The SAX (Satellite per Astronomia X) mission is a major joint program

of the Italian Space Agency (ASI) and the Netherlands Agency for Space Programs

(NIVR). In the framework of past and future X-raymissions the SAX satellite, launched

in April 1996, stands out for its very wide spectral coverage from 0.1 to 200 keV, with

well-balanced performances of the low-energy and high-energy instrumentation. The

sensitivity of the scientific payload will allow the exploitation of the full band of SAX

also for weak sources (1/20 of 3C273), opening new perspectives in the study of spectral

shape and variability of several classes of objects. In this paper we describe the main

aspects of the mission, the instruments, the scientific objectives and operations

Di Cocco, G.

Scientific Open-access Literature Archive and Repository

IL NUOVO CIMENTO VOL. 20 C, N. 5 Settembre-Ottobre 1997The SAX mission for wide-band X-ray astronomy ()()G. DI COCCOfor the SAX collaboration()Istituto TESRE/CNR - via P. Gobetti 101, 40129 Bologna, Italy(ricevuto il 4 Aprile 1996; approvato il 24 Maggio 1996)Summary. — The SAX (Satellite per Astronomia X) mission is a major joint programof the Italian Space Agency (ASI) and the Netherlands Agency for Space Programs(NIVR). In the framework of past and future X-ray missions the SAX satellite, launchedin April 1996, stands out for its very wide spectral coverage from 0.1 to 200 keV, withwell-balanced performances of the low-energy and high-energy instrumentation. Thesensitivity of the scientific payload will allow the exploitation of the full band of SAXalso for weak sources (1/20 of 3C273), opening new perspectives in the study of spectralshape and variability of several classes of objects. In this paper we describe the mainaspects of the mission, the instruments, the scientific objectives and operations.PACS 96.40 – Cosmic rays.PACS 95.55 – Astronomical and space-research instrumentation.PACS 01.30.Cc – Conference proceedings.1. – Outline of the missionThe study of the spectral behaviour of celestial sources over a wide range of energiesis of primary importance in understanding comprehensively the emission mechanismsthat, in several instances, produce spectral features localized in different regions of theelectromagnetic spectrum.() Paper presented at the VII Cosmic Physics National Conference, Rimini, October 26-28, 1994.() The author of this paper has agreed to not receive the proofs for correction.() SAX Consortium:Istituto per le Tecnologie e lo Studio Radiazioni Extraterrestri, CNR, Bologna (Istituto TESRE);Istituto Astrofisica Spaziale, CNR, Frascati, (IAS);Istituto Fisica Cosmica e Tecnologie Relative, CNR (IFCTR) ed Unita` GIFCO, Milano;Istituto di Fisica Cosmica ed Appl. Informatica (IFCAI) ed Unita` GIFCO, Palermo;Istituto Osservatorio Astronomico, Universita` di Roma “La Sapienza”;Space Research Institute of Utrecht/SRON (SRU), The Netherlands;Space Science Departments (SSD), ESA, Noordwijk, The Netherlands.G Societa` Italiana di Fisica 789790 G. DI COCCOFig. 1. – SAX instruments accommodation.The Italian-Dutch X-ray Satellite SAX will be the first X-ray mission that will havethe capability of observing sources over more than three decades of energy —from 0.1 to200 keV— with a relatively large area, a good energy resolution, associated with imagingcapabilities (resolution of about 10) in the range of 0.1–10 keV.The instrument complement dedicated to such purpose (fig. 1) is composed ofa medium-energy (1–10 keV) concentrator optics/spectrometer, MECS, consisting ofthree 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 PhoswichDetector System (15–300 keV), PDS, all of which have narrow fields and point in the samedirection (Narrow-Field Instruments, NFI).The other characterization of the mission is its capability of monitoring large regionsof the sky with a resolution of 50 in the range 2–30 keV to study long-term variability ofsources down to 1 mCrab and to detect X-ray transient phenomena. This is realized bymeans of two coded mask proportional counters (Wide Field Cameras, WFC) pointing indiametrically opposed directions perpendicular to the NFI (fig. 1). Finally, the anticoin-cidence scintillator shields of the PDS will be used as a gamma-ray burst monitor in therange 60–600 keV. The main characteristics of the satellite onboard experiments are givenin table I.SAX has been launched by an ATLAS G-Centaur into a 600 km orbit at 3 degreesinclination in April 1996.During each orbit up to 450 Mbits of data will be stored onboard and relayed to groundon the station contact. The average data rate available to instruments will be about60 kbits/s, but peak rates of up to 1100 kbits/s can be retained. The satellite passes abovethe ground station, placed near the equator in Malindi, once every orbit. This will allow aprompt operation and control on the satellite.THE SAX MISSION FOR WIDE-BAND X-RAY ASTRONOMY 791TABLE I. – SAX instruments.Instrument Range FOV Angular res. Area Energy res.