Mission Description

The BeppoSAX mission (acronym for the Italian Satellite per Astronomia in raggi X, SAX, later renamed BeppoSAX in honor of Giuseppe Occhialini) is a program of the Italian Space Agency (ASI) with participation of the Netherlands Agency for Aerospace Programs (NIVR), developed with the support of a consortium of Institutes in Italy and in The Netherlands and the Space Science Department of ESA.

BeppoSAX is the first X-ray mission offering a very wide spectral coverage from 0.1 and up to 300 keV, with well balanced performances of the low energy and high energy instrumentation, a good energy resolution (8%@6 keV, 4% or 15%@60 keV) and imaging capabilities (resolution of the order of about 1.2 arcmin HPR at 6 keV) in the 0.1-10 keV range. The scientific payload sensitivity allows the exploitation of the full band of BeppoSAX for relatively weak sources (1 mCrab, or $\approx2\times10^{-11}$ erg cm$^{-2}$ s$^{-1}$). The MECS and LECS sensitivity goes down to $\approx 10^{-13}$ erg cm$^{-2}$ s$^{-1}$.

A detailed description of the mission, necessary for proposal preparation, is given in the BeppoSAX Observers' Handbook and in its Addendum . Both documents can be retrieved from the following anonymous ftp services:

• ftp ftp.asdc.asi.it

  cd pub/sax/doc/handbook

or from the mirrors

• ftp legacy.gsfc.nasa.gov

  cd sax/ao_info/handbook

Detailed and frequently updated information about this AO and the mission in general is also available on-line from the BeppoSAX Web pages at the ASI Science Data Center (ASDC)

• /bepposax/

  and the HEASARC:

• http://heasarc.gsfc.nasa.gov/docs/sax/saxgof.html

The instrument complement is composed of:

• Four identical X-ray telescopes (double cone approximations of the Wolter 1 configuration and referred to as Concentrators) with Gas Scintillation Proportional Counters in their focal plane: three of these work in the 1-10 keV energy range, the fourth one (with a thin entrance window) operates in the 0.1-10 keV energy range. The first set of instruments is referred to as the MECS (Medium Energy Concentrator System), the second as the LECS (Low Energy Concentrator System). Since May 1997 only two of the three MECS are operative.

• A High Pressure Gas Scintillation Proportional Counter (3-120 keV), HPGSPC.

• A Phoswich Detector System (15-300 keV), PDS.

All these instruments have narrow fields of view (of about 1 degree) and point in the same direction (Narrow Field Instruments, or NFI).

The BeppoSAX mission is also capable of monitoring large regions ($\sim 3000$ square degrees) of the sky with a resolution better than 5 arcmin in the 2-30 keV energy band, and therefore to study long term variability of sources down to about 1 mCrab and to detect X-ray transient phenomena for follow up observations with the NFI. This is obtained by means of two coded mask proportional counters (Wide Field Cameras; WFC) pointing in opposite directions perpendicular to the NFI.

Finally, the anticoincidence scintillator shields of the PDS is used as a gamma-ray burst monitor in the range 40-700 keV.

BeppoSAX was launched on April 30, 1996 by an Atlas 1-Centaur from Cape Canaveral into a 600 km orbit at 4 degrees inclination. The satellite thus nearly avoids the South Atlantic Anomaly and takes full advantage of the screening effect of the Earth magnetic field in reducing the cosmic ray induced background. This is important to achieve the necessary sensitivity for observations of weak sources with the high energy experiments, particularly with the PDS.

The satellite achieves 1 arcmin pointing stability. The main attitude constraint derives from the need to maintain the normal to the solar arrays (X axis) within 30 degrees from the Sun. The scientific instruments are pointing in a direction perpendicular to the X axis. Therefore at any time the sky region accessible to NFI is a band 60 degrees wide (50 % of the sky), and for the WFC a slightly larger region. Depending on target position, the observing efficiency will be limited to 50 % on average, by Earth eclipses (which subtends an angle of about 130 degrees at 600 km) and by passages in high background regions. Further constraints are imposed by attitude operations (see Addendum)

The satellite is operated from an Operational Control Center (OCC), while the quick look of scientific data is performed at the Scientific Operation Center (SOC). Both are located in the same site in Rome, and are connected to the ground station in Malindi, Kenya by a relay satellite (Intelsat). The satellite passes over the ground station every orbit. This allows a prompt operation and control of the satellite, particularly important in the case of Targets Of Opportunity (TOO) that require timely follow up observations. During each orbit up to 450 Mbits of data are stored onboard and relayed to the ground during station passage. The average data rate available to instruments will be about 60 kbit/s, but peak rates of up to 100 kbit/s can be accommodated. A Science Data Center (SDC), partly co-located with the OCC, takes care of mission scheduling, raw data archiving (about 1 GByte of data produced every day) and FOT (Final Observing Tape) production.

Support to the scientific community is provided by the newly established ASI Science Data Center (ASDC). This includes FOT distribution to PI's, software maintenance and calibration archiving.

After launch, all satellite sub-systems have been checked-out (Commissioning phase); the first calibration phase (Science Verification Phase, SVP) has been carried out in the period Jul. - Sept. 1996, after which the AO1 program started.

In flight verification showed that scientific instruments and system components performed nominally (or better) with the following modifications :

• MECS unit 1 is not operative since May 1997.

• Due to sunlight contamination the LECS can only be operated in earth shadow : this introduces further constraints on the observing window to achieve an observing efficiency that is typically $30\%-60\%$ of that of MECS.
• Attitude system : due to a series of gyro failures, a new pointing mode - making use of 1 gyro only - (1 gyro mode) has been implemented and successfully uploaded on board on Aug. 1997. For part of the AO5 phase we expect to operate the satellite in gyro-less mode. This mode will rely primarily on the use of star trackers, with further constraints on scheduling that, along with the constraint from the LECS operation, will limit time critical programs (see next section).

• HPGSPC operating policy is changed since July 1999. In order to maximize the operational life time of the HPGSPC (which occasionally shows an anomalous behaviour) it has been decided to use the instrument only for bright sources, when the expected target flux exceeds 10 mCrab. Furthermore, the collimator will be kept fixed in the on position. Upon request, and only for target with declination $>70\deg$ (or $<-70\deg$), the collimator will be set in rocking mode.

  Furthermore the typical manoeuvre will last a minimum of 1-2 orbits. This sets the minimum pointing duration to 40 ksec.

See the Addendum to the SAX Observers' Handbook for further details on instruments' performances.