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PS reprint
M. C. Maccarone
Ist. IFCAI/CNR, Piazza G. Verdi 6, 90138 Palermo, Italy
The Italian-Dutch astronomical X-ray satellite SAX is devoted to systematic, integrated, and comprehensive studies of galactic and extra-galactic sources in the energy band 0.1--300 keV. Its scientific payload is comprised of four Narrow Field Instruments NFI (Low Energy Concentrator Spectrometer LECS, Medium Energy Concentrator Spectrometer MECS, High Pressure Gas Scintillation Proportional Counter HPGSPC, Phoswich Detection System PDS) all pointing in a same direction, and of two Wide Field Cameras WFC, pointing in diametrically opposed directions perpendicular to the NFI common axis.
SAX is scheduled for launch in April 1996, by an Atlas G-Centaur into a circular equatorial orbit of about 3 degrees inclination and 600 Km altitude.
A bi-directional Intelsat link between the Operational Control Center (situated in Rome, Italy) and the equatorial Ground Station (in Malindi, Kenya) will control the satellite and collect the SAX data during each of 15 ground contacts per day. SAX data will be transferred to the Scientific Data Center SDC (situated in Rome, near the Operational Control Center) to be archived on optical disks and distributed to the Scientific Community in the form of Final Observation Tapes (FOT). At SDC the SAX scientific software, necessary to access the FOT data, will be maintained and made available to observers.
The aim of this presentation is to illustrate the main features of the SAX scientific software to reduce the observational data coming from all the instruments onboard SAX. The software will be available at SDC for network retrieval starting from Spring 1996, together with documentation and an installation guide.
The scientific software is specifically designed to the SAX data reduction; it allows the reading of FOTs and calibration data and includes specific reduction and processing tasks such as the accumulation of photon lists, spectra, images and time profiles, and the creation of instrument response matrices.
The general processing system includes a number of stages. FOTs produced at SDC are re-organized on disk via a FOT filing procedure and therefore made available to the accumulation programs which essentially produce selected photon lists. Through particular reduction modules (e.g., instrument specific background subtraction, dead time correction, and so on) the photon lists are converted in a net/corrected form, ready to be passed to the spectral, timing, and image scientific analysis procedures. The path between accumulation and reduction steps is not necessarily a regular pipeline, but it can be an iterative and interactive process with a feedback loop between steps.
Response matrix generation is a further step necessary for spectral analysis of SAX data and for data deconvolution purposes. Programs to generate the response matrix of the experiments onboard SAX, appropriate to the observations under analysis, using calibration and housekeeping data and particular instrument settings, are part of the SAX scientific software.
The SAX scientific software has been developed along two different guidelines: XAS (X-ray astronomy Analysis System), a prototype software written at the Italian Institutes in the context of the SAX mission, and FTOOLS, a collection of utility programs provided by HEASARC/NASA, to manipulate data files in FITS format. SAX reduced data related to LECS and WFC instruments are directly written in FITS format and the reduction software is based on FTOOLS and related packages. Data coming from HPGSPC, MECS and PDS instruments are primarily written via the XAS software in XAS format (directly readable by IDL and SAOimage packages) but specific programs are available to convert them to FITS format. In addition to this, scientific software related to MECS data will also have a part directly developed via FTOOLS for uniformity with the LECS data analysis system.
SAX reduced data will be then available in the specific XAS format as well as in FITS form. Reduced SAX data can be therefore analyzed via the major existing packages widely used in the astronomical community for spatial, spectral and timing analysis (e.g., XIMAGE, XSPEC, XRONOS, IDL, MIDAS/EXSAS, IRAF/PROS, ...), according to the final user's choice.
XAS scientific software, primarily developed in the SAX Italian Institutes, will be integrated care of SDC. Data from HPGSPC, MECS, and PDS will be reduced with this software and written in XAS format.
FOT filing is the first step of the data reduction phase. SAX observational data in a FOT are structured as observations in a given observing period; the FOT filing program will recreate from FOTs on disk the original ``packet'' files with appropriate record length and will produce ``telemetry'' files per observation and per data type, plus an ``observation directory'' file.
Accumulation is the second step of the data reduction phase. It decodes observation directories and accumulates photon lists (but also images, spectra, time and HK profiles, where appropriate), from the telemetry files by selecting data on a proper parameter set (energy ranges, SAA regions, ...) and by applying basic corrections (position dependent energy correction, ...).
