Adv. Space Res. Vol. 11, No.8, pp. (8)281—(8)285, 1991 0273—1177/91 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright © 1991 COSPAR THE HIGH ENERGY X-RAY EXPERIMENT PDS ON BOARD THE SAX SATELLITE F Frontera,*,*** D. Dal Fiume,* M. Pamini,* J. M. Poulsen,* A. Basili,* T. Franceschini,* G. Landini,* S. Silvestri,* E. Costa,** D. Cardini,** A. Emanuele** and A. Rubini** *Istjtuto TE.S.R.E. (C.N.R.), Bologna, Italy **Jstitu(o de Astrofisica Spaziale (C.N.R.), Frascati, Italy ***Dipardmento de Fisica, Universitd di Ferrara, Ferrara, Italy ABSTRACT The high energy X-ray experiment PDS (Phoswich Detection System) is one of the four narrow field instruments on board the SAX satellite. The experiment will be dedicated to temporal and spectral studies of celestial X-ray sources in the 15-300 keY energy band. The PDS detector is composed of 4 actively shielded NaI(Tl)/CsI(Na) phoswich scintillators with a field of view of 1~4 (FWHM). The total geometric area is 800 cm 2. The design of the instrument is made with the goal to achieve an energy resolution better than 17% at 60 keV and a 3 ~ flux sensitivity for an observation time of io~ s of about 2 inilliCrab in one energy band 40 keV wide centered at 100 keV. The expected 3 ~ sensitivity of the PDS to cydotron lines is about 5x10~ photons/cm2 s at 60 keV in io~ a. The CsI(Na) lateral shields of the PDS will also be utilized as a gamma—ray burst monitor. Details of the experiment design are described and expected performance is given. INTRODUCTION The three—axis—stabilized satellite for X—ray astronomy SAX is under development by the Italian Space Agency with an industrial contribution by the Netherlands Agency for Aerospace Programs (NflTR). SAX will be lauiiched at the end of 1993 into a circular orbit at 600 km altitude with 2° inclination. During each orbit up to 450 Mbits of information will be stored on board and relayed to the ground during passage over the San Marco (Malindi, Kenya) ground station. The minimum mission lifetime wifi be two years, extendable to four years. The SAX payload is built under the scientific responsibility of a Consortium of Institutes in Italy with Institutes in the Netherlands and the Space Science Departement of the European Space Agency. An overview of the SAX mission and scientific consortium can be found elsewhere /1/. The payload includes Narrow Field Instruments (NFl) and Wide Field Cameras (WFC). The NFIs are four Concentrator Spectrometers (C/S) with 3 units operating in the 1—10 keY energy band and 1 unit operating in 0.1—10 keY, one High Pressure Gas Scintillation Proportional Counter (HP— GSPC) operating in the 3—120 keV energy band and a Phoswich Detection System (PDS) with four units operating in the 15—300 keY energy band. All NFIs are coaligned, while the two WFCs are orthogonal to them, at 180°from each other. The main scientific goal of the SAX mission is to perform systematic temporal and spectral studies of celestial X—ray sources in the broad 0.1—300 keY energy band. In the hard X—ray range (>15 keY) very sensitive observations will be performed with the PDS experiment, which will achieve in the 15-200 keV energy band an unprecedented sensitivity, never before achieved by hard X—ray experi- ments in already flown satelilte missions. The scientific objectives of SAX achievable with the PDS experiment include the study of the accretion processes on cataclismic variables, X—ray pulsars and black hole candidates; the study of the role of the magnetic field in determining the temporal and (8)281