XMM-Newton Optical Monitor Serendipitous UV Source Survey

Source Properties: Sky Coverage

Figure 1 plots the distribution of XMM-OM pointings between the dates 2000 February 3 and 2007 March 31. Only those pointings which contribute sources to the SUSS catalogue are included. The size of the pointing symbols are not related to field-of-view but increase in radius and vary in hue as the detected source density increases, N = unique sources/pointing.

XMM-OM pointing distribution in Galactic coordinates.
Fig 1: The observation content of the SUSS catalogue (blue) mapped onto the sky in Galactic longitude and latitude. The colour image in the background is the Infra-Red Astronomy Satellite (IRAS) 100μm all-sky map.

Mirroring the wide variety of scientific experiments performed by XMM for it's Guest Observers, there are sub-clusters of pointings around the Galactic plane, concentrated towards the Galactic center, and the Magellanic Clouds. Between the Galactic plane and poles the pointings have a more random distribution (in a serendipitous sense) for extra-Galactic and Galactic-neighbourhood targets. The FITS file housing the XMM-OM catalogue includes a table extension summarizing pointing-related data.

The following table contains the total area of sky observed through each filter and the fraction of observations, fobs which contain a specific filter. Nsrc is the number of sources found with a detection significance > 3σ.

Filter       Sky coverage (deg2)       Nsrc (> 3σ)       fobs (%)
UVW2 43.7 119,805 15.9
UVM2 29.0 145,210 19.3
UVW1 54.0 618,226 82.0
U 21.6 177,569 23.6
B 14.3   81,191 10.8
V 12.6   78,160 10.4

Table 1: Sky coverage statistics for each of the six XMM-OM lenticular filters included within the catalogue.

Exposure maps for each filters can be downloaded. Since the observations cover a small fraction of total sky, the maps are provided in table form for ease-of-use. The resolution of the maps is 1 x 1 arcmin2; and the maximum exposure time, total exposure time, number of visits and number of observations are provided for each pixel.

Plotting in Figure 2 the distribution of sources as a function of Galactic latitude reveals several clusters, one associated with the Galactic plane at b = 0° and the other at b = 30° associated with the Magellanic Clouds.

XMM-OM pointing distribution across Galactic latitudes.
Fig 2: The normalized source number content of the UV catalogue plotted as a function of Galactic latitude.

Table 2 provides some statistics for source numbers within ±5° of the Galactic Plane, confined within a cone of radius 4°, centered around the Magellanic Clouds at l = 278°, b = -33°, and at high Galactic latitude |b| > 30°, excluding the Magellanic Cloud cone.

Filter       NGalactic       NMagellanic       NHigh-b       NTotal
UVW2   25,097 50,158  24,439 119,805
UVM2   25,625 53,208  35,311 145,210
UVW1 124,188 71,948 199,934 618,266
U   34,080    4,887   74,815 177,569
B     8,738          0   42,644   81,191
V   27,366          0   23,073   78,160

Table 2: Detected source numbers within three sky regions. NGalactic is the number of sources contained within the region −5° < b < +5°. NMagellanic is the number of sources contained within a cone of radius 4° centered on l = 278°, b = −33°. NHigh-b is the number of sources in regions b < -30° or b > +30°, excluding sources inside the Magellanic Cloud cone.

U, B and V sources are under-represented in table 2 because only those optical objects with UV counterparts are included within the catalogue and fields with > 5,000 source detections are rejected in order to avoid crowding confusion during fine-aspect correction. Since the effective area of XMM-OM peaks in the optical, the UBV data contain a larger fraction of crowded fields compared to the UV sample. The optical fields of the Magellanic clouds in particular do not make this cut and don't appear within the catalogue.