The study of the spectral behaviour of celestial sources over a wide range of energies is of primary importance in understanding comprehensively the emission mechanisms that, in several instances, produce spectral features localized in different regions of the electromagnetic spectrum. This fact has become more evident in the last years, when the efforts made by scientists in organizing complex campaigns of simultaneous observations have been rewarded by excellent results.
The Italian-Dutch X-ray Satellite BeppoSAX will be the first X-ray mission (and, of the coming near-future satellites, the only one) that will have the capability of observing sources over more than tree decades of energy - from 0.1 to 200 keV - with a relatively large area, a good energy resolution, associated with imaging capabilities (resolution of about 1') in the range of 0.1-10 keV. The instrument complement dedicated to such purpose is composed by a medium energy (1-10 keV) concentrator optics/spectrometer, MEC S, consisting of three units, a low energy (0.1-10 keV) concentrator optics/spectrometer, LECS , a high pressure gas scintillation proportional counter (3-120 keV), HPGSPC , and a phoswich detector system (15-300 keV), PDS , all of which have narrow fields and point in the same direction (Narrow Field Instruments, NFI).
The other characterization of the mission is its capability of monitoring large regions of the sky with a resolution of 5' in the range 2-30 keV to study long term variability of sources down to 1 mCrab and to detect X-ray transient phenomena. This is realized by means of two coded mask proportional counters ( Wide Field Cameras, WFC) pointing in diametrically opposed directions perpendicular to the NFI Finally, the anticoincidence scintillator shields of the PDS will be used as a gamma-ray burst monitor in the range 60-600 keV.
The primary characteristic of BeppoSAX among past and future missions is its very wide spectral coverage, with well balanced performances between the low-medium (0.1-10 keV) and medium-high (10-200 keV) energy bands. For example the complex spectrum of a Seyfert 1 galaxy, namely MCG-6-30-15, can be observed by BeppoSAX-NFI in 40,000 s with all the spectral component and features detected by several satellites in the past. Starting from the low energy part there is: a soft excess observed by EXOSAT (Pounds et al. 1986), an OVII edge around 0.8 keV observed by ROSAT (Nandra and Pounds 1992) and ASCA (Fabian et al. 1994), an iron line at 6.4 keV and a high energy bump above 10 keV detected by GINGA (Matsuoka et al. 1990). All those components can be measured with good accuracy with SAX in a single shot for the first time.
Concerning near future missions, they will generally have an approach complementary to that
of BeppoSAX, like the Russian Spectrum X-gamma (a complex one with emphasis on spectroscopy
below 10 keV) and the American XTE (timing in the 2-200 keV band).
Given the instrument capabilities over the wide energy range described in the previous sections,
SAX can provide a significant contribution (and unique contribution for science involving the
exploitation of the wide band) in several areas of X-ray astronomy such as:
- Compact galactic sources; shape and variability of the continuum: narrow spectral features (iron line, cyclotron lines) as a function of the orbital and rotational phases. Ultra-soft sources; discovery and study of X-ray transients.
- Active Galactic Nuclei: spectral shape and dynamics of the variable continuum and of the narrow and broad components from 0.1 to 200 keV in bright objects (soft excess, warm and cold absorption and related O and Fe edges, iron line and high energy bump, high energy cut-off ); spectral shape of objects down to 1/20 of 3C273 up to 100-200 keV; spectra of high redshift objects up to 10 keV (fig.4.2).
- Clusters of galaxies: spatially resolved spectra of nearby objects and the study of temperature gradients and cooling flows; chemical composition and temperature distribution as a function of redshift
- Supernova remnants: spatially resolved spectra of extended remnants; spectra of Magellanic Cloud remnants
- Normal galaxies: spectra from 0.1 to 10 keV of the extended emission
- Stars: multi-temperature spectra of stellar coronae from 0.1 to 10 keV
- Gamma-ray bursts: temporal profile with 1 msec resolution from 60 to 600 keV. X-ray counterparts of a subset with positional accuracy < 5'.
With its minimum lifetime of two years (extendible to four years), BeppoSAX will be able to perform more than 2000 pointings. While the NFI will be the prime instruments most of the time, the WFC will be periodically used to scan the galactic plane to monitor the temporal behavior of sources above 1 mCrab and to detect transient phenomena; thanks to their large field of view, the WFC will be operated to monitor selected objects when the NFI perform their sequence of pointed observations. We expect to detect about 10-20 bright X-ray transients per year during the WFC observations.
The observing program will be held flexible in order to accommodate those TOO for follow-up observations with the NFI. Furthermore the operation capability of BeppoSAX will allow to acquire the target within a few hours of its discovery. The observations will be organized on the basis of a "Core Program", devoted to systematic studies with particular regard to scientific objectives that exploit SAX capabilities, and a "Guest Observer" program.
The time reserved to the Core Program is 80% the first year of the mission, decreasing in the following years. Participation in the Core Program is open to members of the national communities participating in the mission and will be regulated, along with applications for Guest Observer time, by the selection of proposals. This policy will allow the achievement of complex and systematic programs, leaving however ample time for smaller projects.
Guest Observer time is open to the members of the world wide community. The Announcement of Opportunity for BeppoSAX observations was released in the second half of 1995.
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