spectral fits with general
continuum models and lightcurves all corrected for deadtime.
Variability tests and searches for X-ray bursts must be performed in
the pipeline analysis.
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R. Jager, J. Heise, M. H. F. Savenije, J. J. Schuurmans, C. P. de Vries, G. J. Wiersma
SRON, Utrecht, The Netherlands
The data from the SAX satellite, which will be placed in a low earth orbit, will be downlinked every orbit during 10 minutes to the SAX groundstation at Malindi, Kenya. From there an Intelsat link exists to the Rome based Operations Control Center at Nuova Telespazio premises. After a quick look analysis for possible transient X-ray sources, the raw data is archived and transferred to the adjacent Scientific Data Center (SDC) where it is archived and where Final Observation Tapes (FOTs) are produced for the observer (Maccarone 1996).
Each SAX hardware institute is responsible for the delivery of a software package that as a minimum enables the user to reduce raw data to basic astronomical data products like calibrated images, spectra and lightcurves. The distribution of software to local centers in Italy and the Netherlands and to the rest of the astronomical community is one of the tasks of the SDC.
The NeSDaC has been set up at SRON Utrecht to act as the Dutch local center. At the NeSDaC SAX software will be available for retrieval or can be used. Software development for the WFCs as well as calibration activities also take place at SRON in close cooperation with the SDC. The complete WFC public data archive will be on line after one year in order to facilitate a systematic and homogeneous analysis of the data and to produce a WFC skymap and catalogue. Tools for planning SAX observations will be made available like source visibility calculations and instrument simulations.
The requirements for the WFC data analysis software are mentioned here. This software shall be used in two ways: in a fully automated standard pipeline and interactive.
The WFCs are coded mask instruments so the imaging is performed indirectly. This implies the use of image reconstruction techniques. The baseline technique used at the NeSDaC is cross-correlation of the detector image with the coded mask pattern. Also Iterative Removal Of Sources has to be applied (In 't Zand 1992; Jager et al. 1995). Other promising methods like the Maximum Entropy Method or Maximum Likelihood Fitting (Skinner & Nottingham 1993) are under investigation.
Instrument parameters will be retrieved from calibration measurements, both from ground as from inflight measurements. They include WFC alignment and geometry, detector Point Spread Function as a function of position and energy, Field Of View, effective area as a function of energy, detector energy resolution and energy conversion, dead time correction.
The analysis will result in: source positions in proper coordinate
systems with position accuracies, source fluxes and accuracies in
physical units, hardness ratios from four energy bands, spectra in
physical units including spectral fits with general
continuum models and lightcurves all corrected for deadtime.
Variability tests and searches for X-ray bursts must be performed in
the pipeline analysis.
The standard analysis pipeline that will be set up should facilitate an average processing time of maximum 10 percent of real time in order to allow for various reprocessing runs. Main aims of this analysis are: automatic searches for long duration transients using full exposure time as well as searches for bursts and short lasting transients. Furthermore an overview of all observations and a skymap will be produced. The pipeline results will be automatically stored in a database. By using the same software in an interactive way parameters can be adjusted to optimize the analysis.
The expected WFC mean data rate (background plus weak sources) is of the order of 500 c/s or 15 kbit/s. This amounts to a needed storage capacity of roughly 250 Mbyte per day or 90 Gbyte per year. Together with analysis results an average of 120 Gbyte of data must be stored each year.
Additional requirements are to provide parameters to check and plot the WFCs state of health and functioning and to plot the satellite attitude and ephemeris. For the purpose of external users and in particular the Italian SAX institutes and the SAX SDC, portability to other UNIX platforms like DEC Ultrix and DEC OSF/1 is required.
The WFC data analysis software is developed and running on HP-9000 workstations and is written in ANSI C and FORTRAN-77. File formats follow the FITS standard. For the user interface Tcl/Tk is used. FTOOLS and the FITS I/O library are used wherever practical. This should allow for a reasonably portable system. Configuration control is achieved through a combination of the Revision Control System and an Oracle database. All programs use parameter files (IRAF compatible) for input, they can read parameters from the command line, prompt for possible missing parameters and provide range and type checking.
Access to NeSDaC will be possible via telnet and the X Window system. For public data, software downloading and retrieval of results, an anonymous ftp and WWW server can be used. Via WWW forms data can be located and collected, in addition invoking analysis runs will be possible. The documentation will be menu and hyperlink driven like WWW. This enables remote access and provides an easy to use interface. It will comprise manual pages and software descriptions.
The NeSDaC is embedded in a general environment based on a relational database; the environment is depicted in the figure. Raw data, result data as well as all software are stored on an archive. Full traceability of the data and software is thus obtained. Automatic reruns with new software issues are possible. All file formats follow the FITS standard. Tasks can be run with parameter files like in the IRAF environment. A user interface is provided with easy access to parameter files and to job scripts that enable automated task execution at several levels.
This software environment allows for integration of external software, as long as files are produced with headers (preferably FITS files) that contain the critical information about the software task itself (name and version) as well as all relevant parameters needed to run that task.
The WFC data analysis is divided in three stages. In stage I the following functions are incorporated:
Testing of the software will be done at several levels. At the highest level the analysis chain can be tested by using pre-flight calibration data, fake FOTs from Nuova Telespazio and the use of COMIS/TTM data, a coded mask precursor instrument onboard MIR/KVANT (Mels et al. 1988).
The NFI data analysis software in principle reads FOTs, produces cleaned event lists and produces calibrated fluxes, spectra and lightcurves (Maccarone 1996). Some of the instruments use the SAX specific XAS file formats but conversion tools are available for conversion to/from FITS files. Machine dependencies are concentrated in a so called VOS (Virtual Operating System) library. This implies that this environment will be made available in the NeSDaC.
We would like to thank Dr. J. W. den Herder for his critical reading of the manuscript and providing of useful comments.
Jager, R., et al. 1992, in Astronomical Data Analysis Software and Systems I, ASP Conf. Ser., Vol. 25, eds. D. M. Worrall, C. Biemesderfer, & J. Barnes (San Francisco, ASP), p. 154
Jager, R., et al. 1995, Proceedings SPIE 2517-17
Maccarone, M. C. 1996, this volume
Mels, W., et al. 1988, Nucl. Instr. and Meth., A273, 689
Skinner, G. K., & Nottingham, M. R. 1993, Nucl. Instr. and Meth., A333, 540
