Triple-Double Radio Galaxies: Clues for understanding recurrent AGN activity

Chavan, Kshitij; Dabhade, Pratik; Saikia, D. J.
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42nd meeting of the Astronomical Society of India (ASI

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Using LOFAR Two-metre Sky Survey (LoTSS), we discovered three cycles of episodic jet activity (J1225+4011, Chavan et al. 2023, MNRAS) in a giant radio galaxy, which is an extremely rare class of restarted Active Galactic Nuclei (AGN), making it only fourth of its kind. It is also called 'Triple-Double Radio Galaxy' (TDRG). J1225+4011 extends up to 1.35 mpc in projected size, with the second and the most recent episode extending up to 572 and 118 kpc, respectively. Studying TDRGs is essential to gain insights into recurrent activity by AGN, which helps us understand the evolution of host galaxies, their environment, and heat flow processes in the intracluster medium. We measure a steep spectral index for the outer lobes using available data, proving that it is an earlier or older activity. A comparison of the radio properties of all TDRGs is made for the first time to understand the evolution of these TDRGs. This comparison revealed that the sense of asymmetry is in the opposite sense for inner doubles in two of the four sources. We also updated the size of previously known TDRG-J0929+4146 using LoTSS data, which unveiled its larger extent and making it the largest TDRG. Studying the PD diagram for all episodes in known TDRGs suggests a new trend, requiring further studies. Propagation of jets in relic lobes is not yet well understood. Hence, we have started a systematic observational program with uGMRT, LOFAR and JVLA to study their spectral ageing properties from 50 MHz to 10 GHz. Full-time has been already awarded for uGMRT observation to study TDRG J1225+4011. We will discuss our studies on TDRGs, providing insights into duty cycles, the propagation of the jets in the cocoon created by the relic jet, and physical properties of these sources.