A deep study of A399-401: An application for wide-field facet calibration

TL;DR

This study uses deep LOFAR radio observations to investigate the particle acceleration mechanisms in the bridge between galaxy clusters Abell 399 and Abell 401, favoring second-order Fermi re-acceleration as the primary process.

Contribution

It provides the first detailed analysis of the particle acceleration mechanism in the A399-401 bridge using deep, multi-resolution LOFAR data with improved calibration techniques.

Findings

Second-order Fermi re-acceleration explains the radio emission.

Past AGN activity supplies fossil plasma for re-acceleration.

Radio halos are consistent with second-order Fermi processes.

Abstract

We examine the particle acceleration mechanism in the Mpc-scale bridge between Abell 399 and Abell 401 and assess in particular if the synchrotron emission originates from first-order or second-order Fermi re-acceleration. We use deep (~40 hours) LOw-Frequency ARray (LOFAR) observations from Abell 399 and Abell 401 and apply improved direction-dependent calibration to produce deep radio images at three different resolutions at 144 MHz. With a point-to-point analysis we find in the bridge trends between the radio emission from our new maps and X-ray emission from an XMM Newton observation. By analyzing our observations and results, we argue that second-order Fermi re-acceleration is currently the most favoured process to explain the emission from the radio bridge, where past AGN activity may be responsible for the supply of fossil plasma needed for in-situ re-acceleration. The radio halos from Abell 401 and Abell 399 are also consistent with a second-order Fermi re-acceleration model.