The Ionization Fraction in the Obscuring ``Torus'' of an Active Galactic Nucleus
Abstract
The LINER galaxy NGC 2639 contains a water vapor megamaser, suggesting the presence of a nuclear accretion disk or torus viewed close to edge-on. 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 ergs 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 n2edl/nHdl 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, and Conger for X-ray heated molecular gas. The two values may be reconciled if the molecular gas is very dense: nH2>~109 cm-3. The measured ionization fraction is also consistent with the idea that both absorptions occur in a hot (~6000 K), weakly ionized (ionization fraction a few times 10-2) atomic region that may coexist 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 1998
- DOI:
- 10.1086/306210
- arXiv:
- arXiv:astro-ph/9805279
- Bibcode:
- 1998ApJ...505..587W
- Keywords:
-
- ACCRETION;
- ACCRETION DISKS;
- GALAXIES: ACTIVE;
- GALAXIES: INDIVIDUAL: NGC NUMBER: NGC 2639;
- GALAXIES: NUCLEI;
- GALAXIES: SEYFERT;
- RADIO CONTINUUM: GALAXIES;
- Accretion;
- Accretion Disks;
- Galaxies: Active;
- Galaxies: Individual: NGC Number: NGC 2639;
- Galaxies: Nuclei;
- Galaxies: Seyfert;
- Radio Continuum: Galaxies;
- Astrophysics
- E-Print:
- doi:10.1086/306210