LOFAR imaging of Cygnus A -- Direct detection of a turnover in the hotspot radio spectra

Abstract

The low-frequency radio spectra of the hotspots within powerful radio galaxies can provide valuable information about the physical processes operating at the site of the jet termination. These processes are responsible for the dissipation of jet kinetic energy, particle acceleration, and magnetic-field generation. Here we report new observations of the powerful radio galaxy Cygnus A using the Low Frequency Array (LOFAR) between 109 and 183 MHz, at an angular resolution of ~3.5 arcsec. The radio emission of the lobes is found to have a complex spectral index distribution, with a spectral steepening found towards the centre of the source. For the first time, a turnover in the radio spectrum of the two main hotspots of Cygnus A has been directly observed. By combining our LOFAR imaging with data from the Very Large Array at higher frequencies, we show that the very rapid turnover in the hotspot spectra cannot be explained by a low-energy cut-off in the electron energy distribution, as has been previously suggested. Thermal (free-free) absorption or synchrotron self absorption models are able to describe the low-frequency spectral shape of the hotspots, however, as with previous studies, we find that the implied model parameters are unlikely, and interpreting the spectra of the hotspots remains problematic.

9 pages, 3 figures, accepted for publication in MNRAS

References in corpus (4)

• Some Aspects of Measurement Error in Linear Regression of Astronomical Data
• Post-correlation radio frequency interference classification methods
• The LOFAR radio environment
• Cygnus A: A Long Wavelength Resolution of the Hot Spots

Cited by in corpus (42)

• A radio spectral index map and catalogue at 147-1400 MHz covering 80 per cent of the sky
• The LOFAR LBA Sky Survey I. survey description and preliminary data release
• Particle acceleration in astrophysical jets
• RAiSE III: 3C radio AGN energetics and composition
• On the population of remnant FRII radio galaxies and implications for radio source dynamics
• Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies
• LOFAR Observations of X-ray Cavity Systems
• Nearby galaxies in LoTSS-DR2: insights into the non-linearity of the radio-SFR relation
• Interplanetary Scintillation studies with the Murchison Wide-field Array II: Properties of sub-arcsecond compact sources at low radio frequencies
• Radio spectral properties and jet duty cycle in the restarted radio galaxy 3C388
• The galaxy group NGC 507: newly detected AGN remnant plasma transported by sloshing
• Carbon and hydrogen radio recombination lines from the cold clouds towards Cassiopeia A
• Two striking head-tail galaxies in the galaxy cluster IIZW108: insights into transition to turbulence, magnetic fields and particle re-acceleration
• The impact of red giant/AGB winds on AGN jet propagation
• Long-term FRII jet evolution in dense environments
• An updated estimate of the cosmic radio background and implications for ultra-high-energy photon propagation
• LOFAR MSSS: The Scaling Relation between AGN Cavity Power and Radio Luminosity at Low Radio Frequencies
• On the maximum energy of non-thermal particles in the primary hotspot of Cygnus A
• High-resolution international LOFAR observations of 4C~43.15 -- Spectral ages and injection indices in a high-z radio galaxy
• Gravitational lensing in LoTSS DR2 -- Extremely faint 144-MHz radio emission from two highly magnified quasars
• Filamentary structures of ionized gas in Cygnus X
• Pushing subarcsecond resolution imaging down to 30 MHz with the trans-European International LOFAR Telescope
• On the significance of relativistically hot pairs in the jets of FR II radio galaxies
• LOFAR observations of 4C+19.44. On the discovery of low frequency spectral curvature in relativistic jet knots
• A Space-based Decametric Wavelength Radio Telescope Concept
• A Polarimetric Study of 9 PG Quasars with the VLA
• Particle acceleration in low-power hotspots: modelling the broad-band spectral energy distribution
• A Numerical Study of the Impact of Jet Magnetic Topology on Radio Galaxy Evolution
• Simulations of the merging cluster of galaxies Cygnus A
• Spectral modelling of Cygnus A between 110 and 250 MHz. Impact on the LOFAR 21-cm signal power spectrum
• Jansky VLA observations of synchrotron emitting optical hotspots of 3C 227 and 3C 445 radio galaxies
• Lunar Far-Side Radio Arrays: A Preliminary Site Survey
• $\text{Herschel}$ discovery of far-infrared emission from the hot spot D in the radio galaxy Cygnus A
• Magnetic fields and extraordinarily bright radio emission in the X-ray faint galaxy group MRC 0116+111
• Strong low-frequency radio flaring from Cygnus X-3 observed with LOFAR
• A novel radio imaging method for physical spectral index modelling
• A Polarization Study of 3 Blazars using the uGMRT at ~600 MHz
• Tied-array holography with LOFAR
• The 'spectral index-flux density relation' for extragalactic radio sources selected at metre and decametre wavelengths
• A Multi-Mechanism Hybrid Model of Peaked-Spectrum Radio Sources
• The resolved jet of 3C 273 at 150 MHz
• Spectral analysis of spatially-resolved 3C295 (sub-arcsecond resolution) with the International LOFAR Telescope