Abell 0399-Abell 0401 radio bridge spectral index: the first multifrequency detection

Abstract

Recent low-frequency radio observations at 140 MHz discovered a 3 Mpc-long bridge of diffuse emission connecting the galaxy clusters Abell 0399 and Abell 0401. We present follow-up observations at 60 MHz to constrain the spectral index of the bridge, which so far has only been detected at 140 and 144 MHz. We analysed deep (~18 hours) LOw Frequency ARray (LOFAR) Low Band Antenna (LBA) data at 60 MHz to detect the bridge at very low frequencies. We then conducted a multi-frequency study with LOFAR HBA data at 144 MHz and uGMRT data at 400 MHz. Assuming second-order Fermi mechanisms for the re-acceleration of relativistic electrons driven by turbulence in the radio bridge regions, we compare the observed radio spectrum with theoretical synchrotron models. The bridge is detected in the 75'' resolution LOFAR image at 60 MHz and its emission fully connects the region between the two galaxy clusters. Between 60 MHz and 144 MHz we found an integrated spectral index value of -1.44 +\- 0.16 for the bridge emission. For the first time, we produced spectral index and related uncertainties maps for a radio bridge. We produce a radio spectrum, which show significant steepening between 144 and 400 MHz. This detection at low frequencies provides important information on the models of particle acceleration and magnetic field structure on very extended scales. The spectral index gives important clues to the origin of inter-cluster diffuse emission. The steepening of the spectrum above 144 MHz can be explained in a turbulent re-acceleration framework, assuming that the acceleration timescales are longer than ~200 Myr.

Letter to the Editor: 7 pages, 4 figures, 2 tables; accepted for publication in Astronomy & Astrophysics (A&A)

References in corpus (20)

• WSClean: an implementation of a fast, generic wide-field imager for radio astronomy
• Shocks and cold fronts in galaxy clusters
• The LOFAR Two-metre Sky Survey -- V. Second data release
• Diffuse Radio Emission from Galaxy Clusters
• GMRT Radio Halo Survey in galaxy clusters at z = 0.2 -- 0.4. II.The eBCS clusters and analysis of the complete sample
• A radio ridge connecting two galaxy clusters in a filament of the cosmic web
• LOFAR observations of galaxy clusters in HETDEX
• The LOFAR LBA Sky Survey I. survey description and preliminary data release
• Second order Fermi reacceleration mechanisms and large scale synchrotron radio emission in intra-cluster bridges
• Gas and galaxies in filament between clusters of galaxies: The study of A399-A401
• Dynamical evolution of magnetic fields in the intracluster medium
• On the absence of radio halos in clusters with double relics
• The Coma cluster at LOFAR frequencies I: insights into particle acceleration mechanisms in the radio bridge
• Radio footprints of a minor merger in the Shapley Supercluster: From supercluster down to galactic scales
• The ultra-steep diffuse radio emission observed in the cool-core cluster RX J1720.1+2638 with LOFAR at 54 MHz
• Detecting shocked intergalactic gas with X-ray and radio observations
• Particle re-acceleration and diffuse radio sources in the galaxy cluster Abell 1550
• The merging galaxy cluster Abell 3266 at low radio frequencies
• The thermal and non-thermal components within and between galaxy clusters Abell 399 and Abell 401
• Radio multifrequency observations of galaxy clusters. The Abell 399$-$401 pair

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