XMM-Newton Optical Monitor Serendipitous UV Source Survey

Source Properties: Time Sampling

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. Segregated by filter, figure 1 illustrates the distribution of repeat detections within individual pointings. The differences between UV and optical distributions is a consequence of data selection - only those optical sources with UV counterparts within the catalogue are selected for inclusion.

Multiple exposure distribution of XMM-OM sources.

Fig 1: Distribution of source exposures during a single pointing. N_exp is the number of images obtained during a pointing in which a particular source has been detected. N_src is the number of sources detected N_exp times. Objects which are detected only once, N_exp = 1, are excluded from the sample.

Filter       N_e       f_e (%)       N_o       f_o (%)

UVW2   46,031 38.4   11,657 12.0

UVM2   33,660 23.2   13,010   11.8

UVW1 137,309 22.2   58,280 11.2

U   24,627 13.9   18,625 12.7

B   12,732 15.7     9,702 15.4

V   11,724 15.0   12,022 20.9

Table 1: Number of sources detected in multiple images. N_e is the number of sources detected in more than one exposure through a specific filter during a single spacecraft pointing. f_e is the fraction of sources detected more than once per pointing. N_o is the number of sources detected multiple times during different spacecraft pointings. f_e is the fraction of unique sources detected during different pointings.

The second timescale for source sampling is of the order years and associated with different pointings to the same field of stars. Figure 2 presents the number of sources N_src detected during multiple observations, N_pnt is the number of pointings.

Multiple pointing distribution of XMM-OM sources.

Fig 2: Distribution of sources detected during more than one spacecraft pointing. N_pnt is the number of pointing over which a specific source was found.

The time domain of detections included in the catalogue span just over 8 years between Feb 24, 2000 and Mar 29, 2007. Figure 3 plots the distribution of time-spans between first and last source detections. The apparent biannual sampling is the result of seasonal pointing constraints imposed by sun angle and efficiency of the solar panel power supply.

Time domains sampled by XMM-OM sources with multiple pointings.

Fig 3: Distribution of intervals between the first and last detections of a specific source, Δt_pnt, in units of years. The lower panels of each pair contain the source number distributions. The upper panels contain the number of pointings with a specific source detection plotted against the time interval over which the detections occur.