The radio band is where the class of BL Lacertae objects was discovered. For many years the great majority of the BL Lacs known were found among the sources detected in large radio surveys. As a consequence, all the classical and well-studied objects have been discovered at these frequencies.
2.1.1 The 1-Jy sample
This is currently the largest complete radio sample of BL Lacs. Described in
Stickel et
al. (1991) and Stickel, Fried & Kühr (1993), it includes 34
objects extracted from the 1-Jy catalogue (radio flux
fr 1 Jy at 5 GHz:
Kühr et
al. 1981a), a flux-limited catalogue which covers essentially the
whole sky excluding the galactic plane (|b| < 10°) and the
Magellanic Clouds, according to the following criteria: (1) flat radio
spectrum between
2.7 and 5 GHz (
0.5, f
-
); (2)
magnitude brighter than 20; (3) emission lines in the optical spectrum absent
or weak with a rest-frame equivalent width of the strongest line < 5 Å.
Three more objects are included in the updated version of the 1-Jy catalogue
(Stickel,
Meisenheimer & Kühr 1994). Two of these have V
20, while
the third one (PKS 2149+173) has V = 18.9, although during the
period of the spectroscopic observations it was below the 20th magnitude
limit (Stickel &
Kühr 1993a). PKS 0521-365, an object classified as
a BL Lac in the
literature but originally excluded from the 1-Jy sample because some of its
lines had W
> 5 Å, has been included as an uncertain BL Lac,
since its [O III] luminosity is more typical of BL Lacs than of
quasars (see discussion in Urry & Padovani 1995).
Brunner et al. (1994) have reported on a ROSAT observation of S5 0454+844 (which belongs both to the 1-Jy and S5 samples), which shows the BL Lac object to be only 48 arcsec away from a source about five times brighter. They therefore suggest that previous reports of X-ray emission from this object (which are referred to in Table 1) could be due to a misidentification.
It has been suggested (Perlman et al. 1995a) that the requirement on the radio
spectral index (r
0.5), imposed as a criterion for the
selection of the 1-Jy BL Lacs to exclude the bulk of the radio galaxies, might
have resulted in the loss of some objects, since BL Lacs with steeper radio
spectral indices are known. To estimate the magnitude of this
effect, we performed the following simple calculation: out of the 119
confirmed BL Lacs which, to our knowledge, have 2.7 - 5 GHz spectral index
information, only 8 have
r > 0.5. Out of these, one (S5 1749+701) was
included nevertheless in the 1-Jy sample because the steep radio
spectrum was clearly due to variability and non-simultaneous measurements
(Stickel et
al. 1991). As regards the remaining ones, variability cannot be
the explanation for the steepness of
r in the case of PKS 0548-322,
MS1207.9+3945 and
MS1402.3+0416 since the multifrequency radio
measurements were contemporaneous
(Stocke et
al. 1985), while it cannot be
excluded in the remaining four objects, ON 231,
MS1407.9+5954,
RXJ00079+4711
and RXJ16442+4546. We then estimate that, out of the 85
BL Lacs not included in
the 1-Jy, S4 and S5 samples (which were selected to have flat radio spectra),
between 3 (4 per cent) and 7 (8 per cent) have 2.7-5 GHz spectral indices
steeper than 0.5. This suggests that the condition requiring a flat radio
spectrum might cause the loss of only 1 to 3 BL Lacs in the 1-Jy sample.
2.1.2 The S4 sample
The S4 sample includes 14 objects extracted from the S4 catalogue
(fr 0.5 Jy at 5
GHz, 35°
70° and |b|
10°: Pauliny-Toth et al. 1978;
Stickel &
Kühr 1994) by
Stickel &
Kühr (1994) using the same criteria as those applied to 1-Jy BL
Lacs. Note
that S4 1652+398 (Mrk 501) and S4 1823+568 have been mistakenly
classified as a normal galaxy and a QSO respectively in Stickel & Kühr
(1994): both are
in fact confirmed 1-Jy BL Lacs. About 10 per cent of the S4 sources are still
classified as empty fields so a small number of BL Lacs could still be
unidentified.
2.1.3 The S5 sample
The S5 sample includes 13 objects extracted from the S5 catalogue
(fr 0.25 Jy at 5
GHz,
70° and |b|
10°:
Kühr et
al. 1981b) by Kühr & Schmidt (1990). The selection criteria are
slightly different from those adopted for the 1-Jy and S4 samples: they
include in fact maximum optical polarization Pmax
larger than 3 per
cent on at least one occasion, while it is not clear what is the equivalent
width limit adopted to separate BL Lacs from quasars. (Note that all but two
1-Jy BL Lacs have Pmax > 3 per cent
[Stickel et
al. 1994], although
this was not one of the selection criteria.) We have excluded S5 1053+81 from
the sample because its spectrum shows emission lines (Xu et al. 1994), and we
have added a BL Lac candidate, S5 2353+81 (Stickel & Kühr
1993b). The S5
catalogue is currently being updated by Stickel & Kühr (in preparation).
We note that the dynamical range of source flux in radio surveys is small
and of order 10. This
demonstrates that the study of BL Lacs is still at a very early stage
even in the part of the electromagnetic spectrum where
these objects were first discovered more than 25 years ago.