4U 1820-30

M. Garcia, R. Burg, and J. Grindlay, Center for Astrophysics, write: "We searched Einstein Monitor Proportional Counter (MPC) observations, made in 1979 Mar., Apr., and Sept. of the x-ray-burst source in NGC 6624, for the 685-s period discovered by Priedhorsky et al. (IAUC 4247). Due to sporadic variability of the source on ~ 10-min timescales, we found a number of apparently significant periods. However, on Apr. 5 and 7 the strongest periods are 683.0 +/- 9 s (1 sigma) and 684.0 +/- 1.8 s, respectively; phasing the data over the two days creates aliasing at delta-P = 3 s, but only one period (684.9 +/- 0.12s) is consistent with both the SAS and EXOSAT period determinations. The pulse fraction is 2.8 percent and the pulse shape is smooth with a minimum lasting about 135 deg. Comparing our period to the weighted mean of the 4 EXOSAT values reported in IAUC 4247 (685.3 +/- 0.4 s) and the 6 SAS values reported in IAUC 4254 (684.99 +/- 0.07 s) yields P-dot/P = 1.0 (+/- 1.0) x 10E-4 and -0.54 (+/- 0.67) x 10E-4 yrE-1, respectively. Both are consistent with P-dot/P = 0.0 and the 3-sigma upper limit of P-dot/P < 2.3 x 10E-4 yrE-1 reported by Morgan and Remillard (IAUC 4254). The 691.5-s period observed 1978 Oct. 6 with the HEAO A-1 satellite (Norris et al., IAUC 4257) does not appear consistent with our period and P-dot/P limits (above). Comparing our period to that derived from the HEAO A-1 observations made 6 months earlier would imply P-dot/P = -1.9 (+/- 0.6) x 10E-2 yrE-1; this inconsistency suggests that the HEAO A-1 period determination may be affected by sporadic variability of the sort seen with the MPC. Our upper limits on P-dot/P support the interpretation by Priedhorsky et al. that 685 s is the system's binary period."