J/A+A/688/A175 LOFAR Observations of Abell 2256 (Osinga+, 2024)
Probing particle acceleration in Abell 2256: From 16 MHz to gamma rays.
Osinga E., van Weeren R.J., Brunetti G., Adam R., Rajpurohit K., Botteon A.,
Callingham J.R., Cuciti V., de Gasperin F., Miley G.K., Roettgering H.J.A.,
Shimwell T.W.
<Astron. Astrophys. 688, A175 (2024)>
=2024A&A...688A.175O 2024A&A...688A.175O (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, galaxy ; Radio continuum ; Gamma rays
Keywords: radiation mechanisms: non-thermal - galaxies: clusters: general -
galaxies: clusters: intracluster medium -
gamma rays: galaxies - gamma rays: galaxies: clusters -
radio continuum: general
Abstract:
Merging galaxy clusters often host spectacular diffuse radio
synchrotron sources. These sources can be explained by a non-thermal
pool of relativistic electrons accelerated by shocks and turbulence in
the intracluster medium. The origin of the pool and details of the
cosmic ray transport and acceleration mechanisms in clusters are still
open questions. Due to the often extremely steep spectral indices of
diffuse radio emission, it is best studied at low frequencies.
However, the lowest frequency window available to ground-based
telescopes (10-30MHz) has remained largely unexplored, as radio
frequency interference and calibration problems related to the
ionosphere become severe. Here, we present LOFAR observations from 16
to 168MHz targeting the famous cluster Abell 2256. In the deepest-ever
images at decametre wavelengths, we detect and resolve the radio halo,
radio shock and various steep spectrum sources. We measure standard
single power-law behaviour for the radio halo and radio shock spectra
and find significant spectral index and curvature fluctuations across
the radio halo, indicating an inhomogeneous emitting volume. In
contrast to the straight power-law spectra of the large-scale diffuse
sources, the various AGN-related sources often show extreme steepening
towards higher frequencies and flattening towards low frequencies. We
also discover a new fossil plasma source with a steep spectrum between
23 and 144MHz, with α=-1.9±0.1. Finally, by comparing radio
and gamma-ray observations, we rule out purely hadronic models for the
radio halo origin in Abell 2256, unless the magnetic field strength in
the cluster is exceptionally high, which is unsupportable by energetic
arguments and inconsistent with the knowledge of other cluster
magnetic fields.
Description:
In this paper, we present LOFAR observations from 16 to 168MHz
targeting the famous cluster Abell 2256. In the deepest-ever images at
decametre wavelengths, we detect and resolve the radio halo, radio
shock, and various steep spectrum sources. We measure standard single
power-law behaviour for the radio halo and radio shock spectra and
find significant spectral index and curvature fluctuations across the
radio halo, indicating an inhomogeneous emitting volume. In contrast
to the straight power-law spectra of the large-scale diffuse sources,
the various AGN-related sources often show extreme steepening towards
higher frequencies and flattening towards low frequencies. We also
discover a new fossil plasma source with a steep spectrum between 23
and 144MHz, with α=-1.9±0.1. Finally, by comparing radio and
gamma-ray observations, we rule out purely hadronic models for the
radio halo origin in Abell 2256, unless the magnetic field strength in
the cluster is exceptionally high, which is unsupportable by energetic
arguments and inconsistent with the knowledge of other cluster
magnetic fields.
Objects:
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RA (2000) DE Designation(s)
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17 03 09.36 +78 39 35.9 ACO 2256 = 1RXS J170351.0+783835
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 137 6 List of fits images
fits/* . 6 Individual fits images
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 23 I4 --- Nx Number of pixels along X-axis
25- 28 I4 --- Ny Number of pixels along Y-axis
30- 50 A21 "datime" Obs.date Observation date
52- 59 F8.4 MHz Freq Observed frequency
61- 65 I5 Kibyte size Size of FITS file
67-101 A35 --- FileName Name of FITS file, in subdirectory fits
103-137 A35 --- Title Title of the FITS file
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Acknowledgements:
Erik Osinga, erik.osinga(at)utoronto.ca
(End) Erik Osinga [UofT], Patricia Vannier [CDS] 03-Aug-2024