Radio relic and the diffuse emission trail discovered in low-mass galaxy cluster Abell 1697

¹ Department of Physics, Savitribai Phule Pune University, Pune 411007, India
e-mail: surajit@physics.unipune.ac.in
² National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Post Bag 3, S. P. Pune University Campus, Ganeshkhind, Pune 411007, Maharashtra, India
³ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁴ Inter University Centre for Astronomy and Astrophysics, Pune 411007, India

Received: 28 August 2019
Accepted: 31 October 2019

Abstract

We report the discovery of a putative radio relic, 830 kpc in length, and found towards the outskirts of galaxy cluster Abell 1697 (z = 0.181), using the LOFAR Two Meter Sky Survey at 144 MHz. An X-ray-inferred mass of M₅₀₀^X-ray = 2.9_−0.7^+0.8 × 10¹⁴ M_⊙ places Abell 1697 among the least massive relic hosts. The relic is also detected at 325 MHz in the Westerbork Northern Sky Survey and at 1.4 GHz in the NRAO VLA Sky Survey, with an average spectral index of α(144, 325, 1400 MHz) = − 0.98 ± 0.01, and magnetic field of B_eq ∼ 0.6 μG. This relic, located in the north-east periphery of the cluster, is 300 kpc wide, exhibits a gradual spectral steepening across the width (α_{144 MHz}^{1.4 GHz}(inj) = −0.70 ± 0.11 to α_{144 MHz}^{1.4 GHz}(edge) = −1.19 ± 0.15), as well as indications of a co-spatial X-ray (ROSAT) shock and the radio relic emission. The radio power of the relic is P_1.4 GHz = 8.5 ± 1.1 × 10²³ W Hz⁻¹, which is found to be in good agreement with the expected empirical correlation between the radio power and largest linear size of relics. The relic is trailed by extended (790 × 550 kpc) diffuse radio emission towards the cluster centre, which is likely an ultra-steep spectrum (α_{144 MHz}^{1.4 GHz} < −1.84) radio source. This structure is also found to be older by at least 190 Myr, has a very low surface brightness of 0.3 μJy arcsec⁻² and magnetic field B_eq ∼ 0.8 μG, similar to that of a radio phoenix. Finally, we discuss the possible mechanisms responsible for the relic and the trailing diffuse radio emission, invoking re-acceleration due to wake turbulence, as well as the revival of fossil electrons in the remnant radio lobes of active galactic nuclei by the cluster merger shocks.