Cross-accumulation allows further selection or filtering on the primary accumulated data (i.e., access to FOT data is no longer required). Cross-accumulation programs are experiment independent and they mainly deal with list of photons to produce other reduced data structures as, for example, to create an image from a photon list with selection in energy and time windows, simultaneously.
Reduction proper is strictly related to the instrument specific accumulations, and it only depends on experiment specific functions, as dead time correction, background subtraction, point spread function correction, and so on. The result of this step can obviously advise the observer to go back and to repeat the accumulation step, along an iterative and interactive process.
Format conversion programs can convert data from/to XAS format to/from plain and OGIP FITS formats. The XAS format is essentially composed of a header (that includes specific information on the given kind of data), and of the actual data in binary format (using the system-dependent native internal representation). The XAS format can be immediately mapped to FITS format.
Some support programs will be also available, devoted to several different purposes: access to the file content to show/print/edit/modify header information and/or content of a XAS data file, display data files in graphical form, generate time window reading cursor co-ordinates from a time profile plot, perform algebraic operations, and so on.
Response matrix generation programs are also part of the XAS software. They generate the response matrix (in FITS format) of the HPGSPC, MECS and PDS experiments for the given observations. Calibration and housekeeping data and particular instrument settings are used here.
All programs of the XAS software will adopt a consistent user interface, supporting command line arguments, command/parameter files and interactive input. Each XAS program is able to access the results of a previous one (programs will be normally running sequentially in the same session), either via files as well as via global variables held in memory. A small package of environment access modules will be available to set/modify any variable, perform programmatic access to the environment, and save global variables.
The portability of the XAS system is assured by the definition of a reduced subset of system calls, and their isolation in a Virtual Operating System (VOS) library. These library routines will be the only part of the code which is Operating System dependent. Currently, at the SAX Institutes, UNIX/Ultrix is supported; VOS for VAX/VMS, Sun and HP-UX systems are used too, but they are not officially supported. It is planned to add (or replace the existing) official support to AXP/OSF1 on Alpha platforms.
The LECS data reduction software system is being developed at ESA/ESTEC on a SunOS 4.1 platform. All data files generated and used in it are in FITS format. The package re-uses external data analysis software whenever possible (FTOOLS, XSELECT, and SAOimage) integrating modules written ad hoc within the FTOOLS framework.
The LECS package allows the user to go from the FOT to the final scientific analysis. The first step is the FOT decoding that, together with the selection and concatenation of the observation intervals, produces on disk a set of FITS files containing event lists, HK parameters, attitude and ephemeris data and auxiliary information. Calibration, ephemeris and attitude data are then used in the linearization programs (which are not part of the standard FTOOLS package as distributed by HEASARC/NASA); they convert from PHA to PI channels, from spacecraft to UT time, and from raw pixel position to mm and to sky co-ordinates. The result is a FITS event list with PI, UT, RA and DEC information, and a FITS HK file in physical units. The next step is the so-called screening of the data performed by a mix of ad hoc and existing software. It can, among other things, plot the time series of HK and auxiliary parameters and interactively select good time intervals (GTIs). Pre-defined sets of screening criteria can also be used to define GTI files and then to select events. Existing software tools are then used for the extraction of the data products. This extraction can be performed either non-interactively, by using FTOOLS utility programs, or interactively through a combination of the XSELECT and SAOimage packages. The result is a spectrum, image or light curve in FITS standard format. The response matrix generation package produces a response matrix tailored for an individual spectrum, generated in the previous step, by using the calibration data. Finally, spectral fitting and light curve analysis can be performed within the XSPEC and XRONOS packages, or with any other fitting package accepting FITS data files.
The software for the reduction and analysis of WFC data is being developed at SRON/Utrecht on a HP-UX platform (portable on Ultrix). All result data files are in FITS format. The user interface, based on FTOOLS, emulates the IRAF interface for standalone programs.
The WFC software system allows the observer to produce cleaned photon lists, images, spectra and light curves, starting from the observational WFC data on FOTs. The first step of the standard pipeline data reduction is always the FOT reading to produce on disk a set of FITS files, in principal a list of events labelled with detector position, energy channel and time of detection. Filtering and correction of the events (according to WFC and satellite HK parameters, time and energy windows), plotting the HK parameters, and reconstruction of the WFC images are all part of the reduction system, followed by the analysis of the final images and the production of light curves and spectra.
A more detailed description of the WFC data reduction and analysis system at the Netherlands SAX Data Center is given elsewhere in this Proceedings book.