Variable X-Ray Sources

L. J. Kaluzienski and S. S. Holt, Goddard Space Flight Center, communicate the following: (1) The Ariel 5 all-sky monitor has detected a flare from the region centered on l = 357o.5 +/- 0o.5, b = -3o.2 +/- 2o.0 (R.A. = 17h49m, Decl. = -32o.7, equinox 1950.0; 90-percent-confidence error box). Observations in the instrument fine spatial resolution mode on the latter half of Aug. 19 yield a source flux (3-6 kev; half-day averages) of 0.4 +/- 0.05 times that of the Crab Nebula and early on Aug. 22 of 0.6 +/- 0.1 times the Crab. Similar observations on Aug. 12 yield a possible detection (0.1 +/- 0.1 times the Crab), while course-mode observations show the possible presence of a source beginning Aug. 14 (although confusion with known nearby sources cannot be ruled out). The derived position of the flare is consistent with that of 4U 1755-33 and, if due to this source, the flare represents an order of magnitude increase above the peak level observed by Uhuru. (2) Continued monitoring of the Norma-Circinus region has revealed a decline in the average intensity of the flaring source 4U 1608-52 (IAUC 3090, 3094). Following the initial rapid rise phase (half-day-average maximum flux at 3-6 keV ~ 1.0 times the Crab) during July 5-10 the source maintained a relatively stable plateau level of ~ 0.6 times the Crab through early August. During this time no significant variations on timescales >~ 0.5 day were exhibited, although orbit-to-orbit variability was observed. The average intensity during Aug. 12-22 decreased to ~ 0.45 times the Crab, and this may thus signal the commencement of the decline phase. The duration of outburst is already considerably longer than that of Nov. 1975 (IAUC 2859), which lasted less than one month. (3) From additional observations of turn-offs in the 16.6-day cycle of Cir X-1 (IAUC 2939) we have refined the ephemeris to T = 1977 May 28.618 (+/- 0.09) UT + 16.595 (+/- 0.004) E. T is the time of abrupt transition from high to low flux, and errors are 3-sigma.