A New Enigmatic Radio Relic in the Low-mass Cluster Abell 2108

Abstract

We report the disco v ery ofã radio relic in the north-eastern periphery of the galaxy cluster Abell 2108 (A2108). A2108 is part of the uGMRT LOw-MAss Galaxy Cluster Surv e y (GLOMACS), where our mainãim is to search for diffuse radio emission signatures in very sparsely explored low-mass galaxy clusters using uGMRT band-3 (central frequency 400 MHz). We used our uGMRT band-3 observationsãlong with the existingãrchival band-3 uGMRT data to impro v e image sensitivity. Along with the previously reported south-western relic, the disco v ery of the new relic makes A2108 one of the few low-mass clusters hostingã double relic. The new relic spans o v erã re gion of 610 kpc ×310 kpcãnd, interestingly, differs considerably in sizeãnd morphology from the other relic. Using XMM-Newton scienceãrchive data, weãlso report the tentative detection ofã mildly supersonic shock of Mach number M SB = 1 . 42ãnd M T = 1 . 43 from the surface brightnessãnd temperature discontinuities, respecti vely, near this ne wly found relic. Both the relics in A2108ãre found to be significantly under-luminous compared with other double-relic systems in the mass-luminosity plane. Moreo v er, the north-eastern relic is remarkably under-luminous in the size-luminosity plane. Although mild supersonic shocks resulting fromãn off-axis merger could have influenced the origin of both relics, we hypothesize that local environments have playedã crucial role in shaping their morphologies. © 2024 The Author(s).