Observations of cosmic X-ray sources in the 10–250 keV range

Abstract

Observations of cosmic X-ray sources have been made from high-altitude balloons over Palestine, Texas, using actively collimated detectors. In this technique, a thin NaI central counter 10 to 50 cm2 in area is surrounded by a CsI well crystal shield several centimeters thick. The aperture, about 8° to 20° FWHM, is determined by either the well opening or an active honeycomb collimator. The background is determined mainly by diffuse cosmic and atmospheric X-rays entering the forward aperture. The detector is usually either servo-controlled to track the source or operated as a meridian device. Data are telemetered over the 20–250 keV range in a digital format from a 128-channel pulse-height analyzer. Several strong sources in the northern hemispheric sky have been observed. The Crab nebula has a power-law differential number spectrum with an index of –1.9 ± 0.1 and an intensity of about 10−2 photons/cm2-s-keV at 20 keV. Two observations in September 1965 and September 1966 on this object give the same flux and spectral index within about 5% over the 20–100 keV range. The source Cygnus XR-1 also has a power-law shape, very similar in slope and intensity to the Crab, which extends above background to at least 180 keV. These measurements are in general agreement with those of other workers. The power-law spectrum of the Crab and Cygnus XR-1 may be contrasted with that of SCO XR-1, which has an exponential spectrum, typical of a hot gas at 50 × 106 °K.