Estimates of the Free‐Free Optical Depth toward the Subparsec‐Scale Radio Source in Centaurus A

Abstract

Multifrequency, near-simultaneous VLBA observations have been used to achieve matched resolution images of the inner 8 pc of Centaurus A, including the jet, the nucleus, and the counterjet, at a spatial resolution of approximately 0.1 pc. By comparing the three images, at 2.2, 5.0, and 8.4 GHz, and assuming a constant intrinsic spectral index between 2.2 and 8.4 GHz along each line of sight, it is possible, in principle, to separate the effects of synchrotron self-absorption intrinsic to the radio source from free-free absorption, which is extrinsic. Toward the unresolved Centaurus A nucleus, the observed spectral index between 2.2 and 5.0 GHz is inverted to an extent where free-free absorption is unavoidable, α = 3.8 ±, at its peak. Toward this region the optical depth to free-free absorption is estimated to be τff = 0.9 ± 0.4 at 2.2 GHz, giving an intrinsic spectral index of α = 2.0 ± 0.5, within the bounds explainable by synchrotron self-absorption. Away from the nucleus the remainder of the bright subparsec-scale radio jet has a spectral index close to -0.7 and is not affected by free-free absorption. We adopt a simple spherical geometry for the nuclear absorber with an upper limit on its radius of 0.016 pc, giving a constraint on the electron density and temperature, nT 9.3, where ne4 is the electron density in units of 104 cm-3 and T4 is the electron temperature in units of 104 K. Assuming T4 = 1 gives a lower limit for the total mass of the absorber of 0.01 M☉. Future observations of higher resolution and sensitivity will be required to more accurately constrain the properties of the nuclear absorber and detect any absorption toward the counterjet.