BALLOON-BORNE TELESCOPE SYSTEM FOR HARD X-RAY ASTRONOMY

For the study of hard X-rays (18-150 keV) from celestial X-ray sources, NaI/CsI phoswich scintillator telescopes have been flown from Hyderabad during the last decade. The 'phoswich' technique reduces the non X-ray background by a factor approximately 10 at balloon altitudes. The telescope with 1000 cm2 area [5 mm thick NaI(Tl) and 50 mm thick CsI(Na) phoswich] had a sensitivity to detect a minimum flux of 2.2 x 10(-6) photons cm-2 s-1 keV-1 in the energy range 18-150 keV for 10(4)s expolsure; this allows study of several bright active galactic nuclei (AGNs) in hard X-rays. The telescope had a microprocessor-based tracking system to automatically follow celestial sources in elevation and azimuth according to a pre-determined schedule. Several galactic X-ray sources-mostly binary pulsars-and the quasar 3C273 have been studied in the last few years. The telescope system and some of the significant results are described.