The instrument launched in this flight was the second generation of a high sensitivity Nal/Csl phoswich X-ray telescope (area 1000 cm2) composed by two banks of five detectors each, collimated to a field of view of 5° x 5° FWHM.
Each detector consists of a Nal(TI) scintillator (5mm) and a Csl (Na) scintillator (50 mm) of area 100 cm2 viewed by a single 127 mm diameter photomultiplier and equipped with a graded shield collimator.
Non-X-ray background photons are rejected by requiring that only signals characteristic of Nal (and not Csl) -as revealed by the pulse shape- are accepted. This "phoswich" technique reduces the background at ceiling altitude by a factor 10 to 20. The detectors have good efficiency in 20-150 keV range. The two banks of detectors are mounted, one on each side of central column, on a platform steerable in both the azimuth and the zenith to track the X-ray sources as they move on the sky. An Am-241 radio-active source is used to periodically calibrate each detector. Signals from each detector, the telescope aspect and the house-keeping information are telemetered to ground. The arrival time of an X-ray photon can be determined with great accuracy. This high time resolution is especially valuable in search for short (millisecond) periodicities in the X-ray sources.
Balloon launched on: 12/18/1988 at 5:15 IST
Launch site: TIFR National Balloon Facility, Hyderabad, India
Balloon launched by: National Balloon Facility, Tata Institute of Fundamental Research
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen Stratofilm (tow+main) 242.900 m3
Flight identification number: 398
End of flight (L for landing time, W for last contact, otherwise termination time): 12/18/1988
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 10 h
Payload weight: 661 kgs
Overall weight: 1.155 kgs
The telescope weighting 476 kg. had its maiden flight on 18 December 1988 at 5:15 Indian Standard Time from Hyderabad on a 8.58 million cubic feet Winzen balloon. The payload floated at the ceiling altitude of 37.5 km for 10 hours.
Eight galactic X-ray sources were tracked for different time intervals according to an optimum "viewing schedule" fed into the memory of an on-board microprocessor. Because of the low signal-to-noise ratio of most X-ray sources, substantial time had to be devoted to the measurement of noise (background) in the different elevation and azimuth ranges traversed by the X-ray sources due to earth's rotation. Maximum exposure was obtained for Sco X-1 (115 min.). Cyg X-l, PSR 1907+9.7, GX1+4, GX5-1 and the radiopulsar 1951-32 were each scanned for 45-60 minutes. Her X-l and the radio pulsar PSR 1737+21 were tracked for 30 minutes each.
Preliminary analysis shows that the instrument performed according to the expectation. The detector count rates at the ceiling are about 40% less than those obtained with the Mark I telescope flown earlier. This, combined with the large area and reduced dead-time compared to the earlier telescope, will allow higher sensitivity for the X-ray observations.
A major objective in this flight was to confirm the earlier discovery of a transient millisecond pulsar (period 2.93 ms) during a 13 min. observation in December 1984 (IAUC 4485). The sensitivity of the present observation is several times better than the previous observation.