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We present broadband observations and spectral modeling of PKS B0008-421, andidentify it as an extreme gigahertz-peaked spectrum (GPS) source. PKS B0008-421is characterized by the steepest known spectral slope below the turnover, closeto the theoretical limit of synchrotron self-absorption, and the smallest knownspectral 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 thewide bandpass receivers on the Australia Telescope Compact Array. We haveimplemented a Bayesian inference model fitting routine to fit the data withvarious absorption models. We find that without the inclusion of ahigh-frequency exponential break the absorption models can not accurately fitthe data, with significant deviations above and below the peak in the radiospectrum. The addition of a high-frequency break provides acceptable spectralfits for the inhomogeneous free-free absorption and double-componentsynchrotron self-absorption models, with the inhomogeneous free-free absorptionmodel statistically favored. The requirement of a high-frequency spectral breakimplies that the source has ceased injecting fresh particles. Additionalsupport for the inhomogeneous free-free absorption model as being responsiblefor the turnover in the spectrum is given by the consistency between thephysical parameters derived from the model fit and the implications of theexponential spectral break, such as the necessity of the source beingsurrounded by a dense ambient medium to maintain the peak frequency near thegigahertz region. The discovery of PKS B0008-421 suggests that the nextgeneration of low radio frequency surveys could reveal a large population ofGPS sources that have ceased activity, and that a portion of the ultra-steepspectrum source population could be composed of these GPS sources in a relicphase.

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