XMM-Newton Data Files Handbook

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2 An overview of the XMM-Newton data file subsystems

The nomenclature used for activities performed by XMM-Newton is as follows: A proposal is a group of activities that pursue the same specific goal such as a science programme proposed by a Guest Observer (GO) or a calibration campaign proposed by an instrument team. Each proposal consists of one or more observations within which there are several measurements with different instruments, called exposures. An observation is defined to be the science data and required auxiliary information collected during a continuous period of time and registered under a common indentifier. It usually contains data from all science instruments (EPIC, RGS and OM), but not necessarily.

In the vast majority of cases, XMM-Newton is pointing to a fixed position in the sky during an observation. Exceptions are EPIC Mosaic, RGS Multipointing and slew observations. A slew is the period of time elapsed during a single manoeuvre between two sky pointings, and the EPIC-pn remains open with the medium filter and the observing mode of the previous observation. XMM-Newton slews through the sky at $\sim 90$ degrees/hour, which means that a source passes through the detector field-of-view in about 15 seconds. Only the EPIC-pn camera allows slew sources to be imaged essentially as point sources while the EPIC-MOS cameras detects sources as streaks in the sky. The slew catalog therefore concentrates only on EPIC-pn data and ignores data taken in small window or timing mode. EPIC mosaic observations consist of several sky pointings with different coordinates during which data are continuously taken with the X-ray cameras. RGS multipointing observations consist of five pointings with slightly different sky positions for better sampling of the RGS spectra.

The time interval between the start and the end time of an observation is called the duration of the observation. In turn, an observation consists of a sequence of one or more exposures for each instrument. For an individual instrument, exposures form a non-overlapping sequence. During an exposure, the commanded state of the individual instrument is unchanged, no changes in data acquisition mode or filter (if applicable) occur, and the data thus have a fixed format. An exposure is necessarily a concept which refers to a single individual instrument.

For each observation period (i.e., the period of time from the start of configuring the spacecraft and instruments for an observation until the end of the observation) or per slew period, a set of files is produced by the XMM-Newton Observation Data Subsystem (ODS). The ODS provides the facilities necessary to accept all inputs needed for the generation of the Observation Data File (ODF) and Slew Data File (SDF). These terms stand for a set of files generated for each specific observation or slew, respectively. They are stored in a single directory for each observation or slew and contain data from all operative instruments. Basically, their component files contain raw, uncalibrated science data from each active instrument mode and spacecraft information data.

Each ODF requires several components:

  1. Spacecraft and science telemetry
  2. Reconstructed attitude data
  3. Reconstructed orbit data

For EPIC mosaic observations, the ODS creates a single ODF for the entire observation, out of which additional separate ODFs have been produced. These "pseudo ODFs" follow the same format as ODFs produced by the ODS. The coordinates quoted for each of the "pseudo ODFs" are the central coordinates of that pointing in the sequence. The contents are the normal ODF data corresponding solely to the observation during the time that pointing was stable. Typically, there is one exposure per instrument per pointing, except for OM, where several exposures could have been taken within a single pointing. "Pseudo ODFs" can be recognised by the file names as shown in Sect. 4.1.1.

All scientific data included in the ODF/SDF (including the pseudo ODFs for mosaic observations) are systematically and automatically analysed by the Processing Pipeline Subsystem (PPS), and PPS products are generated for each instrument mode, using some specific SAS tasks.

The collection of calibration files necessary to reduce and analyse the data collected with the scientific payload of XMM-Newton is called Current Calibration File (CCF). A CCF contains all the calibration files ever created. The reason for keeping all files is that a given observation does not necessarily have to be calibrated with the last calibration files, but oftentimes with the files that are nearest in time to the time of the observation. Each file contains a time stamp to indicate the particular time for which it is valid.

For most purposes the PPS treats individual exposures separately, most PPS products are derived from data taken during a single exposure. However, there are some significant exceptions: all EPIC exposures are input to a joint source detection process; OM source information from different exposures are combined to provide colour information; potential OM counterparts to EPIC sources are identified; the RGS zero order position is determined from the EPIC source location. EPIC source lists are cross-correlated with multiwavelength archival data in the Archival Catalogue and Database Subsystem (ACDS) at the Observatoire Astronomique de Strasbourg (OAS).

The EPIC Radiation Monitor (ERM) is used for detailed monitoring of the space radiative environment, constituting a reference for the development of detectors to be used in future missions. ERM data could also be useful to define good-time intervals. Calibrated ERM data can be produced with SAS, version 9 or higher (see SAS manual for details) and will be included as part of the PPS products.

next up previous contents
Next: 2.1 The XMM-Newton Science Archive Up: XMM-Newton Data Files Handbook Previous: 1.3 A summary of the data flow
European Space Agency - XMM-Newton Science Operations Centre