Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Ionization Fraction in the Obscuring `Torus' of an Active Galactic
  Nucleus   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.

28
The categories of this paper are: alg-geom
29
The categories of this paper are: funct-an
14
The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
3
The categories of this paper are: physics
18
The categories of this paper are: econ
11
The categories of this paper are: hep-lat
15
The categories of this paper are: nucl-ex
9
The categories of this paper are: nlin
19
The categories of this paper are: eess
4
The categories of this paper are: cond-mat
22
The categories of this paper are: chao-dyn
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th