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A description of the raw data files obtained from XMM-OM is available in the XMM-Newton Data File Handbook and details of pipeline products for the OM are provided within the SSC Products Specification. Both documents can be retrieved from the XMM-Newton Science Operations Center.
Both the position and arrival time of photons are recorded on-board. The OM instrument operates in two science readout modes which are tailored to optimize the telemetry bandwidth available:
- Imaging mode provides the largest field-of-view available at the expense of tossing away the timing information. The largest unbinned image available is 2048x2048 pixel2 or 17x17 arcmin2.
- Fast mode retains the arrival time of each event, to an accuracy of 0.5s, but only transmits data within two small detector windows of dimensions 22 x 23 pixel2.
Currently the catalogue contains only sources detected within Imaging mode data. A maximum of five windows can be defined and operated by the OM simultaneously, provided they meet bandwidth constraints. The windows can be a mixture of Imaging and Fast modes. XMM users have the option to use standard mode permutations or define their own mode configuration. Excluding non-standard exceptions, the bandwidth-limited maximum exposure time is 5 ksec. Modes are discussed more fully in the XMM-Newton Users' Handbook.
XMM's 48h orbit allows for long uninterrupted pointings of science targets. The OM typically takes multiple observations of the same field with a sequence of differing Imaging mode windows. This allows both the full 17 x 17 arcmin2 field-of-view to be mosaiced over time and the field to be observed in multiple colours. Individual sources are often recorded more than once through the same filter during the observing sequence. In the new version of the catalogue the individual images are searched for sources and then the images are summed up and the detection routine is re-run to find any faint sources missed in the individual images.
The spacecraft pointing history is also telemetered down with the OM data. This data originates from independent on-board star trackers and is sampled every 30s. The OM also records tracking history data from its own dedicated tracking mode windows, but the stability of XMM's pointing is excellent and they have not been needed.
Data required to convert the brightness, position and extent of detected OM sources from detector units to physical units have been calibrated by the XMM Operation Team and stored in the Current Calibration Layer available from the XMM SOC.
All raw data employed to construct the OM source catalogue are in the public domain and can be downloaded from the XMM-Newton Science Archive.