The XMM-OM samples source properties on two distinctive time scales. Multiple exposures through a particular filter are often obtained during one pointing. They provide sampling on timescales between hours and a day. The longest observations are curtailed by radiation constraints at the perigee of the 48 hour spacecraft orbit and individual exposures are limited typically to durations < 5 ksec in order to avoid potential memory corruption after cosmic ray hits. For this version of the catalogue the repeated exposures within a single pointing are averaged together.
The second timescale for source sampling is of the order years and is associated with different pointings to the same field of stars. 692,000 sources have multiple entries and Figure 1 presents the number of sources Nsrc detected during multiple observations, where Npnt is the number of pointings.
The dates at the start and end of each observation are recorded in the second table. Figure 2 shows a histogram of the time intervals between repeated observations of the same source. 113,000 sources identified as having more than one entry (i.e. multiple observations) in the first table of catalogue, are included but there is no distinction between the filters. The longest possible interval would be just over 12 years; the length of time between the first observation in the catalogue and the last. The apparent biannual sampling is the result of seasonal pointing constraints imposed by sun angle and efficiency of the solar panel power supply. It is interesting to compare this plot with the equivalent one for Swift-UVOT (UVOTSSC time sampling page) where the very different orbit (90 minutes to XMM's 48 hours) and observing strategy makes for a completely different plot.
It is possible to estimate how many of the sources are variable by comparing the variability in the count rates with the expected errors. For this only sources observed four or more times were included. A source is considered to be variable if the standard deviation in the measurements is more than three times the mean (1 sigma) error, see Table 1. The number of sources found in this way to be variable in at least two UV filters is 3668, and all three UV filters is 429.
|Filter||No. sources observed more than 3 times||No. variable sources||Percent|
Table 1: showing the number of possibly variable sources in XMM-SUSS2 in the UV filters