(keV) (FWHM) (arcmin) (cm2) (% FWHM)Concentrators1 LECS 0.1–10 0.5 1:5 E6 0:520 at 0.25 keV 8 E6 0:53 MECS 1–10 150 at 6 keVHPGSPC 3–120 1(coll.) – 450 (geom.) 4 E60 0:5PDS 15–300 1.5(coll.) – 800 (geom.) 18 E60 0:52 WFC 2–30 20 20 (a) 5 250 (a) (through mask) 18 E6 0:5(a) per unit.2. – The instrumentsThe main characteristics of the SAX instruments (see also Butler and Scarsi, 1990 andreferences therein) are give in table I and fig. 2.3. – The concentrator/spectrometer systemThe concentrator/spectrometer system consists of four separate concentrator mirrorassemblies with a focal length of 185 cm, each one with a position-sensitive, xenon-filled,GSPC in the focal plane, three covering the 1–10 keV range (MECS), the fourth one ex-tending the range down to 0.1 keV (LECS).The mirror assemblies, for which the development and testing has been performed atIFCTR and IFCAI, are composed of 30 nested gold-coated mirrors, which are confocaldouble-cone approximations to a Woltjer 1 configuration. X-ray test on the prototypemirrors (Citterio et al., 1990, Conti et al., 1993; 1994) and on the flight unit in the Pantercalibration facility have shown a resolution of 4000 (HPR), better than the design goal of 1arcmin and confirmed the prediction on the effective area (fig. 2). The concentrators aredesigned to maximize their effective area around 7 keV for studies of iron line.The four focal plane detectors are imaging Gas Scintillation Proportional Counters.Three of the GSPCs have 50 m beryllium windows, thus being sensitive in the range of1–10 keV (ME) (Giarrusso et al., 1990, Bonura et al. 1992). The fourth GSPC developed bySSD/ESTEC (Parmar et al., 1990) is similar to the ME, except that a thin window (Bavdazet al., 1994) allows for transmission of low-energy X-rays into the detector.4. – The high-energy instrumentsThe high-energy instruments onboard SAX are, respectively, a 5 atm xenon-filled GasScintillation Proportional Counter (High-Pressure GSPC) and a NaI/CsI phoswich scin-tillator (Phoswich Detector System, PDS).Great care has been put in the design to control any sources of systematic effects. Bothinstruments adopt the technique of rocking collimators, continuously pointing on and offsource (with a period of the order of a minute) to monitor the background. Furthermore,in the case of PDS, which is composed of four units, the two halves of the experiment will792 G. DI COCCOFig. 2. – Effective area (scale on the right-hand side) of the SAX Narrow-Field Instruments. Thecontinuous lines labelled as ME and LE give the effective, area of the three telescopes and of onetelescope, respectively. The dotted lines give the focal plane detectors efficiency (left-hand scale).alternatively point source and background regions, thus providing a continuous monitor-ing of the background. The two experiments have also an equalization gain system, basedon tagged radioactive source (time-tagged for the PDS, position tagged for the HPGSPC),that will automatically adjust the high voltage in order to keep the gain of the detectorswithin 0.25–0.5%.The High-Pressure Gas Scintillation Proportional Counter is a new development of aGSPC filled with 5 atm Xenon/He mixture, which allows the detector to be sensitive upto 120 keV. The cell of the HPGSPC is seen by an Anger camera arrangement of sevenphotomultiplier tubes (PMT). The event position is used to correct the event energy withan overall improvement of a factor of two in energy resolution compared to proportionalcounters (Giarrusso et al., 1989).The Phoswich Detector Sytem (PDS) consists of a square array of four independentNaI(Tl)/CsI(Na) phoswich scintillation detectors (Frontera et al., 1991). Each of the fourdetectors is made of the two crystals optically coupled and forming what is known asPHOSWICH (acronym of PHOSphor and sandWICH). The scintillation light produced ineach phoswich is viewed, through a light guide of quartz, by a PMT. The NaI(Tl) acts asTHE SAX MISSION FOR WIDE-BAND X-RAY ASTRONOMY 793Fig. 3. – Spectrum from all the SAX-NFI in T = 105 s for a Seyfert 1 like MCG 6 30 15. Notethe sensitivity up to 200 keV.X-ray detector, while the CsI(Na) scintillator acts as an active shield. The phoswich con-figuration is well known to provide a detector with high efficiency at hard X-ray energiesand a very low background level.In order to further reduce the phoswhich background level, both a lateral and a topanticoincidence shield are provided. The lateral shielding system is made of 4 CsI(Na)scintillators 10 mm thick that rejects a relevant fraction of the unwanted X-ray photonsand charged particles impinging on the sides of the instruments. The top shield is madeof an organic scintillator 1 mm thick that covers the X-ray entrance aperture. It rejectsthe background events due to charged particles, in particular high-energy electrons gen-erated by interactions of cosmic rays with the materials of the instruments or with thesatellite.Two calibration systems are provided. One as gain control of each detection unit whichis performed by using a time-tagged calibration source of 241Am. A continuous and au-tomatic correction is performed by acting on the high-voltage supplies of the phoswichPMTs.The other calibration system consists of two radioactive sources of 57Co (lines at 14,122, 136 keV) distributed along a wire 141 mm long. Upon command, the sources, one for794 G. DI COCCOFig. 