Context : The majority of bright extragalactic \gamma -ray sources are blazars .
Only a few radio galaxies have been detected by Fermi /LAT .
Recently , the GHz-peaked spectrum source PKS 1718–649 was confirmed to be \gamma -ray bright , providing further evidence for the existence of a population of \gamma -ray loud , compact radio galaxies .
A spectral turnover in the radio spectrum in the MHz to GHz range is a characteristic feature of these objects , which are thought to be young due to their small linear sizes .
The multiwavelength properties of the \gamma -ray source PMN J1603–4904 suggest that it is a member of this source class .

Aims : The known radio spectrum of PMN J1603–4904 can be described by a power law above 1 GHz .
Using observations from the Giant Metrewave Radio Telescope ( GMRT ) at 150 , 325 , and 610 MHz , we investigate the behavior of the spectrum at lower frequencies to search for a low-frequency turnover .

Methods : Data from the TIFR GMRT Sky Survey ( TGSS ADR ) catalog and archival GMRT observations were used to construct the first MHz to GHz spectrum of PMN J1603–4904 .

Results : We detect a low-frequency turnover of the spectrum and measure the peak position at about 490 MHz ( rest-frame ) , which , using the known relation of peak frequency and linear size , translates into a maximum linear source size of \sim 1.4 kpc .

Conclusions : The detection of the MHz peak indicates that PMN J1603–4904 is part of this population of radio galaxies with turnover frequencies in the MHz to GHz regime .
Therefore it can be considered the second confirmed object of this kind detected in \gamma -rays .
Establishing this \gamma -ray source class will help to investigate the \gamma -ray production sites and to test broadband emission models .