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Subsections

2 Target selection

AGN candidates were selected with the criterion $B\mathrel{\mathchoice {\vcenter{\offinterlineskip\halign{\hfil
$\displaystyle ... from 3 different sources: [1] the OMHR survey of UV-excess objects at the North Galactic pole [(Moreau & Reboul 1995)], [2] the Hamburg Quasar Survey (HQS) by [Hagen etal. (1995)] and [3] the ROSAT All-Sky Survey (RASS) using identifications on HQS plates [(Bade etal. 1998)].

We also selected the object US 370 [(Usher 1981)], which was classified as a QSO candidate with V=18 and U-V=-0.55 by [Berger etal. (1991)]. The proper motion of this target could not be measured due to the proximity of a bright star as mentioned by Moreau & Reboul (1995). A slit-less observation was performed by [Weedman (1985)] and we provide here a spectrum of US 370 to confirm its nature and measure its redshift properly.

2.1 OMHR candidates

[Berger etal. (1991)] performed a photometric (U, B, V) and astrometric analysis of 1221 UV-excess unresolved objects in a 40.5$^{\ifmmode\hbox{\rlap{$\sqcap$}$\sqcup$}\else{\unskip\nobreak\hfil
\penalty50\h...
 ...ox{\rlap{$\sqcap$}$\sqcup$}
\parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi}$field almost centred on the selected area SA 57:
\begin{eqnarraystar}
12^{\rm h}52^{\rm m}08.6^{\rm s}&<\alpha_{{\rm J}2000}<&
13...
 ...32\hbox{$^\circ$}14\hbox{$^\prime$}39\hbox{$^{\prime\prime}$}.\end{eqnarraystar}
They used the MAMA (Machine Automatique à Mesurer pour l'Astronomie) machine to scan three plates taken in 1962 at the Palomar 48-inch Schmidt. This field was fully reduced in U, B and V by Moreau & Reboul (1995) with a visual limiting magnitude 20.3. Amongst the $\sim 130\,000$ detected objects, the authors extracted a list of 1759 quasar candidates selected mainly as UV-excess unresolved objects. The colour criterion was U-V<0.1 with a typical accuracy of 0.1 for the U and V magnitudes. After rejection of the objects with previous spectroscopic identifications, a total of 1681 quasar candidates remained.

A recent comparison of the MAMA catalogues extracted from the Schmidt plates taken in 1962 with those obtained in the same way from OCA (Observatoire de la Côte d'Azur) Schmidt plates taken in 1991 and 1994 provides measures of proper motions of the above OMHR quasar candidates, among many other objects (Moreau & Reboul, in preparation). The limited field of view of the OCA telescope constrained this proper motion survey to a 26.7$^{\ifmmode\hbox{\rlap{$\sqcap$}$\sqcup$}\else{\unskip\nobreak\hfil
\penalty50\h...
 ...ox{\rlap{$\sqcap$}$\sqcup$}
\parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi}$ field within:
\begin{eqnarraystar}
12^{\rm h}56^{\rm m}42.1^{\rm s}&<\alpha_{{\rm J}2000}<&13^...
 ...31\hbox{$^\circ$}57\hbox{$^\prime$}51\hbox{$^{\prime\prime}$}.\end{eqnarraystar}
The $1\sigma$ sensitivity of the measurement of proper motions is 5 to 8 mas yr-1 for the magnitude range $16<V\mathrel{\mathchoice {\vcenter{\offinterlineskip\halign{\hfil
$\displaystyl...
 ...{\offinterlineskip\halign{\hfil$\scriptscriptstyle ... used in the present work.

The combined requirements of [1] unresolved morphology, [2] UV-excess and [3] absence of proper motion larger than about 10 mas yr-1 (i.e. a transverse velocity less than 25 km s-1 at 500 pc) give a priori high-quality quasar candidates. However, the constraint imposed by medium resolution spectroscopy with the HST restricted our choice to objects brighter than magnitude 18. Quasars this bright are drastically scarcer (and consequently samples are far more polluted by stars) than for visual magnitudes fainter than V=18 where the best and most numerous quasar candidates are expected to lie.

The sample of OMHR objects selected for observation finally contained the 18 brightest and bluest targets ($B\mathrel{\mathchoice {\vcenter{\offinterlineskip\halign{\hfil
$\displaystyle ... , $U-V\leq 0$) with no proper motion detected at the 0.68 confidence level.

2.2 HQS and RASS candidates

Quasar candidates were selected from the HQS digitized objective-prism plates in two steps (Hagen et al. 1995, 1999). In the first step, prism spectra with a blue continuum are selected automatically from the digitized data which are available only at low resolution. The selected spectra are rescanned with the Hamburg PDS machine to yield high-resolution spectra having a dispersion of 139 nm mm-1 at H$\gamma$. These spectra are interactively classified on a vector graphics screen. Non-blue spectra with stellar absorption features are discarded while the others are kept in an archive.

Blue stellar-like spectra are classified as "QSO/HS", meaning quasar or hot star candidate. Spectra with apparent emission lines or unusual continua are classified as "QSO" or "Narrow Emission". If the spectra are blue and featureless they enter the category "Unid" for "unidentified". The number of entries in these classes increases towards the "unidentified" sources, while the efficiency to find quasars in these classes decreases from "QSO" towards the other classes.

We selected from the archive all the objects with $B\le 18$ contained in the above mentioned classes within about $4\hbox{$^\circ$}$ from the centre of the Coma cluster. Hagen et al. (1999) estimate that for z>0.1 and $B\le 17.5$less than 10% of the quasars are missed. Objects already known from the literature were discarded by cross-correlation with the NED and SIMBAD databases. We checked for overlaps with the OMHR survey, and discarded as well the objects with a significant proper motion (confidence level >0.68) or colour U-V>0. The final HQS list used to obtain follow-up spectroscopy contains 31 objects.

The HQS list was augmented by two AGN candidates from the RASS. X-ray sources from this survey are identified on the Schmidt plates of the HQS. The confirmation rate of the AGN candidates is very high, about >90% [(Bade etal. 1992)], but most of them are optically fainter than our selection limit or were already known. The first RASS source RXJ1252.6+3002 will be contained in the RASS Faint Source Catalog (Voges et al., in preparation) while the second one RXJ1303.7+2633 is published in the RASS Bright Source Catalogue [(Voges et al. 1997)]. Also, HS 1312+2735 is listed as X-ray source in the latter catalogue (namely RXJ1314.3+2719).


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