How do we know which of the few photons are ours? 
Or if the question asks which of the ones detected by the instrument come from IC 2497 (our intended source), there are two answers. For "softer" X-rays, at lower energies,
Suzaku has an X-ray telescope which uses nested grazing-incidence mirrors to focus X-rays into the detectors, so there is an image of the sky. For higher-energy "hard" X-rays, which go right through even the grazing mirrors, a set of metal baffles limits the field of view to about half a degree on a side. We already know from the
Swift X-ray telescope that there aren't other confusing sources in this region. For the hard X-ray detector, it is especially important to know the overall X-ray background as seen with that system to we can tell (statistically) how many at each energy are from the source of interest. This is easier with the focussing lower-energy system, since we see plenty of "blank" sky in the field of view around the source.
The hard X-rays are especially interesting - if the central quasar is surrounded by a large enough amount of gas, it could absorb even the soft X-rays leaving this as our most direct window to the core. X-ray detectors are especially good at measuring the energies of each detected photon, so we know at what energies they all arrive.
