Estimates of the free-free optical depth toward the subparsec-scale radio source in Centaurus A

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, alpha = 3.8 +/- (0.6)(0.5), at its peak. Toward this region the optical depth to free-free absorption is estimated to be tau (ff) = 0.90 +/- 0.4 at 2.2 GHz, giving an intrinsic spectral index of alpha = 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, where is the electron density in units of 10(4) cm(-3) and T-4 is the electron temperature in units of 10(4) K. Assuming T-4 = 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.