X-ray cavities provide diagnostic information on the central engine of the AGN and its energetics. In particular, determining the duty cycle (or fraction of time when the AGN is active) and age of the radio plasma require observations with high resolution and high sensitivity over a wide frequency range. Previous Very Large Array data at 325 MHz (in green; G. Swarup Indian J. Radio Space Phys. 19, 493–505; 1990) showed radio lobes that only partially filled the X-ray cavities, which suggested that the gas was displaced and compressed by bubbles of radio-emitting plasma inflated by the jet. Now the 144 MHz LOFAR observations (in orange) reveal radio lobes that fill in the cavities completely. The blue core is imaged by the Chandra X-ray Observatory, and the white regions by the Hubble Space Telescope in the near-infrared.
LOFAR’s sensitivity and resolution enable Biava and co-workers to characterize in detail the morphology, spectral properties and radiative age of the source. The steepness of the radio spectrum being much greater than for normal AGN cores, they believe the measured central emission is a superposition of the actual core and the inner part of the jets. By using all available X-ray and radio observations in their modelling, the authors further estimate that the age of the outer lobe, corresponding to the first active jet phase that lasted 35–55 Myr, is between 106 Myr and 170 Myr. The newly identified intermediate cavity is less constrained because of the poorer resolution, but is of comparable age to the first, starting some 103–122 Myr ago, with an inactive period of a few to ten million years between the two. The off-axis nature of the intermediate lobe reveals a change of direction in the reactivated jet. The latest period of jet activity, which created the inner cavities, lasted less than 78 Myr. From these estimates Biava and co-workers conclude that the AGN is active for most of the time and continuously heating the halo gas, not giving it a chance to cool. Hence the duty cycle is close to unity. Finally, calculations of the pressure inside the cavities reveal the need for an additional pressure source to maintain equilibrium of the plasma bubbles, perhaps in the form of hot thermal gas entrained by the jet. When the Square Kilometre Array comes online, similar studies of more sources will help us further understand the impact of AGN activity on its surroundings.
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