Radio images of NGC 6086

To these, we add two Very Large Array archival observations at higher frequencies (1400 and 4700MHz). In the new observations in the frequency range 144-675MHz, we detect a second pair of larger lobes and three regions within the remnant emission with a filamentary morphology. We analysed the spectral index trend in the inner remnant lobes and see systematically steeper values {alpha}_low_~1.1-1.3) at the lower frequencies compared to the gigahertz frequencies ({alpha}_high_~0.8-0.9).

Steeper spectral indices are found in the newly detected outer lobes (up to {alpha}_outer_~2.1), as expected if they trace a previous phase of activity of the AGN. However, the differences between the spectra of the two outer lobes suggest different dynamical evolutions within the intra-group medium during their expansion and/or different magnetic field values. Using a single-injection radiative model and assuming equipartition conditions, we place constraints on the age of the inner and outer lobes and derive the duty cycle of the source.

We estimate that the duration of the two active phases was 45Myr and 18Myr and the duration of the two inactive phases was 66Myr and 33Myr. This results in a total active time of t_on_~39%. The filamentary structures have a steep spectral index (~1) without any spectral index trend, and only one of them shows a steepening in the spectrum.

Their origin is not yet clear, but they may have formed due to the compression of the plasma or due to magnetic field substructures.