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Constraints on the Parsec-Scale Environment in NGC 1275
Levinson, A.; Laor, A.; Vermeulen, R. C.
Astrophysical Journal v.448, p.589 (ApJ Homepage)
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Recent VLBI observations of 3C 84 (NGC 1275) by Vermeulen, Readhead, & Backer and Walker, Romney, & Benson reveal a northern feature which is interpreted as a receding milliarcsecond counterjet and strongly suggest that the counterjet spectrum is severely attenuated at low frequencies (below 8 GHz).

It is argued that the inverted counterjet spectrum is most likely due to free-free absorption by some gas surrounding the central active galactic nucleus (AGN). Different physical models for free-free absorption are considered, and constraints on their parameters are derived, using available radio, optical, UV, and X-ray data.

We first consider absorption by gas in a disk aligned at roughly a right angle to the radio jet axis. We show that the heating rate attainable through viscous dissipation in an accretion disk is substantially smaller than the heating rate required to balance line cooling and sustain the gas in the disk in an ionized state. We conclude that some external heating source is needed. A plausible ionizing source is the central continuum radiation. The photoionizing flux required is determined uniquely by the free-free optical depth. Two distinct sources of ionizing photons are considered: direct illumination by the central source and Thomson scattering of the nuclear radiation by some material surrounding the disk. Direct illumination may occur if the disk is warped or flared and, in the case of a flared disk, requires the mass of the black hole to be a few times 106 Msun and the luminosity of the central source to be at about the Eddington limit. In the case of illumination by Thomson-scattered radiation, the intrinsic continuum luminosity must exceed the observed, implying either intrinsic extinction of E(B - V) ≳ 0.2 or nonisotropic emission. The dominant coolant of the disk is the [O III] lambdalambda4959, 5007 doublet, and the [O III] line flux predicted by these photoionization models is in good agreement with the observed flux.

The possibility that the free-free absorbing gas is distributed spherically around the central source is also examined. We find that such a model is in conflict with the observations. Finally, we consider free-free absorption in a torus-like distribution of matter and derive constraints on the mean density and geometry of the torus.

We predict that if the free-free absorption interpretation is true, then (1) the core [O III] emission should remain spatially unresolved in Hubble Space Telescope (HST) observations, (2) variations in the central AGN UV continuum should be followed by corresponding variations in the [O III] flux with a time lag of ˜10 yr, and (3) the free-free optical depth should track with no time-delay variations in the [O III] flux, as both originate from the same region.

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