The innermost region of the water megamaser radio galaxy 3C 403

¹ INAF-Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy e-mail: atarchi@ca.astro.it
² INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁵ IRAM, Avenida Divina Pastora 7, 18012 Granada, Spain

Received: 19 July 2007
Accepted: 18 September 2007

Abstract

Context.The standard unified scheme of active galactic nuclei requires the presence of high column densities of gas and dust potentially obscuring the central engine. So far, few direct subarcsecond resolution studies of this material have been performed toward radio galaxies.

Aims.The goal of this paper is to elucidate the nuclear environment of the prototypical X-shaped Fanaroff-Riley type II radio galaxy 3C 403, the only powerful radio galaxy known to host an H₂O megamaser.

Methods.Very Large Array A-array and single-dish Green Bank and Effelsberg 1.3 cm measurements were performed to locate and monitor the water maser emission. Very Long Baseline Interferometry 6 cm continuum observations were taken to analyze the spatial structure of the nuclear environment at even smaller scales, while the CO J = 1–0 and 2–1 transitions were observed with the IRAM 30-m telescope to search for thermal emission from a spatially extended, moderately dense gas component.

Results.Positions of the H₂O maser features and the continuum emission from the core coincide within 5 mas (5.5 pc). Intensities of the two main maser components with (isotropic) luminosities sometimes surpassing 1000  appear to be anti-correlated, with typical timescales for strong variations of one year. If the variations are intrinsic to the cloud(s), the implied angular source size would be 0.3 mas and the brightness temperature 510 K. The VLBI continuum observations support a scenario where a nuclear core, represented by the dominant central radio continuum component, is accompanied by a jet and counterjet, directed toward the western and eastern large scale lobes of the galaxy. CO remains undetected, providing a maximum scale size of ~50 pc (500 K/T_b)^1/2, with T_b denoting the brightness temperature of the CO J = 1–0 line. Possible scenarios that could produce the observed maser emission are outlined. Adopting a mass of several 10⁸ for the nuclear engine, the observed maser features can only be interpreted in terms of an accretion disk as in NGC 4258, if they solely represent the systemic velocity components. The receding and approaching parts of the putative maser disk are, however, not seen and a secular velocity drift of the observed features is not (yet) apparent. Most likely, the two main maser components mark shocked molecular gas interacting with the nuclear jets. The X-shaped morphology of the radio galaxy may point at a binary nuclear engine. This possibility, greatly complicating the nuclear environment of 3C 403, should motivate a number of worthwhile follow-up studies.