Speaker
Description
Radio relics play a pivotal role in unravelling the intricate dynamics of galaxy cluster mergers, serving as key indicators of shock waves and particle acceleration processes. While extensively studied in the radio band, the investigation of radio relics at ultra-low frequencies (<100 MHz) remains limited due to technical challenges, such as ionospheric disturbances and high sky temperature (resulting in observation with low signal-to-noise ratios).
In this talk, I will present two compelling cases of radio relics observed below 100 MHz with LOFAR LBA data. These observations are crucial for studying the emission from low-energy populations of cosmic rays in disturbed environments and complex morphologies of extended sources like relics.
Firstly, I delve into the observations of Abell 746, a complex merging galaxy cluster exhibiting a rich variety of radio structures.
The multi-frequency approach, including radio observations from 50 MHz to 1.4 GHz, alongside X-ray data from the XMM-Newton satellite, unveils a morphology with several substructures and discontinuities. In particular, thanks to radio low-frequency observations at 53 and 144 MHz we detect a double relic with filamentary structures and two more additional relic-like structures. The large radio frequency coverage allowed to perform a detailed spectral index study via spectral index maps and profiles to unveil the nature of diffuse sources in this system.
I will discuss a scenario involving multiple mergers and a double relic system. In this scenario, the shock creating the northwest relic opposes the shock associated with the southern relic. Additionally, I will examine a challenging third western relic, which may also be linked to this double merger.
Secondly, I will present preliminary results of one of the most studied systems hosting relics in the northern sky, the Sausage galaxy cluster, observed for the first time with LOFAR at 50 MHz. Moreover, I will also show the challenges encountered during the calibration process of such a target, in the proximity of the extremely bright radio source Cassiopeia A and the ultra-low-frequency regime.
Through these case studies, we emphasize the significance of ultra-low frequency observations in advancing our understanding of radio relics and their role in shaping the large-scale structure of galaxy clusters.
