Optical identification of radio-loud active galactic nuclei in the ROSAT-Green-Bank sample with SDSS spectroscopy

TL;DR

This study refines optical identification of radio/X-ray sources in the RGB catalog using SDSS spectroscopy, confirming most are radio-loud AGNs and distinguishing various subclasses with detailed spectral analysis.

Contribution

It provides the first optical identifications for 72 sources and refines classification of NLRGs into Seyfert II and LINERs using high-quality spectral diagnostics.

Findings

Majority are radio-loud AGNs, especially blazars.

Most NLRGs are LINERs, with few Seyfert II.

Identified rare radio-loud NLS1s and flat-spectrum quasars.

Abstract

Results are presented of extended and refined optical identification of 181 radio/X-ray sources in the RASS-Green Bank (RGB) catalog which have been spectroscopically observed in the Sloan digital sky survey (SDSS) DR5. For 72 sources, the identifications for are presented for the first time. It is confirmed that the majority of strong radio/X-ray emitters are radio-loud active galactic nuclei (AGNs), particularly blazars. Taking advantage of the high spectral quality and resolution and our refined spectral modeling, we are able to disentangle narrow line radio galaxies (NLRGs), as vaguely termed in most previous identification work, into Seyfert II galaxies and LINERs, based on the standard emission line diagnostics. The NLRGs in the RGB sample, mostly belonging to `weak line radio galaxies', are found have optical spectra consistent predominantly with LINERs, and only a small fraction with Seyfert II galaxies. A small number of LINERs have radio power as high as $10^{23}-10^{26}$ \whz at 5 GHz, being among the strongest radio emitting LINERs known so far. Two sources are identified with radio-loud narrow line Seyfert 1 galaxies (NLS1s), a class of rare objects. The presence is also confirmed of flat-spectrum radio quasars with the radio--optical--X-ray effective spectral indices similar to those of High-energy peaked BL Lac (HBL), albeit it is yet a debate as to whether this is the case for their actual spectral energy distributions.