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Lobe-dominated gamma-ray Emission of Compact Symmetric Objects

The $γ$-ray emitting compact symmetric objects (CSOs) PKS 1718--649, NGC 3894, and TXS 0128+554 are lobe-dominated in the radio emission. In order to investigate their $γ$-ray radiation properties, we analyze the $\sim$14-yr Fermi/LAT observation data of the three CSOs. They all show the low luminosity ($10^{41}-10^{43}$ erg s$^{-1}$) and no significant variability in the $γ$-ray band. Their $γ$-ray average spectra can be well fitted by a power-law function. These properties of $γ$-rays are clearly different from the $γ$-ray emitting CSOs CTD 135 and PKS 1413+135, for which the $γ$-rays are produced by a restarted aligned jet. In the $L_γ-Γ_γ$ plane, the three CSOs are also located at the region occupied by radio galaxies (RGs) while CTD 135 and PKS 1413+135 display the similar feature to blazars. Together with the similar radio emission property to $γ$-ray emitting RGs Cen A and Fornax A, we speculate that the $γ$-rays of the three CSOs stem from their extended mini-lobes. The broadband spectral energy distributions of the three CSOs can be well explained by the two-zone leptonic model, where their $γ$-rays are produced by the inverse Compton process of the relativistic electrons in extended region. By extrapolating the observed Fermi/LAT spectra to the very high energy band, we find that TXS 0128+554 among the three CSOs may be detected by the Cherenkov Telescope Array in future.