TY - JOUR
T1 - The ionization fraction in the obscuring "torus" of an active galactic nucleus
AU - Wilson, A. S.
AU - Roy, A. L.
AU - Ulvestad, J. S.
AU - Colbert, E. J.M.
AU - Weaver, K. A.
AU - Braatz, J. A.
AU - Henkel, C.
AU - Matsuoka, M.
AU - Xue, S.
AU - Iyomoto, N.
AU - Okada, K.
N1 - Copyright:
Copyright 2015 Elsevier B.V., All rights reserved.
PY - 1998
Y1 - 1998
N2 - 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.
AB - 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.
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U2 - 10.1086/306210
DO - 10.1086/306210
M3 - Article
AN - SCOPUS:22444456374
VL - 505
SP - 587
EP - 593
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 2 PART I
ER -