We discuss the properties of BL Lacs and radio galaxies discovered in the REX survey. In particular, we have found ... 40%), that we call BL Lac candidates.
BL LAC AND RADIO GALAXIES IN THE REX SAMPLE A. CACCIANIGA1, R. DELLA CECA1 , T. MACCACARO1, A. WOLTER1 , I. M. GIOIA2 3 1 Osservatorio Astronomico di Brera, Milano, Italy 2 Istituto di Radioastronomia del CNR, Bologna, Italy 3 Institute for Astronomy, Hawaii, USA ;
ABSTRACT. We discuss the properties of BL Lacs and radio galaxies discovered in the REX survey.
In particular, we have found \weak" BL Lac candidates, with luminosities in the radio, in the optical and in the X-ray band intermediate between radio galaxies and \classical" BL Lacs. The existence of these intermediate objects, predicted by the Beaming Model, indicates a continuity between the two classes of sources and imposes a re-de nition of the BL Lac classi cation. 1. The REX survey
The REX sample (Maccacaro et al. 1997; Caccianiga et al. 1998) contains about 1600 Radio Emitting X-ray sources (REX) selected through a positional cross-correlation between the NVSS (NRAO VLA Sky Survey, NVSS, Condon et al. 1998) and an X-ray catalog of serendipitous sources detected in about 1200 pointed ROSAT PSPC elds. Results from the spectroscopic follow-up of the rst 130 REX show that a large fraction of them (58) is represented by BL Lacs and radio galaxies, while the remaining sources are principally radio loud QSOs. Thus, we are in the position of studying in the same sample both BL Lacs and radio galaxies that are thought to be the same class of sources seen at di erent angles in the context of the Beaming Model (e.g. Urry & Padovani 1995). 2. BL Lacs and radio galaxies
We discuss here only the objects without emission lines (EW5 A) in the optical spectrum. In this class are included \classical" BL Lacs, i.e. objects that show in the optical spectrum a Ca II contrast (KCa II ) at 4000 A below 25%, radio galaxies (with KCa II 40%) and intermediate objects (25%KCa II 40%), that we call BL Lac candidates. The luminosities at 1.4 GHz of the BL Lacs and of the radio galaxies discovered in the REX survey range from 1031 to 1033 erg s 1 Hz 1 , consistent with those of \classical" FR I and BL Lacs. All the BL Lacs and candidates for which we have information on the radio ux at 5 GHz are characterized by a slope R 0.6 (f/ ) while radio galaxies cover a wider range of values, from {0.2 to 1.2. The overall distribution of the slopes does not reveal any bimodality. The objects with KCa II 40% show X-ray luminosities (0.5{2.0 keV) up to 1044 erg s 1 while some of the objects with KCa II below 40% reach luminosities of 51044 - 1045 erg s 1 consistent (but at the low luminosity end) with the
Fig. 1. The values of K as a function of the X-ray luminosity for the objects in cluster (a) and the isolated objects (b). Ca II
range of luminosities typical of BL Lac objects (1044 < LX 95%), while Figure 1a does not show any correlation. 3. Discussion and conclusion
We have found a continuity in the properties of BL Lacs and radio galaxies discovered in the REX sample and we have discovered several intermediate objects, whose existence is predicted by the Beaming Model. Altogether, the two classes of sources show a signi cant anti-correlation between the values of KCa II and the X-ray luminosities. This result suggests a common origin of the high X-ray luminosities and the low values of KCa II , probably related to the presence of a non-thermal nucleus in the host galaxy. The di erences in the observed luminosities can be connected either to a di erent intrinsic power of the non-thermal nucleus or to the e ect of di erent orientation in the framework of the beaming model. In the case of clusters of galaxies, the strong X-ray luminosities are mainly related to the intracluster gas and not to the activity of the nucleus and thus the values of KCa II and the X-ray luminosities are not expected to be correlated. References
Caccianiga, A. et al. 1998, submitted to ApJ Condon, J.J., et al. 1998, AJ, 115, 1693 Maccacaro, T., Caccianiga, A., Della Ceca, R., Wolter, A., Gioia, I. M. 1998, Astron. Nach., 319, 15 Urry, M. & Padovani, P. 1995, PASP, 107, 803
