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 (A4686) 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 c..' 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 2 300 pc seems well established. The total X-ray power is c..'2 X 1(}'!7 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 tc = 3 X 10-10 h-1 R 312 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. < 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.

Some possible implications of the radio emission of Sco X-1

“…Several speakers at this Symposium have expressed the view that binary models are basically unsupported by observational evidence, but this is far from being a uniformly agreed conclusion. Wilson (1969) has even proposed a new binary model for Cyg X-2, an X-ray object similar to Sco X-l with an optical counterpart that seems to be a short-period spectroscopic binary (Burbidge et al, 1967;Kristian et al, 1967; but see Kraft and Demoulin, 1967). The existence of a measurable degree of circular polarization in the radio emission of Sco X-l might provide a possibility of confirming the essential correctness of such models.…”

Sco X-1

“…The first case would be appropriate if the optical variation were caused by partial eclipse, or more likely by a reflection, that is, the light curve in case of (Hiltner and Mook, 1967b). (b) Fine structure of a 'flicker' (Westphal et aL, 1968), The shape of the decrease from the bright period (B = 12.5) to the dark period (B = 13.5) shown in (a-I) seems to occur often, and this decrease may be a characteristic of Sco X-1. When the source is bright (B = 12.5), it often flickers (a-2).…”

“…The distance of Sco X-1 has been estimated by various method (Westphal et al, 1968;Hiltner and Mook, 1970;Felten and Humphreys, 1973;Oda and Matsuoka, 1971), but it has not been well established.…”

Properties of individual X-ray sources: (Invited discourse)

“…The optical observations were made by Westphal et al (1968) on the 100-inch telescope on the night of 3 June 1967. Figure 11 shows the Scorpius flare of 0910 on the 3 June observation by the satellite OSO-III and also the correlated optical data.…”