The Italian Dutch Satellite for X-ray Astronomy SAX, Satellite
Astronomia X (Scarsi 1993), renamed BeppoSAX in
honour of Giuseppe Occhialini, has been carried into a low-Earth
orbit (height of 600 km and inclination 
) by an
Atlas Centaurus Rocket on April 30, 1996. The BeppoSAX payload
consists of four Narrow Field Instruments (NFIs) coaligned along the
Z axis of the spacecraft and two Wide Field Cameras (WFCs,
Jager et al. 1995) which point along the +y and
-y axis respectively. The NFIs package includes an imaging Low
Energy Concentrator System (LECS, Martin et al.
1995), three units of the imaging Medium Energy
Concentrator System (MECS, Boella et al. 1995), the
High Pressure Gas Scintillation Proportional Counter (HPGSPC,
Giarrusso et al. 1995) and the Phoswhich Detector
System (PDS, Frontera et al. 1995). Using the NFIs
BeppoSAX will be able to perform detailed spectroscopic and timing
studies of more than 2000 celestial X-ray sources with an
unprecedented wide energy band that ranges from a fraction of keV
(0.1 keV is the lower limit of the LECS) up to 300 keV (the upper
limit of the PDS). More details on the mission and its scientific
objectives can be found in Scarsi (1993) and
Piro et al. (1995).
Observation of cyclotron features, that are present in the X-ray
spectrum of many celestial sources is the primary scientific goal of
the HPGSPC. Cyclotron features provide a powerful means of
understanding the physics of radiative transport in highly magnetic
plasmas. We note that Ginga observations (Makishima et al.
1991) of cyclotron lines in several accreting X-ray pulsars
have confirmed and enlarged previous positive detections by other
experiments in Her 
(Trümper et al.
1978) and 
(Wheaton et al.
1979), and have provided a great impetus for attempts at
modelling the emission region of these objects. With the HPGSPC and
PDS, BeppoSAX will be able to address open crucial points such as
the presence and nature of multiple harmonics and the shape and
energy of lines. Sensitive in the 4-120 keV energy band the
HPGSPC will, also, complement the source continuum observation of
concentrators/spectrometers and the PDS and the iron line studies of
concentrators/spectrometers. In the anticipated 4 year lifetime of
the mission the HPGSPC will study the hard band of the spectrum of
many astrophysical objects such as the Seyfert 2 galaxies, some of
which have been already detected by OSSE and/or BATSE in the hard
X-ray band (Johnson et al. 1994), and Seyfert 1
galaxies (to study the high energy bump above 10 keV and the
presence of the break above about 50 keV). In addition the HPGSPC
will address the problem of the hard tails in the spectrum of black
hole candidates.
Funded by the Italian Space Agency (ASI), the HPGSPC was invented and designed by the Istituto di Fisica Cosmica ed Applicazioni dell'Informatica (IFCAI, Palermo) of the Consiglio Nazionale delle Ricerche and has been developed by Laben, Milan and Alenia Spazio, Turin, under the scientific responsibility of IFCAI and ASI.
Both the Flight Model and the Spare Flight Unit have been successfully integrated, tested and calibrated at the Laben premises respectively during October/November 1994 and July/August 1995.