Dissecting nonthermal emission in the complex multiple-merger galaxy
  cluster Abell 2744: Radio and X-ray analysis
  We present the first deep low frequency radio observations of the massive and
highly disturbed galaxy cluster Abell 2744 using the upgraded Giant Metrewave
Radio Telescope (uGMRT). The cluster is experiencing a very complex multiple
merger and hosts a giant halo and four radio relics. The uGMRT observations,
together with existing VLA and Chandra observations, allow us to study the
complexity of the physical mechanisms active in this system. Our new images
reveal that the central halo emission is more extended toward low frequencies.
We find that the integrated spectrum of the halo follows a power-law between
150 MHz and 3 GHz, while its subregions show significantly different spectra,
also featuring high frequency spectral steepening. The halo also shows local
regions in which the spectral index is significantly different from the average
value. Our results highlight that an overall power-law spectrum, as observed in
many radio halos, may also arise from the superposition of different
subcomponents. The comparison of the radio surface brightness and spectral
index with the X-ray brightness and temperature reveals for the first time
different trends, indicating that the halo consists of two main components. All
four relics in this system follow a power-law radio spectrum, compatible with
shocks with Mach numbers in the range $3.0-4.5$. All relics are also highly
polarized from 1-4 GHz and show low Faraday dispersion measures, suggesting
that they are located in the outermost regions of the cluster. The complexity
in the distribution and properties of nonthermal components in Abell 2744
supports a multiple merger scenario, as also highlighted by previous X-ray and
lensing studies. Our unique results demonstrate the importance of sensitive and
high-resolution, multi-frequency radio observations for understanding the
interplay between the thermal and non-thermal components of the ICM.
