Circinus X-1

L. J. Kaluzienski and S. S. Holt, Goddard Space Flight Center, report the detection of a strong increase in x-ray flux from Cir X-1 on Sept. 10.1 +/- 0.1 UT, the 3-6 keV flux increasing to that of the Crab Nebula on Sept. 11.3 (integrations over ~ 0.3 day). Fine-mode observations on Sept. 9.6 and again on Sept. 12.2 yield upper limits of approximately 0.08 times the Crab. This flare is significant in that it represents the first positive detection of the source at high levels (> 0.2 times the Crab) early in the phase of the 16.6-day cycle (turn-on was at phase ~ 0.3) since Ariel 5 was launched in Oct. 1974. This behavior is reminiscent of that observed by Uhuru in 1971-72 and suggests a possible return to the source conditions prevailing at that epoch.