The ionization fraction in the obscuring "torus" of an active galactic nucleus

A. S. Wilson, A. L. Roy, J. S. Ulvestad, E. J.M. Colbert, K. A. Weaver, J. A. Braatz, C. Henkel, M. Matsuoka, S. Xue, N. Iyomoto, K. Okada

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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 ∫ ne2 dl/∫ nH 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, 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.

Original languageEnglish
Pages (from-to)587-593
Number of pages7
JournalAstrophysical Journal
Volume505
Issue number2 PART I
DOIs
Publication statusPublished - Jan 1 1998

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All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Wilson, A. S., Roy, A. L., Ulvestad, J. S., Colbert, E. J. M., Weaver, K. A., Braatz, J. A., ... Okada, K. (1998). The ionization fraction in the obscuring "torus" of an active galactic nucleus. Astrophysical Journal, 505(2 PART I), 587-593. https://doi.org/10.1086/306210