| ISDCINTEGRALPlanckGaiaFACT |
| LOFTSAFARIATHENACAPHEAVENS |
| INTEGRAL Science Data Centre |
1. Introduction
The INTEGRAL spacecraft incorporates (1) two Gamma-ray instruments, the IBIS imager and the SPI spectrometer, (2) the JEM-X X-ray monitors, and (3) the OMC optical monitor. The three high-energy instruments all rely on so-called coded-mask imaging techniques. Gamma-ray sources illuminate the detector plane, made of a number of independent units, through a mask composed of a pattern of transparent and opaque elements. The main task of the data analysis is to process the recorded mask shadows (sometimes called shadowgrams) to reconstruct images of the sky, and to extract source spectra and lightcurves. This reconstruction, or extraction, process is complicated because the background is very intense and because the signal is not always perfectly "coded". Gaps in the detector plane or too small a number of measurements (the SPI spectrometer contains "only" 19 detectors) can lead to systematic errors in the reconstructed images.
To improve the situation, the INTEGRAL observations are carried out following a dithering strategy. Instead of staring, the spacecraft is moved every 20 to 60 minutes to a new location around the target, gradually covering a 7 point hexagonal pattern, or a 5x5 rectangular grid. A stable attitude period is called a "pointing" in ISDC terminology, while a transition period during which the spacecraft is moving is referred to as a "slew". The typical separation of the "pointings" is of the order of 2 degrees. In this way, the number of "measurements" is increased and this helps to reduce the systematic errors in the deconvolution process.
The continuous flow of data received from INTEGRAL is cut by the initial ISDC pipeline processing at every pointing-slew, or slew-pointing, transition. All data produced during a pointing or a slew are stored into a set of FITS files, which are grouped together into an entity called a "Science Window Group (SWG)". A SWG can therefore either be related to a slew or a pointing. Technically, a SWG is a (FITS grouping) table containing the list of member files used for data access in our software. A SWG data set represents a basic atomic unit of INTEGRAL data. All data belonging to one SWG are stored below the same location in the archive.
The first task of an INTEGRAL data analysis is to identify and collect the relevant pointings (ISDC does not currently provides any tool to analyze slews). Guest observers receiving their private data may want to process the complete lot, but more generally, all available pointings including the target of interest in the instrument field-of-views should be selected. This can be achieved with the ISDC archive browser.
Once the list of ScWs is available, a so-called Observation Group (OG) can be created with the program OG_CREATE. Again, an OG relates all data from all SWGs together, and it is conveniently used for data access. In most cases, the only thing the ISDC software needs to know about input data is the name of the OG.
The ISDC software includes four pipelines (jemx_science_analysis, ibis_science_analysis, spi_science_analysis, omc_science_analysis) which allow to carry out sky image reconstruction, and spectrum and lightcurve extraction, from the OG. Each of these pipelines is documented by a "cookbook" guiding the user through practical simple examples with some (limited) explanatory information.
2. How to use further ISDC documentation
Users are advised to start by exercising simple examples following the "cookbooks" (JEMX, IBIS, OMC, for SPI see below) instructions. You may then start your own analysis, and come back, when you are confronted with problems or difficult decisions to the "User Manuals" or additional documents available from the documentation page
http://www.isdc.unige.ch/integral/download/osa_doc
In addition, a guide about how to use the SPI data analysis specific documentation is provided in the following page
http://www.isdc.unige.ch/support/spi/index
If you do not find adequate answers from the documentation, you may consult our FAQ
http://www.isdc.unige.ch/support/faq
or send a request to the INTEGRAL help desk
http://www.isdc.unige.ch/integral/support/helpdesk
