Although this work is primarily interested in X-ray emission, especially of AGN, the project started with a photographic plate from the Hamburg Quasar Survey (HQS). The HQS is an objective prism plate based survey of the northern sky. Additional direct plates were taken for exact position determination. A detailed description can be found in Hagen et al. (1995), and a first list of quasars will be given in Engels et al. (1997).
It is obvious that the magnitude limited samples obtained as test samples for the automated quasar search are of astronomical interest as well. They fill in the magnitude gap between deep surveys which cover necessarily only small areas and extended surveys which are cut off at high magnitudes.
The field HS 47.5/22, centred around 
(B1950.0), at a galactic latitude of 
was selected for
follow-up observations in several wavelength ranges. The selection was based
solely on the extraordinary plate quality. Telescope condition, weather,
and plate material, were all at their best when the plates were taken, which
allows the classification of objective prism spectra, and the recognition of
optical counterparts on the direct plate, to much fainter magnitudes than the
average HQS field. This "technical" selection holds the
advantage of a total random choice regarding astronomical properties.
Another major advantage of the field is its location in an area with very low
column density of galactic neutral hydrogen 
. Low
resolution surveys like the Stark Survey (Stark et al. 1992) give an
average value of 
, corresponding
to a visual extinction of only 
. This is not much higher than the
values in the so called "Lockman Hole'' which contains the lowest
of the whole northern sky:
(Lockman et al.
1986). Absorption of X-rays in the interstellar medium will
therefore be a comparatively minor problem.
Previous X-ray surveys have shown that the majority of the faint sources
outside the galactic plane are AGN, or at least extragalactic, the fraction
growing with galactic latitude (Boyle et al. 1994). Therefore the high
galactic latitude of HS 47.5/22, together with the low ,
predestinate the field for a search of extragalactic X-ray sources.
The central part (
) of HS 47.5/22 was surveyed with
ROSAT (Trümper 1983). The observations consist of a series of
overlapping PSPC pointings, the exposure times chosen so as to give net values
between those of large scale surveys (e.g. RASS, Voges 1992;
EMSS, Gioia et al. 1990) and deep observations in small areas (e.g.
Hasinger et al. 1993) to cover the intermediate flux range.
The X-ray sources were identified with the aid of the HQS plates, published
catalogues, and follow-up spectroscopy. The ratio of X-ray to optical flux,
, played an important role since the
different types of X-ray emitters were found in the EMSS to have different,
well defined ranges (Gioia et al. 1990). ROSAT observations confirmed
these albeit with slight changes due to the different energy bands
of the two satellites (Bade 1993).
In this paper, we present the results of the X-ray survey. The next section presents the X-ray data and analysis. The third section overviews some additional data in other wavelength bands. Identification of X-ray sources is described in the fourth, and the last contains some concluding remarks.