New active galactic nuclei detected in ROSAT All Sky Survey galaxies - The complete dataset

TL;DR

This study identifies new active galactic nuclei in the ROSAT All Sky Survey, providing a comprehensive dataset and analysis of their optical and X-ray properties, revealing a high prevalence of Seyfert 1 galaxies among X-ray sources.

Contribution

The paper presents the complete dataset of newly identified AGN from ROSAT survey galaxies, including optical spectra and detailed distribution analysis, highlighting the dominance of Seyfert 1 galaxies in X-ray selected samples.

Findings

139 of 166 X-ray counterparts are AGN, mostly Seyfert 1.

High ratio of Seyfert 1 to Seyfert 2 galaxies (11/1).

ROSAT's sensitivity favors detection of soft X-ray, heavily absorbed in type 2 AGN.

Abstract

The ROSAT ALL Sky Survey Bright Source Catalogue (RASS-BSC) has been correlated with the Catalogue of Principal Galaxies (PGC) to identify new extragalactic counterparts. 550 reliable optical counterparts have been detected. We took optical spectra of 176 X-ray candidates and companions at ESO, Calar Alto observatory and McDonald observatory. We discuss the redshift-, linewidth-, as well as optical and X-ray luminosity distribution of our ROSAT selected sample. 139 galaxies of our 166 X-ray counterparts have been identified as AGN with 93 being Seyfert 1 galaxies (61%). Eighteen of them (20%) are Narrow Line Seyfert 1 galaxies. 34 X-ray candidates (21%) are LINERs and only eight candidates (5%) are Seyfert 2. The ratio of the number of Seyfert 1 galaxies to Seyfert 2 galaxies is about 11/1. Optical surveys result in ratios of 1/1.4. The high fraction of detected Seyfert 1 galaxies is explained by the sensitivity of the ROSAT to soft X-rays which are heavily absorbed in type 2 AGN. Two X-ray candidates are HII-galaxies and 25 candidates (15%) show no signs of spectral activity. The AGN in our RASS selected sample exhibit slightly higher optical luminosities M_B = (-20.71 +- 1.75) mag and similar X-ray luminosities (log(L_X [erg/s]) = 42.9 +- 1.7) compared to other AGN surveys. The Halpha line width distribution (FWHM) of our newly identified ROSAT AGN sample is similar to the line widths distribution based on SDSS AGN. However, our newly identified RASS AGN have rather reddish colors explaining why they have not been detected before in ultraviolet or blue excess surveys.