Balloon-borne telescope system for hard X-ray astronomy

Abstract

For the study of hard X-rays (18-150 keY) 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 ~ 10 at balloon altitudes. The telescope with 1000 cm² area [5 mm thick NaI(TI) 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} keY^-¹ in the energy range 18-150 keY for 10⁴/s exposure; 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.