Investigating X-ray emission in the GeV-emitting compact symmetric objects PKS 1718–649 and TXS 1146+596

¹ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
² INAF, Astrophysics and Space Science Observatory Bologna, Via P. Gobetti 93/3, 40129 Bologna, Italy
e-mail: ettore.bronzini@inaf.it
³ INAF, Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
⁶ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁷ INFN, Sezione di Trieste, 34127 Trieste, Italy
⁸ National Tsing Hua University, Kuang-Fu Road, Hsinchu 30013, Taiwan, ROC

Received: 9 October 2023
Accepted: 29 January 2024

Abstract

Aims. Compact symmetric objects (CSOs) are thought to represent the first step in the evolutionary path of radio galaxies. In the present study, we investigated the X-ray emission of two CSOs confirmed to emit at GeV energies: PKS 1718–649 and TXS 1146+596. Unveiling the origin of their observed high-energy emission is crucial to establishing the physical parameters of the radio source and understanding how CSOs interact with the surrounding medium.

Methods. We combined archival and new NuSTAR observations of PKS 1718–649 and TXS 1146+596 to have broadband X-ray coverage. For both sources, we modeled the broadband spectral energy distribution (SED) from the radio band up to γ-rays in order to derive their physical parameters. We also discuss the role of the ambient medium in confining the source expansion, which we investigate using X-ray obscuration.

Results. For the first time, we report X-ray detections of PKS 1718–649 and 1146+596 with NuSTAR at energies higher than 10 keV. Combining Chandra and NuSTAR observations of TXS 1146+596, we reveal the presence of a multitemperature thermal component dominating the soft X-ray spectrum, and we interpret this finding as indicative of an AGN feedback process in action in this source. In addition, we show that two emitting electron populations are necessary to reproduce the observed broadband SED of TXS 1146+596: in our models, the X-ray emission could either be produced by synchrotron radiation or by a weak X-ray corona, or could be an ADAF-type emission. Interestingly, an additional X-ray component, namely a weak corona, is also required for PKS 1718–649. Moreover, we argue that heavily obscured and possibly frustrated sources tend to show different radio sizes with respect to those that are unobscured and free to expand.