Constraining the AGN duty cycle in the cool-core cluster MS 0735.6+7421 with LOFAR data

Abstract

Context. MS 0735.6+7421 is a galaxy cluster which hosts a central radio galaxy with a very steep spectrum, produced by one of the most powerful known jetted active galactic nuclei (AGN). The radio plasma, ejected at nearly light speed from the central AGN, have displaced the intra-cluster medium, leaving two pairs of cavities observable in the X-ray, associated to two different outbursts, and have distributed energy to the surrounding medium. While the age of the cavities has previously been estimated from the X-rays, no confirmation from the radio data is available. Furthermore, to-date the radio spectrum has only been derived from integrated flux density measurements, without providing the spatial distribution which could help us to understand the nature of this source. Aims. The aim of this work is to perform for the first time a detailed, high-resolution spectral study of the source at radio frequencies and investigate its duty cycle to be compared with previous X-ray estimates. Methods. We use new observations at 144 MHz produced with the LOw Frequency ARray (LOFAR) together with archival data at higher frequencies (235, 325, 610, 1400 and 8500 MHz), to investigate the spectral properties of the source, and we use radiative models to constrain the age of the source. Results. At LOFAR frequency, the source presents two large outer radio lobes, wider than at higher frequencies, and a smaller Intermediate lobe located south-west of the core. A new inspection of X-ray data, allowed us to identify an intermediate cavity, associated with that lobe, indicating the presence of a further phase of jet activity. The radio lobes have a steep spectrum even at LOFAR frequencies, reaching α 610 144 = 2.9 in the outer lobes and α 610 144 = 2.1 in the Intermediate lobe. Fitting the lobe spectra using a single injection model of particle ageing, we derived a total age of the source between 170 and 106 Myr, in agreement with the buoyancy and sound crossing time-scales derived from X-ray data. Thanks to the resolution of the spectral age map we performed, we are able to reconstruct the duty cycle of the source. There were three phases of jet activity, with the AGN being active for most of the time with only brief quiescent phases, ensuring the repeated heating of the central gas. Finally, we estimated the minimum energy inside the outer lobes, finding that a source of additional pressure support must be present to sustain the bubbles against the pressure of the external medium.