A large, uniform sample of X-ray emitting AGN: selection approach and an initial catalog from the ROSAT All-Sky and Sloan Digital Sky Surveys

Abstract

Many open questions in X-ray astronomy are limited by the relatively small number of objects in uniform optically-identified and observed samples, especially when rare subclasses are considered, or when subsets are isolated to search for evolution or correlations between wavebands. We describe initial results of a new program aimed to ultimately yield 104 fully characterized X-ray source identifications—a sample about an order of magnitude larger than earlier efforts. The technique is detailed, and employs X-ray data from the ROSAT All-Sky Survey (RASS), and optical imaging and spectroscopic follow-up from the Sloan Digital Sky Survey (SDSS); these two surveys prove to be serendipitously very well matched in sensitivity. As part of the SDSS software pipelines, optical objects in the SDSS photometric catalogs are automatically positionally cross-correlated with RASS X-ray sources. Then priorities for follow-on SDSS optical spectra of candidate counterparts are automatically assigned using an algorithm based on the known ratios of fx/fopt for various classes of X-ray emitters at typical RASS fluxes of 10−13 erg cm−2 s−1. SDSS photometric parameters for optical morphology, magnitude, colors, plus FIRST radio information, serve as proxies for object class. Initial application of this approach to RASS/SDSS data from 1400 deg2 of sky provides a catalog of more than 1200 spectroscopically confirmed quasars and other AGN that are probable RASS identifications. Most of these are new identifications, and only a few percent of the AGN counterparts are likely to be random superpositions. The magnitude and redshift ranges of the counterparts are very broad, extending over 15 < m < 21 and 0.03 < z < 3.6, respectively. Although most identifications are quasars and Seyfert 1s, a variety of other AGN subclasses are also sampled. Substantial numbers of rare AGN types are found, including more than 130 narrow-line Seyfert 1s and 45 BL Lac candidates. These early results already provide a very sizeable set of source identifications, demonstrate utility of the sample in multi-waveband investigations, and show the capability of the joint RASS/SDSS approach to efficiently proceed towards the largest homogeneously selected/observed sample of X-ray emitting quasars and other kinds of AGN.