The observation, carried out between the 25th of February and the 3rd of March, was pointed towards RA , DEC (J2000) with a total accepted exposure time of 15889 s. By comparing to optical data enough sources could be identified so far to confirm the absolute positioning of the ROSAT XRT to a few arcseconds.
Fig. 1 shows an overlay of the CMB measurement onto the X-ray map of our observation. The individual points in the X-ray map correspond to single photons in the energy range 0.5--2.0 keV.
Figure 1: Overlay of the CMB measurement onto the X-ray observation. The separation of the quasar pair PC1643+4631 (+) A (right) & B (left) is . Radio sources (), which were identified in the high resolution radio observation and removed from the lower resolution data (shown here) are also marked. No enhancement of the X-ray flux is seen within the inner contours. Coordinates in this figure are B1950.
In Fig. 2 we show and detail in Tab. 1 the detected X-ray sources with a likelihood ratio greater than 5. This low likelihood ratio has been chosen to demonstrate that not even a marginally detected source can be found within the inner contours of the microwave decrement. Secure detections require at least a likelihood of 10.
Figure 2: Source detections with likelihood ratio . No source is found near the center of the decrement. The rings around the center indicate and apertures for the upper limits to the count rate. The coordinate lines are RA = and DEC = (J 2000).
Table 1: Source detections with likelihood > 5 near the center of the decrement.
No firm detection at the quasar positions can be made and we use the marginal detection of sources at the quasar positions, 48 and possibly one component (with half the total count rate) of the ``extended'' source 53, to give upper limits of 7 and for the respective sources.
In determining the upper limits for the decrement we compared various energy intervals and found 0.5--2.0 keV to give the best estimate, due to the enhanced Galactic background rate at lower energies. In the Gaussian approximation for the Poissonian photon distribution we find that 12.5 counts within an radius around the center of the decrement can be excluded with 99.7% confidence for the one-sided limit, which corresponds to a limiting count rate of for the local exposure. Deconvolved with the effective area and response of the instrument, and with Galactic absorption of HI column density taken into account, this correspondes to a bolometric flux limit of for a thermal spectrum with temperature 2.5 keV in the observer frame.