The XMM spectral catalog of SDSS optically selected Seyfert 2 galaxies

TL;DR

This study analyzes X-ray spectra of optically selected Seyfert 2 galaxies from SDSS to investigate obscuration properties and challenges to the unification model, revealing a significant fraction of unobscured Sy2s with low accretion rates.

Contribution

First comprehensive X-ray spectroscopic catalog of SDSS Seyfert 2 galaxies examining obscuration beyond the local universe.

Findings

Approximately 23% are Compton-thick AGN.

X-ray diagnostics agree with mid-IR and [OIII] luminosity ratios.

About 13% are unobscured Sy2 galaxies with low accretion rates.

Abstract

We present an X-ray spectroscopic study of optically selected (SDSS) Seyfert 2 (Sy2) galaxies. The goal is to study the obscuration of Sy2 galaxies beyond the local universe, using good quality X-ray spectra in combination with high S/N optical spectra for their robust classification. We analyzed all available XMM-Newton archival observations of narrow emission line galaxies that meet the above criteria in the redshift range 0.05<z<0.35. We initially selected narrow line AGN using the SDSS optical spectra and the BPT classification diagram. We further modeled and removed the stellar continuum, and we analyzed the residual emission line spectrum to exclude any possible intermediate-type Seyferts. Our final catalog comprises 31 Sy2 galaxies with median redshift z~0.1. X-ray spectroscopy is performed using the available X-ray spectra from the 3XMM and the XMMFITCAT catalogs. Implementing various indicators of obscuration, we find seven (~23%) Compton-thick AGN. The X-ray spectroscopic Compton-thick classification agrees with other commonly used diagnostics, such as the X-ray to mid-IR luminosity ratio and the X-ray to [OIII] luminosity ratio. Most importantly, we find four (~13%) unobscured Sy2 galaxies, at odds with the simplest unification model. Their accretion rates are significantly lower than the rest of our Sy2 sample, in agreement with previous studies that predict the absence of the broad line region below a certain Eddington ratio threshold.