STScI Preprint #1253

The Ionization Fraction in the Obscuring `Torus' of An Active Galactic Nucleus

The LINER galaxy NGC 2639 contains a water vapor megamaser, suggesting the presence of an edge-on nuclear accretion disk or torus. This galaxy is thus a good candidate for revealing absorption by the torus of any compact nuclear continuum emission. In this paper, we report VLBA radio maps at three frequencies and an ASCA X-ray spectrum obtained to search for free-free and photoelectric absorptions, respectively. The radio observations reveal a compact (< 0.2 pc) nuclear source with a spectrum that turns over sharply near 5 GHz. This turnover may reflect either synchrotron self-absorption or free-free absorption. The galaxy is detected by ASCA with an observed luminosity of 1.4 × 10⁴¹ erg s^-1 in the 0.6-10 keV band. The X-ray spectrum shows emission in excess of a power-law model at energies greater than 4 keV; we interpret this excess as compact, nuclear, hard X-ray emission with the lower energies photoelectrically absorbed by an equivalent hydrogen column of 5×10²³ cm^-2. If we assume that the turnover in the radio spectrum is caused by free-free absorption and that both the free-free and photoelectric absorptions are produced by the same gaseous component, the ratio n_e² dl/ n_H dl may be determined. If the masing molecular gas is responsible for both absorptions, the required ionization fraction is 1.3 × 10^-5, which is comparable to the theoretical upper limit derived by Neufeld, Maloney & Conger (1994) for X-ray heated molecular gas. The two values may be reconciled if the molecular gas is very dense n_H2 10⁹ cm^-3. The measured ionization fraction is also consistent with the idea that both absorptions occur in a hot (~6,000 K), weakly ionized (ionization fraction a few times 10^-2) atomic region that may co-exist with the warm molecular gas. If this is the case, the absorbing gas is ~1 pc from the nucleus. We rule out the possibility that both absorptions occur in a fully ionized gas near 10⁴ K. If our line of sight passes through more than one phase, the atomic gas probably dominates the free-free absorption while the molecular gas may dominate the photoelectric absorption.

Status:

Appeared in: The Astrophysical Journal, 505:587-593, 1998

Affiliations:

¹) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
²) National Radio Astronomy Observatory, P. O. Box 0, Socorro, NM 87801
³) Mail Code 662, Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MD 20771
⁴) The Johns Hopkins University, Department of Physics and Astronomy, Baltimore, MD 21218
⁵) National Radio Astronomy Observatory, P. O. Box 2, Green Bank, WV 24944
⁶) Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁷) The Institute of Physical and Chemical Research (RIKEN), 2-1, Hirosawa, Wako-shi, Saitama 351-01, Japan
⁸) Department of Physics, University of Tokyo, 3-1, Hongo 7-chome, Bunkyo-ku, Tokyo 113, Japan
⁹) Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara, Kanagawa 229, Japan
¹⁰) Also Astronomy Department, University of Maryland, College Park, MD 20742