J/A+A/642/A85 RX J0603.3+4214 LOFAR 58GHz images (de Gasperin+, 2020)
Reaching thermal noise at ultra-low radio frequencies.
Toothbrush radio relic downstream of the shock front.
de Gasperin F., Brunetti G., Bruggen M., van Weeren R., Williams W.L.,
Botteon A., Cuciti V., Dijkema T.J., Edler H., Iacobelli M., Kang H.,
Offringa A., Orru E., Pizzo R., Rafferty D., Rottgering H., Shimwell T.
<Astron. Astrophys. 642, A85 (2020)>
=2020A&A...642A..85D 2020A&A...642A..85D (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, galaxy ; Radio sources ; Interferometry
Keywords: radio continuum: general - techniques: interferometric -
galaxies: clusters: individual: RX J0603.3+4214 -
galaxies: clusters: intracluster medium -
radiation mechanisms: non-thermal
Abstract:
Ultra-low frequency observations (<100MHz) are particularly
challenging because they are usually performed in a low
signal-to-noise ratio regime due to the high sky temperature and
because of ionospheric disturbances whose effects are inversely
proportional to the observing frequency. Nonetheless, these
observations are crucial for studying the emission from low-energy
populations of cosmic rays.
We aim to obtain the first thermal-noise limited (∼1.5mJy/beam) deep
continuum radio map using the Low Frequency Array's Low Band Antenna
(LOFAR LBA) system. Our demonstration observation targeted the galaxy
cluster RX J0603.3+4214 (known as the Toothbrush cluster). We used the
resulting ultra-low frequency (39-78MHz) image to study cosmic-ray
acceleration and evolution in the post shock region considering the
presence of a radio halo.
We describe the data reduction we used to calibrate LOFAR LBA
observations. The resulting image was combined with observations at
higher frequencies (LOFAR 150MHz and VLA 1500MHz) to extract spectral
information.Results.We obtained the first thermal-noise limited image
from an observation carried out with the LOFAR LBA system using
allDutch stations at a central frequency of 58MHz. With eight hours of
data, we reached an rms noise of 1.3mJy/beam at a resolution of
18"x11".
The procedure we developed is an important step towards routine
high-fidelity imaging with the LOFAR LBA. Theanalysis of the radio
spectra shows that the radio relic extends to distances of 800kpc
downstream from the shock front, larger than what is allowed by
electron cooling time. Furthermore, the shock wave started
accelerating electrons already at a projected distance of <300kpc from
the crossing point of the two clusters. These results may be explained
by electrons being re-accelerated down stream by background
turbulence, possibly combined with projection effects with respect to
the radio halo.
Description:
Our eight-hour observation was performed using the LOFAR LBA system in
the frequency range of 39-78MHz, where the antennas are most
sensitive. We used 24 Core Stations and 13 Remote Stations. The
correlated data had an integration time of 1s and a frequency
resolution of 64 channels per 0.192MHz SubBand (SB).
Objects:
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RA (2000) DE Designation(s)
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06 03.3 +42 14 RX J0603.3+4214 = NAME Toothbrush Cluster
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 113 2 List of fits images
fits/* . 2 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- 58 F7.4 MHz Freq Observed frequency
60- 63 I4 Kibyte size Size of FITS file
65- 82 A18 --- FileName Name of FITS file, in subdirectory fits
84-113 A30 --- Title Title of the FITS file
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Acknowledgements:
Francesco de Gasperin, fdg(at)hs.uni-hamburg.de
(End) Patricia Vannier [CDS] 15-Sep-2020