Rapid Changes in the Optical Intensity and Radial Velocities of the X-Ray Source SCO X-1

Abstract

The optical flux of Seo X-1 was continuously monitored on five separate nights between April and June 1967. Pulse counting was used with sampling intervals of 5 sec on the 100- and 200-inch telescopes and 15 sec on the 60-inch reflector. The error due to photon statistics per sample interval was smaller than 0.007 mag in all cases. The data were processed entirely by computer, using a variety of special digital techniques. Rapid optical flickering with amplitudes of ~0.02 mag on a time scale of minutes was found on all five nights. These high-frequency fluctuations are superposed on slower continuous variations with amplitudes ~0.15 mag, and on occasional bursts of 0.2 mag which last about 10 min with very short rise and decay times. Radial velocities have been measured by a digital method of cross-correlation of microphotometer records of the original photographic plates (Westphal 1966). Velocity changes of about 100 km sec^(-1) were found on two different singly trailed plates, each of 3 hours' duration. The hydrogen lines and He II (⋋4686) change in opposite directions, similar to the extar Cyg X-2, suggesting, but not proving, binary motion. Systematic changes of velocity from night to night are set out in Table 2. The distance to Seo X-1 is estimated to be D ≃ 500 pc by three methods, based on the strength of interstellar Ca II K, and on the old nova hypothesis. A firm lower limit of D ≥ 300 pc seems well established. The total X-ray power is ≃2 x 10^(37) h^2 ergs sec^1, where his the distance in units of 500 pc. If the Xray energy is due to bremsstrahlung, this level requires Seo X-1 to continuously pump energy into the hot plasma, replenishing the entire energy store of gas in the cooling time of t_c = 3 X 10^(-15) h^(-1) R^(3/2) sec where R is the radius (in centimeters) of the plasma system. Reasonable values of 10^(11) cm < R < 10^(12) cm give 100 h^(-1) < t_o < 3000 h^(-1) sec, which, remarkably, is the characteristic time of the light flicker. Mechanical energy due to some type of low-Q oscillation of the system, similar in many respects to that observed in the Sun, seems capable of providing 10^(37) ergs sec^(-1) to the plasma gas. The crucial experiment remains to simultaneously monitor the optical and the X-ray flux over periods of several hours. Detailed correlation of fluctuations is not expected, but the mean X-ray flux might plausibly be expected to vary in periods of fractions of an hour as the mean amplitude of the optical fluctuations changes.

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