Radio-loud ROSAT sources near the North Ecliptic Pole

¹ Max-Planck Institut für extraterrestrische Physik, P.O. Box 1603, D-85740 Garching, Germany
² Dept. of Physics, Bucknell University, Lewisburg, PA 17837, U.S.A.
³ Fermi National Accelerator Laboratory, Batavia, IL 60510, U.S.A.
⁴ Dept. of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, U.S.A.
⁵ E.O. Hulburt Center for Space Research, Naval Research Laboratory, Washington DC 20375-5320, U.S.A.

Send offprint request to: W. Brinkmann

Received: 6 March 1998
Accepted: 6 August 1998

Abstract

A deep and large-area survey of the North Ecliptic Pole region was made with the ROSAT All-Sky Survey and the VLA to elucidate the population of radio-loud extragalactic objects. A region of 29.3 square degrees was surveyed with sensitivities around erg s^-1 cm^-2 in the soft X-ray band and 1 mJy at 1.5 GHz. Optical counterparts were sought on digitized Schmidt plates from POSS-I and II. Seventy-four reliable RASS-VLA sources were found. The sample is a heterogeneous mixture of Seyfert galaxies, quasars, BL Lac objects, galaxy clusters and groups containing a radio galaxy. Optical magnitudes range from to . Three results are noteworthy: (1) the bimodality in the ratio of radio to optical emission seen in optically- and X-ray-selected AGN samples is also evident in the ROSAT-VLA objects; (2) X-ray and radio selection is an effective method for locating poor galaxy clusters and groups; and (3) a considerable population of optically faint but X-ray/radio-bright objects is present. This last group may be either distant clusters with radio galaxies or "red quasars".