Broadband Spectral Modeling of the Extreme Gigahertz-Peaked Spectrum Radio Source PKS B0008-421

We present broadband observations and spectral modeling of PKS B0008-421, and it as an extreme gigahertz-peaked spectrum (GPS) source. PKS B0008-421 characterized by the steepest known spectral slope below the turnover, close the theoretical limit of synchrotron self-absorption, and the smallest known width of any GPS source. Spectral coverage of the source spans from0.118 to 22 GHz, which includes data from the Murchison Widefield Array and the bandpass receivers on the Australia Telescope Compact Array. We have a Bayesian inference model fitting routine to fit the data with absorption models. We find that without the inclusion of a-frequency exponential break the absorption models can not accurately fit data, with significant deviations above and below the peak in the radio. The addition of a high-frequency break provides acceptable spectral for the inhomogeneous free-free absorption and double-component self-absorption models, with the inhomogeneous free-free absorption statistically favored. The requirement of a high-frequency spectral break that the source has ceased injecting fresh particles. Additional for the inhomogeneous free-free absorption model as being responsible the turnover in the spectrum is given by the consistency between the parameters derived from the model fit and the implications of the spectral break, such as the necessity of the source being by a dense ambient medium to maintain the peak frequency near the region. The discovery of PKS B0008-421 suggests that the next of low radio frequency surveys could reveal a large population of sources that have ceased activity, and that a portion of the ultra-steep source population could be composed of these GPS sources in a relic.