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

Abstract

In this paper, we report VLBA radio maps at three frequencies and an ASCA X-ray spectrum of the water megamaser galaxy NGC 2639, which is believed to contain an edge-on viewed accretion disk. The radio observations reveal a compact ($<$ 0.2 pc) nuclear source with a spectrum that turns over sharply near 5 GHz. 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 $\simeq$ 5 $\times$ 10${}^{23}$ 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 $\int n_e^{2}dl/\int n_{H}dl$ may be determined. If the masing molecular gas is responsible for both absorptions, the required ionization fraction is $\gtrsim 1.3\times 10^{-5}$, 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 -- $n_{H_2}\gtrsim 10^9$ cm${}^{-3}$. The measured ionization fraction is also consistent with the idea that both absorptions occur in a hot ($\sim$ 6,000K), 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 $\sim$ 1 pc from the nucleus. 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.

References in corpus (4)

• The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars II. Final Results
• The Distribution of H2O Maser Emission in the Nucleus of NGC 4945
• Water maser emission and the parsec-scale jet in NGC 3079
• A direct image of the obscuring disk surrounding an active galactic nucleus

Cited by in corpus (21)

• A 3 dimensional diagnostic diagram for Seyfert 2s: probing X-ray absorption and Compton thickness
• Radio Continuum Survey of an Optically-Selected Sample of Nearby Seyfert Galaxies
• A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
• The Parsec-scale Radio Structure of NGC 1068 and the Nature of the Nuclear Radio Source
• X-ray Properties of LINERs and Low-luminosity Seyfert Galaxies Observed with ASCA. I. Observations and Results
• The radio structure of radio-quiet quasars
• Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI
• Sub-Relativistic Radio Jets and Parsec-Scale Absorption in Two Seyfert Galaxies
• Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies
• Extragalactic H_2O masers and X-ray absorbing column densities
• Hard X-ray Emission and the Ionizing Source in LINERs
• What drives the radio slopes in radio quiet quasars?
• Parsec-Scale Images of Flat-Spectrum Radio Sources in Seyfert Galaxies
• VLBA Identification of the Milliarcsecond Active Nucleus in the Seyfert Galaxy NGC 4151
• Discovery of Water Maser Emission in Five AGN and a Possible Correlation Between Water Maser and Nuclear 2-10 keV Luminosities
• Simultaneous Chandra and RXTE observations of the nearby bright Seyfert 2 galaxy NGC4945
• The Size of the Radio-Emitting Region in Low-luminosity Active Galactic Nuclei
• Milliarcsec-scale radio structure of a matched sample of Seyfert 1 and Seyfert 2 galaxies
• On the nuclear obscuration of H2O maser galaxies
• Connecting X-ray absorption and 21cm neutral hydrogen absorption in obscured radio AGN
• The Nature of Jets in Double-Peaked Emission Line AGN in the KISSR Sample