The central engine of low-luminosity radio galaxy 3C 270 (NGC 4261)

TL;DR

This study analyzes the polarization spectra of the low-luminosity radio galaxy 3C 270, revealing that its nucleus shows polarized light consistent with polar scattering and lacks evidence of a broad line region or Big Blue Bump, suggesting a different accretion mode.

Contribution

It provides new polarization measurements of 3C 270, demonstrating the absence of a broad line region and Big Blue Bump, and supports the hypothesis of radiatively inefficient accretion and reflection.

Findings

Polarization angle indicates polar scattering.

No broad line region detected in polarized flux.

Absence of Big Blue Bump suggests different accretion mode.

Abstract

We present the polarization spectra of the nucleus of 3C~270. We confirm that the polarization angle of both the continuum and the emission lines are close to perpendicular to the jet direction after careful correction of interstellar polarization, which indicates polar scattering. The Stokes flux spectrum resembles the total flux spectrum, with no need for a broad component from the Broad Line Region. Over 94\% of a sample of Seyfert I broad line profile would be significantly detected if present in our polarized flux spectrum. We favor the hypothesis that we are observing the continuum including any contribution from Radiatively Inefficient Accretion Flow and beamed synchrotron instead of a Big Blue Bump, as well as the innermost Narrow Line Region, through reflection. This makes 3C~270 the third known case, after NGC~4258 and Centaurus~A, where only narrow lines (and the continuum, if present) are scattered, with no evidence of an underlying Big Blue Bump.