6 Summary

We have presented an unbiased survey of extragalactic objects chosen by their simultaneous presence in flux-limited X-ray and radio continuum surveys. While other studies have penetrated deeper over small fields or surveyed larger areas with lower sensitivity, our effort is unique in its combination of depth (reaching $5 \ 10^{-14}$ erg s^-1 cm^-2 in the 0.1-2.4 keV band and 1 mJy at 1.5 GHz) and areal extent (29.3 square degrees). It was made possible by the repeated scans of the North Ecliptic Pole region during the ROSAT All-Sky Survey and a complementary radio survey with the VLA.

A total of 74 reliable RASS-VLA sources were found (including multiple radio components), with an additional 34 possible matches. Optical counterparts were sought on digitized Schmidt plates of the POSS-I and II surveys. The result is a rather heterogeneous sample with objects ranging in brightness by 2 orders of magnitude in the X-rays, 3 orders of magnitude in the radio and >3 orders of magnitude in the optical. The minority of objects with optical spectroscopy include four quasars, one BL Lac object, two Seyfert 1 galaxies, and two Seyfert 2 galaxies. Unstudied objects include galaxies with $15 \leq B \leq 22$ and unresolved objects which are presumably quasars and BL Lacs. A considerable fraction (approximately one-third) of the radio- and X-ray-loud objects do not have any optical counterparts brighter than B = 22.5.

The previously identified objects do not show any class-specific separation with regard to their radio or X-ray fluxes. Only the optical magnitudes indicate some systematic trend for the various objects (see Fig. 11). For 23 sources only upper limits could be obtained (m_B > 22.5; one source was detected on a red plate). However, seven of these are quite strong radio sources with fluxes $f\rm _r \gt$ 10 mJy.

While the heterogeneity of the sample and incompleteness of redshift information precludes quantitative analysis, a number of interesting findings emerge. They mainly support similar results that emerged from the shallower but large-area RGB survey discussed in Papers I, II and Laurent-Muehleisen et al.(1998).

1.

The distribution of radio-to-optical flux ratios and spectral indices appears to be bimodal, in agreement with optically-selected and other X-ray/radio-selected samples.

2.

Ten likely galaxy groups and clusters are detected, including two Abell clusters and four groups found in previous X-ray surveys with optical (but not radio) follow-up. The X-ray/radio detection strategy appears to be quite effective in uncovering relatively poor clusters which contain a radio galaxy.

3.

Many of the RASS-VLA sources are optically very faint for their radio and X-ray fluxes compared to normal quasars with $\alpha_{\rm ro} \simeq \alpha_{\rm ox}$ or even $\alpha_{\rm ro} \gt \alpha_{\rm ox}$.These are either "red quasars" or new distant clusters containing a radio galaxy.

Acknowledgements

The ROSAT project is supported by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF/DLR) and the Max-Planck-Gesellschaft. We thank Peter Kahabka for his help in programming the source detection routines and our colleagues from the ROSAT group for their support. This research at Penn State was supported by NASA grant NAGW-2120 to EDF. RIK acknowledges support from Fermi National Accelerator Laboratory; MMC acknowledges partial support from Bucknell University. We acknowledge the assistance of W.L.W. Sargent (CalTech) in making POSS-II plates of the NEP available to ROE for Cosmos digital scanning and thank Richard McMahon for providing the APM data. This research has made use of the NASA/IPAC Extragalactic Data Base (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.