NuSTAR observations of water megamaser AGN

¹ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: alberto.masini4@unibo.it
² Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
³ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁴ New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁵ Center for Cosmology and Particle Physics, Department of Physics, New York University – Affiliate Member, 4 Washington Place, New York, NY 10003, USA
⁶ ASDC-ASI, via del Politecnico, 00133 Roma, Italy
⁷ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
⁸ Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA
⁹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, 306 Santiago 22, Chile
¹⁰ Millennium Institute of Astrophysics, MAS, Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago de Chile
¹¹ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
¹² EMBIGGEN Anillo, Concepción, Chile
¹³ Space Science Laboratory, University of California, Berkeley, CA 94720, USA
¹⁴ Department of Astronomy and Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
¹⁵ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹⁶ Department of Physics, 104 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
¹⁷ DTU Space National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Lyngby, Denmark
¹⁸ Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
¹⁹ Centre for Extragalactic Astronomy, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
²⁰ School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
²¹ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
²² Institute for Astronomy, Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
²³ Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
²⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²⁵ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²⁶ SNSF Ambizione Postdoctoral Fellow  

Received: 3 November 2015
Accepted: 9 February 2016

Abstract

Aims. We study the connection between the masing disk and obscuring torus in Seyfert 2 galaxies.

Methods. We present a uniform X-ray spectral analysis of the high energy properties of 14 nearby megamaser active galactic nuclei observed by NuSTAR. We use a simple analytical model to localize the maser disk and understand its connection with the torus by combining NuSTAR spectral parameters with the available physical quantities from VLBI mapping.

Results. Most of the sources that we analyzed are heavily obscured, showing a column density in excess of ~10²³ cm^-2; in particular, 79% are Compton-thick (N_H > 1.5 × 10²⁴ cm^-2). When using column densities measured by NuSTAR with the assumption that the torus is the extension of the maser disk, and further assuming a reasonable density profile, we can predict the torus dimensions. They are found to be consistent with mid-IR interferometry parsec-scale observations of Circinus and NGC 1068. In this picture, the maser disk is intimately connected to the inner part of the torus. It is probably made of a large number of molecular clouds that connect the torus and the outer part of the accretion disk, giving rise to a thin disk rotating in most cases in Keplerian or sub-Keplerian motion. This toy model explains the established close connection between water megamaser emission and nuclear obscuration as a geometric effect.