The European Retrievable Carrier (EURECA) was launched 31 July 1992 by the
Space Shuttle Atlantis, and put into an orbit at an altitude of 508 km. It
began its scientific mission on 7 August 1992. EURECA was retrieved on 1
July 1993 by the Space Shuttle Endeavor and returned to Earth. The
satellite carried a number of experiments for microgravity studies, solar
observations, and material technology investigations. Also on board was the
WATCH or Wide Angle Telescope for Cosmic Hard X-rays instrument.
The WATCH instrument was sensitive to 6-150 keV photons. The total field of
view covered 1/4 of the celestial sphere. During its 11 month lifetime,
EURECA tracked the Sun and WATCH gradually scanned across the entire sky.
The WATCH instrument was developed at the Danish Space Research Institute
and also flown on the Russian Granat mission. WATCH is a wide field monitor
based on the Rotation Modulation Collimator principle. It has a circular
field of view with a radius of about 65 degrees. There are 2 interleaved
detectors which create a phoswich of alternating strips of NaI(Tl) and CsI(Na).
Each strip is 5 mm wide by 2 mm thick The diameter of the phoswich is 110
mm and it is viewed across a 10 mm air gap by a single 125 mm
photomultiplier. The effective area of the NaI and CsI scintillators if about
45 sq-cm. The sensitivity is about 100 mCrab in 1 day. The effective
observation time for the 318 day mission was 120 days. The minimum time
resolution was 64 microseconds.
Some 2 dozen known X-ray sources were monitored - some for more than 100
days. Also, a number of new X-ray transients were discovered. Nineteen
cosmic gamma-ray bursts were detected by the WATCH instrument on EURECA,
with 12 of these being localized to within ~ 1 degree.