4. – The effective area of a WFC.each of the two detectors, cross the field of view of the instrument above the collimatorsand top shield. This monitoring allows to evaluate the absolute gain of the instrument andrecalibrate it for long-term variations.The lateral shields of the PDS are also used as detectors of gamma-ray bursts of cosmicorigin (GRB) with fluence greater than 10 6 erg cm 2. The signals from each of the slabsin a programmable energy band, nominally 60–600 keV, are sent to both a Pulse HeightAnalyzer and the GRBM logic unit, where the trigger condition is checked. A GRB eventaccumulation is triggererd when at least two of the four lateral shields meet the triggercondition.Note that all the Narrow-Field instruments share wide overlapping energy regions.This will allow a good relative calibration, essential to avoid artifacts in the deconvolutionof spectra over the full range of SAX.The sensitivity curves of all instruments can be found in Perola (1990). In fig. 3 weshow the capability of NFI to determine the spectral shape of faint sources over the fullrange of 0.1–200 keV, in this case a Seyfert 1 like MCG–6–30–15.5. – The wide-field camerasThe two identical Wide Field Cameras developed by SRON/SRU (Jager et al., 1989)consist of two position-sensitive proportional counters filled with 2 atm of xenon, whichview the sky through a coded mask perpendicularly to the axis of the NFI and 180 degreesaway from each other. They combine a large field of view (20  20 per unit) with anangular resolution of 5 arcmin (FWHM). The energy band ranges from 1.8 to 30 keV witha resolution of 18% at 6 keV. The effective area of one camera is shown in fig. 4.The sensitivity depends upon the pointing direction in the sky, because each sourcein the FOV contributes to the overall background. Towards high-galactic latitude theCosmic Diffuse X-Ray Background is the main contributor to the background. In thiscase the sensitivity is of the order of a few mCrab in 104.THE SAX MISSION FOR WIDE-BAND X-RAY ASTRONOMY 7956. – Scientific objectivesThe primary characteristics of SAX among past and future missions is its very widespectral coverage, with well-balanced performances between the low-medium (0.1–10keV) and medium-high (1–200 keV) energy bands.Given the instrument capabilities over the wide energy range described in the previoussections, SAX can provide a significant contribution (and a unique contribution for scienceinvolving the exploitation of the wide band) in several areas of X-ray astronomy such ascompact galactic sources, active galactic nuclei, clusters of galaxies, supernova remnants,normal galaxies, stars and gamma-ray bursts.With its minimum lifetime of two years (extendible to four years), SAX will be able toperform more than 2000 pointings. While the NFI will be the prime instruments mostof the time, the WFC will be periodically used to scan the galactic plane to monitor thetemporal behaviour of sources above 1 mCrab and to detect transient phenomena; thanksto their large field of view, the WFC will be operated to monitor selected objects when theNFI perform their sequence of pointed observations. We expect to detect about 10–20bright X–ray transients per year during the WFC observations. The observing programwill be held flexible in order to accommodate those targets of opportunity for follow-upobservations with the NFI.  The SAX program is the result of the efforts of so many and enthusiastic people and itis impossible to thank them all here. Let us honour the memory of the project manager M.CASCIOLA, who gave so much to the mission. Part of this paper was based on the contri-butions to the SAX Observers Handbook by C. BUTLER, L. CHIAPPETTI, G. CONTI,E. COSTA, D. DAL FIUME, F. FRONTERA, S. GIARRUSSO, R. JAGER, C. MAC-CARONE, G. MANZO, G. MATT, T. MINEO, A. PARMAR, G. C. PEROLA, L. PIRO, S. RE,B. SACCO, L. SALOTTI and A. SANTANGELO. Finally I wish to acknowledge the contri-butions to the mission of J. BLEEKER, G. BOELLA, B. TAYLOR, L. SCARSI; F. FAVATAand M. TRIFOGLIO for the Ground Segment activities; G. MANARINI and B. NEGRI ofASI.REFERENCESBAVDAZ M. et al., Nucl. Instrum. Methods, 345 (1994) 549.BONURA A. et al., Proc. SPIE, 1743 (1992) 510.BUTLER C. and SCARSI L., Proc. SPIE, 1344 (1990) 464.CITTERIO O. et al., Nuovo Cimento C, 13 (1990) 375.CONTI G. et al., Proc. SPIE, 2011 (1993) 118.CONTI G. et al., Proc. SPIE, 2279 (1994) 101.FRONTERA F. et al., Adv Space Res., 11 (1991) 281.GIARRUSSO S. et al., Proc. SPIE, 1344 (1990) 106.GIARRUSSO S. et al., Proc. SPIE, 1159 (1989) 514.JAGER R. et al., Proc. SPIE, 1159 (1989) 2.PARMAR A., SMITH A. and BAVDAZ M., in Observatories in Earth orbit and Beyond, edited by Y.KONDO (Kluwer) 1990, p. 456.PEROLA G. C., Adv. Space Res., 10 (1990) 295.

The SAX mission for wide-band X-ray astronomy

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The SAX mission for wide-band X-ray astronomy

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