OSO-7

photo of OSO-7

* Mission Overview

The proportional counters and hard X-ray telescope on board OSO-7 scanned the entire sky and obtained good exposure of point sources through repeated scans. Data from the proportional counters yielded a "multi-color" catalog of 184 sources (Markert et al., 1979) and the detection from Centaurus A of rapid intensity variability (Winkler & White, 1975). The X-ray telescope studied the high energy temporal and spectral behavior of a number of sources including Her X-1 (Ulmer er al., 1973) and Vela X-1 (Ulmer et al., 1972).

* Instrumentation

Just under three years of data on celestial X-ray sources were obtained using the MIT instrumentation aboard the OSO-7 satellite. The detection system consisted of two banks of proportional counters which viewed the sky through 1- and 3-deg tubular collimators (FWHM) and covered a 1 to 60-keV energy range in five broad energy intervals (1-1.5 keV; 1-6 keV; 3-10 keV; 15-40 keV; 30-60 keV). The data were recorded in 256 azimuthal bins as the detector scanned across a small circle on the sky with each spin. The time resolution was 3.2 minutes.

The UCSD OSO-7 X-ray telescope was designed to detect X-rays in the range from about 7 to 550 keV. It consisted of a 1 cm thick NaI(Tl) crystal inside a massive CsI(Na) anticoincidence shield. There were 128 energy loss channels. Each event signature also contained a clock time good to 0.625 ms. The field of view was 6.5 degrees FWHM and the sensitive area was ~ 64 sq-cm. The total detector weight was 85 lbs.

The UNH gamma-ray monitor consisted of a 7.6 cm x 7.6 cm NaI(Tl) scintillator which operated in the range 300 keV - 10 MeV. The detector was surrounded on all sides by a ~ 4 cm thick CsI(Na) scintillator cup. The exception was a forward aperture which was covered by a 0.5 cm thick CsI(Na) slab. The instrument was calibrated regularly each orbit with a 60Co source. The resolution was 7.8% at 662 keV. The instrument was in the wheel section of OSO-7. Data were accumulated in 0.5 s bins when the detector was looking at the Sun/anti-Sun during each 2-s spin. The monitor operated until 25 December 1972, but was usually turned off during the periodic penetration of the satellite into the radiation belts.

* Science

A sky map of the X-ray intensity was derived directly from several hundred days of data from the MIT experiment. The same experiment also produced results for three important compact galactic sources: Sco X-1, Her X-1 and Cyg X-1, as well as results of observations of the supernova remnant IC 443, supernova 1972e, and a unique flare source, GX 354+3.

The UCSD experiment produced results on Cyg X-3 which showed the 4.8-hr period present in the 6-21 keV range, but not in the 21-98 keV range.


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