Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.

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See also the documentation for the 1XMM catalogue which uses a very similar procedure: http://xmmssc-www.star.le.ac.uk/

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... five

Note: Fig. 1a,b shows a sixth bright X-ray source at the lower edge of the image. This is identified with an AGN at z=0.38 and is hence not connected with the galaxy.

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... object

We note that the R-band finding chart for source E (Fig. 2) shows evidence for diffuse emission, including an unusual "bridge'' feature, which may indicate the AGN galaxy is part of an interacting group.

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... average

The location of these sources in the X-ray hardness ratio plot (Fig. 3) demonstrates that the nominal spectrum used to compute the fluxes and luminosities (Table 1) is a reasonable assumption given the quality of the data. The derived fluxes and luminosities are of course sensitive to the spectral assumptions: increasing the assumed column density to $N_{\rm H} \sim 10^{21} \rm ~cm^{-2}$ would increase them by a factor of $\sim$ 1.5, whilst the possible range of spectral indices also introduces similar changes in the derived values, with steeper (flatter) spectra reducing (increasing) the flux and luminosity estimates by factors $\sim$ 1.5-2.

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... limit

We caution that the actual detection limit could be lower, which would lower the estimated contribution from undetected sources, for example if the detection limit is $2 \times 10^{39} \rm ~erg~s^{-1}$ then the estimated missing flux reduces to $1.3 \times 10^{40} \rm ~erg~s^{-1}$ On the other hand, the PSF wings of the bright QSO obscure our view of the southern regions of the galaxy, hence we potentially miss detecting sources in this region which could lead to us underestimating the luminosity of the galaxy. Indeed there is clear evidence that the profile of source E is asymmetric (Fig. 1), implying the presence of one or more further faint objects in the vicinity of source E.

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