The Lockman Hole

The Lockman Hole is actually several degrees across. At first we chose a ^0.3 deg2 sky region close to the absolute minimum of NH, but away from bright stars. Well in advance of the actual X-ray observations we performed optical UBVRI and radio observations in this field.

Later many other groups have picked up this field as a deep study area, so that the Lockman Hole became one of the best studied extragalactic survey fields. It has been covered by deep VLA [10,13,39], SCUBA [75], ISO [16,42], and Spitzer [37] imaging studies, was targeted in deep NIR surveys from Calar Alto, the Hawaii Institute for Astronomy wide field weak lensing studies with CFHT and Subaru, as well in high-z supernova searches. In X-rays, the Lockman Hole has been covered with deep ASCA [38], BeppoSAX [25], Chandra [45], and XMM-Newton [35] observations.

Deep X-ray survey observations in the Lockman Hole commenced in the ROSAT AO-1 (spring of 1991) with a 100 ks PSPC pointing exposure. The PSPC exposure was aimed from the beginning to reach the ultimate instrumental limits, while an HRI raster scan was planned to improve the PSPC positions. Since the spatial homogeneity of ROSAT observations is very good (see above), their ultimate sensitivity limit is set by confusion. To fight confusion, we had to obtain a very good understanding of the PSPC point-spread function, developed a completely new algorithm for X-ray crowded field analysis, and were running large numbers of simulations in order to understand the systematic subtleties and the limitations of the observations. A substantial fraction of this work was already available before the actual observations. The knowledge about instrumental limitations has led us to the final choice of the exposure time and to a conservative selection of flux limits and off-axis ranges for the complete samples to be analyzed with PSPC data.

Because of the expected confusion in the PSPC, it was also clear from the beginning that HRI data would be necessary to augment the PSPC identification process. Because of the HRI smaller field-of-view, lower quantum efficiency, and higher intrinsic background, we figured that it was necessary to invest about a factor four more HRI time than PSPC time to cover the same field with the same sensitivity, based on prelaunch knowledge. We therefore started a raster scan in AO-1, with ^100 pointings of 2ks each across the survey region. The remaining 200 ks of HRI

raster observations were approved in AO-2. When, however, the first HRI in-flight performance figures became available, we realized that the anticipated sensitivity would not be reached with the HRI raster scan. Because of the increased quantum efficiency, compared to the Einstein HRI, the ROSAT HRI is also more susceptible to background induced by particles in orbit. An increased halo of the HRI point-spread function as well as irreproducible attitude errors of about 5" are responsible for a further loss of sensitivity. Knowing this, we were able to trade the 200 ks HRI time remaining for AO-2 into an extra PSPC observation of 100 ks, which was performed in spring of 1992.

Two years later, after the PSPC had run out of gas, we started to apply for an HRI ultradeep survey aimed for a total observing time of 1 Ms in a single pointing direction. This survey was planned to push the unconfused sensitivity limit deeper than the PSPC exposure in a substantial fraction of the PSPC field. To allow an X-ray "shift and add" procedure, correcting for the erratic ROSAT pointing errors, we selected a pointing direction for the ultradeep HRI exposure which is inside the PSPC field of view, but shifted about 8 arcmin to the North-East of the PSPC center, this way covering a region containing about 10 relatively bright X-ray sources known from the PSPC and the HRI raster scan.

The observations in the Lockman Hole represented the deepest X-ray survey ever performed at that time. The total observing time invested (about 1.4 Ms) is quite comparable to that of other major astronomical projects, like, e.g., the Hubble Deep Field, and set the stage for future deep X-ray surveys. Figure 25.2 shows a color composite of the 200 ks ROSAT PSPC, and the 1.2 Ms HRI image of the Lock-man Hole. About 70-80% of the X-ray background has been resolved into discrete sources at a flux limit of ^10~15 erg cm~2 s^1 in the 0.5-2.0keV energy band [33].

Fig. 25.2 Left: ROSAT PSPC/HRI false color image of the Lockman Hole region. The HRI sources are shown in green. Red and blue colors indicate PSPC sources in the 0.1-0.5 keV and 0.5-2.0 keV energy bands, respectively. The field size is ~30 arcmin. North is up, east to the left (from [44]). Right: Color composite image of the ~800 ks XMM-Newton image of the Lockman Hole. The image was obtained combining three energy bands: 0.5-2keV, 2-4.5keV, 4.5-10keV (respectively, red, green, and blue). The image has a size of 43 x 30arcmin2 (from [35])

Fig. 25.2 Left: ROSAT PSPC/HRI false color image of the Lockman Hole region. The HRI sources are shown in green. Red and blue colors indicate PSPC sources in the 0.1-0.5 keV and 0.5-2.0 keV energy bands, respectively. The field size is ~30 arcmin. North is up, east to the left (from [44]). Right: Color composite image of the ~800 ks XMM-Newton image of the Lockman Hole. The image was obtained combining three energy bands: 0.5-2keV, 2-4.5keV, 4.5-10keV (respectively, red, green, and blue). The image has a size of 43 x 30arcmin2 (from [35])

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