrent. Bursts were detected at times near those given by the sequence 1975 May 16.91310 + 0.18192 k UT, with k = 1-10, except that the burst at k = 4 was unobserved on account of earth occultation. The rms deviation between the sequence times and the actual occurrence times of the bursts is 527s, which is equivalent to a phase jitter of 4.1 percent. During 1976 Jan. 16-22 the source has been exceptionally bright (its average intensity at 1-10 keV approximately twice the Uhuru value), and observers are urged to monitor the intensity at various frequencies during this high-intensity phase.