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Published October 1968 | Published
Journal Article Open

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


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.

Additional Information

© 1968 American Astronomical Society. Provided by the NASA Astrophysics Data System. Received March 25, 1968. We are indebted to Edwin Dennison and to the personnel of the Astroelectronics Laboratory for their construction and maintenance of the data readout systems on all telescopes, without which many of the new techniques of data collection would not have been developed. It is a pleasure to thank Guido Munch for discussions on the interstellar Ca II problem. We also had a useful conversation with E. N. Parker on the problem of coronal heating. An early conversation with J. Ostriker and J. E. Hesser at Princeton was useful in suggesting the power of short-sample-time digital recording of stars, and started us thinking about the current problem.

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Published - 1968ApJ___154__139W.pdf


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August 19, 2023
October 25, 2023