X-Ray Sources

N. E. White, H. E. Huckle, K. O. Mason, P. A. Charles, G. Pollard, J. L. Culhane and P. W. Sanford, Mullard Space Science Laboratory, write: "The Copernicus and Ariel 5 (experiment C) satellites have made observations of seven galactic x-ray sources in the energy range 3-8 keV. The results of a search for modulation of the x-ray flux in the period range 2.5-50 min are as follows: Source Satellite Date C Period (min) A 3U 1223-62 Ariel 1975 Jan. 5 39 11.64 +/- 0.02 50 3U 1728-24 Copernicus 1972 Sept.11 47 4.315 +/- 0.004 12 3U 1728-24 " 1973 Mar. 25 31 4.39 +/- 0.07 20 3U 1813-14 " 1975 Aug. 1 254 31.9 +/- 0.1 4 C gives the mean count per min. The amplitude A (percent) quoted is the peak to mean amplitude as a percentage of the mean flux. Four sources, 3U 1702-36, 1728-16, 1757-25 and 1811-17, showed no detectable modulation with upper limits to the amplitude, of 15, 5, 4 and 4 percent, respectively, when observed with Copernicus. The 4.3-min period of 3U 1728-24 is close to twice the 2.3-min period suggested by Lewin et al. (1971, Astrophys. J. 169, L17) from higher energy data."