Lobe-dominated γ-Ray Emission of Compact Symmetric Objects

Abstract

The $\gamma$-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 $\gamma$-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 $\gamma$-ray band. Their $\gamma$-ray average spectra can be well fitted by a power-law function. These properties of $\gamma$-rays are clearly different from the $\gamma$-ray emitting CSOs CTD 135 and PKS 1413+135, for which the $\gamma$-rays are produced by a restarted aligned jet. In the $L_{\gamma}-\Gamma_{\gamma}$ 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 $\gamma$-ray emitting RGs Cen A and Fornax A, we speculate that the $\gamma$-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 $\gamma$-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.

Study of γ-Ray Emission from a Compact Radio Galaxy with the Fermi Large Area Telescope

“…Based on the broadband SED fitting, the γ-rays from three "bona fide" compact symmetric objects (CSOs; PKS 1718-649, NGC 3894, and TXS 0128+554) are suggested to be lobedominated (Sobolewska et al 2022;Gan et al 2024; see also Migliori et al 2016;Principe et al 2020), while the γ-rays from the two "fake" CSOs (CTD 135 and PKS 1413+135; Gan et al 2021Gan et al , 2022; see also Principe et al 2021) and seven CSSs (Zhang et al 2020;Gu et al 2022) are core-jet dominated similar to blazars. Due to limited observational data availability, it is impractical to investigate the origin of γ-rays in PKS 1007+142 using broadband SED fitting based on radiation models, as even a simple one-zone synchrotron-self-Compton homogeneous model requires seven independent parameters (Tavecchio et al 1998).…”

“…In order to further reveal the γ-ray origins of PKS 1007 +142, we compare it with other γ-ray-emitting AGNs in the L γ − Γ γ plane (Figure 5), where L γ is the γ-ray luminosity. This AGN sample consists of 5 CSOs (Migliori et al 2016;Lister et al 2020;Principe et al 2020;Gan et al 2021Gan et al , 2022Gan et al , 2024Principe et al 2021), 7 CSSs (Zhang et al 2020;Gu et al 2022), together with 18 radio galaxies and numerous blazars (from Abdollahi et al 2022); blazars includes flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs). According to the criteria of "bona fide" CSOs (Kiehlmann et al 2024), CTD 135 and PKS 1413+135 should be classified as blazar-types (see also Frey et al 2022;Peirson et al 2022).…”

“…It is well known that γ-rays of blazars are predominantly produced by their core jets and are strongly amplified due to the Doppler boosting effect, while their radio radiations originate from more extended regions. These nonblazars have lower ratios of L γ to L 1.4 GHz due to the weakly relativistic effect or the lobe-dominated γ-ray emission (e.g., Stawarz & Petrosian 2008;Abdo et al 2010a;Ackermann et al 2016;Migliori et al 2016;Principe et al 2020Principe et al , 2021Gan et al 2024;Yu et al 2024). Although it has been confirmed that radio galaxies Cen A, Fornax A, and NGC 6251 are lobe-dominated γ-ray-emitting AGNs (Abdo et al 2010a;Ackermann et al 2016;Yu et al 2024), in general, the origin of γ-rays in radio galaxies is still believed to be from the core-jet (Abdo et al 2009a(Abdo et al , 2009bAleksić et al 2014;Fukazawa et al 2015;Xue et al 2017).…”

“…Up to now, about a dozen γ-ray-emitting CRSs have been identified using the Fermi Large Area Telescope (Fermi-LAT) observations (e.g., Migliori et al 2016;Abdollahi et al 2020;Lister et al 2020;Principe et al 2020;Zhang et al 2020;Gan et al 2021Gan et al , 2022Gan et al , 2024Principe et al 2021;Gu et al 2022). However, the origin of their γ-ray emission is not so clear.…”

“…However, the origin of their γ-ray emission is not so clear. It is speculated that the γ-ray emission of the quasar-type CRSs originates from the jet in the core region, while the γ-ray emission of galaxy-type CRSs is relatively weak and insignificant variation, and it is dominated by the emission from the extended radio lobes (e.g., Stawarz & Petrosian 2008;Migliori et al 2013;Principe et al 2021;Gan et al 2024). The radiation mechanism of γ-rays for these CRSs is normally the inverse Compton scattering of the relativistic electrons in the core-jet or extended lobes (e.g., Stawarz & Petrosian 2008;Migliori et al 2013;Zhang et al 2020;Gan et al 2022;Gu et al 2022).…”