958 research outputs found
Constraints on the magnetic field in the inter-cluster bridge A399-A401
Galaxy cluster mergers are natural consequences of the structure formation in
the Universe. Such events involve a large amount of energy ( erg)
dissipated during the process. Part of this energy can be channelled in
particle acceleration and magnetic field amplification, enhancing non-thermal
emission of the intra- and inter-cluster environment. Recently, low-frequency
observations have detected a bridge of diffuse synchrotron emission connecting
two merging galaxy clusters, Abell 399 and Abell 401. Such a result provides
clear observational evidence of relativistic particles and magnetic fields
in-between clusters. In this work, we have used LOw Frequency ARray (LOFAR)
observations at 144 MHz to study for the first time the polarized emission in
the A399-A401 bridge region. No polarized emission was detected from the bridge
region. Assuming a model where polarization is generated by multiple shocks,
depolarization can be due to Faraday dispersion in the foreground medium with
respect to the shocks. We constrained its Faraday dispersion to be greater than
0.10 rad m at 95% confidence level, which corresponds to an average
magnetic field of the bridge region greater than 0.46 nG (or 0.41 nG if we
include regions of the Faraday spectrum that are contaminated by Galactic
emission). This result is largely consistent with the predictions from
numerical simulations for Mpc regions where the gas density is times
larger than the mean gas density.Comment: 12 pages, 13 figures; accepted in A&
First evidence of diffuse ultra-steep-spectrum radio emission surrounding the cool core of a cluster
Diffuse synchrotron radio emission from cosmic-ray electrons is observed at the center of a number of galaxy clusters. These sources can be classified either as giant radio halos, which occur in merging clusters, or as mini halos, which are found only in cool-core clusters. In this paper, we present the first discovery of a cool-core cluster with an associated mini halo that also shows ultra-steep-spectrum emission extending well beyond the core that resembles radio halo emission. The large-scale component is discovered thanks to LOFAR observations at 144 MHz. We also analyse GMRT observations at 610 MHz to characterise the spectrum of the radio emission. An X-ray analysis reveals that the cluster is slightly disturbed, and we suggest that the steep-spectrum radio emission outside the core could be produced by a minor merger that powers electron re-acceleration without disrupting the cool core. This discovery suggests that, under particular circumstances, both a mini and giant halo could co-exist in a single cluster, opening new perspectives for particle acceleration mechanisms in galaxy clusters
Radio relics in PSZ2 G096.88+24.18: A connection with pre-existing plasma
Giant radio relics are arc-like structures of diffuse, non-thermal
synchrotron radiation that trace shock waves induced by galaxy cluster mergers.
The particle (re-)acceleration mechanism producing such radio relics is
unclear. One major open question is whether relics can be formed directly from
a population of thermal seed electrons, or if pre-existing relativistic seed
electrons are required. In some cases AGN can provide such a population of
sub-GeV electrons. However, it is unclear how common this connection is. In
this paper we present LOFAR 140 MHz and VLA L-band radio observations, as well
as Chandra data of PSZ2 G096.88+24.18, a merging galaxy cluster system hosting
a pair of radio relics. A large patch of diffuse emission connects a bright
radio galaxy with one of the relics, likely affecting the properties of the
relic. We find that the most plausible explanation for the connection is that
the merger shock wave has passed over an AGN lobe. The shock passing over this
seed population of electrons has led to an increased brightness in the relic
only in the region filled with seed electrons.Comment: 20 pages, 9 figures, 4 tables, accepted for publication in MNRA
The LOFAR and JVLA view of the distant steep spectrum radio halo in MACS J1149.5+2223
Radio halos and relics are Mpc-scale diffuse radio sources in galaxy
clusters, with a steep spectral index (). It
has been proposed that they arise from particle acceleration induced by
turbulence and weak shocks, injected in the intracluster medium (ICM) during
mergers. MACS J1149.5+2223 (MACS J1149) is a high redshift () galaxy
cluster possibly hosting a radio halo and a relic. We analysed LOFAR, GMRT, and
JVLA radio data at 144, 323, 1500 MHz, and Chandra X-ray data to characterise
the thermal and non-thermal properties of the cluster. We obtained radio images
at different frequencies to investigate the spectral properties of the radio
halo. We used Chandra X-ray images to constrain the thermal properties of the
cluster. We measured a steep spectrum of the halo, with
between 144 and 1500 MHz. The radio surface brightness distribution across the
halo is found to correlate with the X-ray brightness of the ICM, with a
sub-linear slope in the range 0.4 to 0.6. We also report two possible cold
fronts in north-east and north-west, but deeper X-ray observations are required
to firmly constrain the properties of the upstream emission. We show that the
combination of high redshift, steep radio spectrum, and sub-linear radio-X
scaling of the halo rules out hadronic models. An old ( Gyr ago) major
merger likely induced the formation of the halo through stochastic
re-acceleration of relativistic electrons. We suggest that the two possible
X-ray discontinuities may actually be part of the same cold front. In this
case, the coolest gas pushed towards the north-west might be associated with
the cool core of a sub-cluster involved in the major merger. The peculiar
orientation of the south-east relic might indicate a different nature of this
source and requires further investigation.Comment: 15 pages, accepted for publication in A&
The Planck clusters in the LOFAR sky. II. LoTSS-DR2: Recovering diffuse extended emission with LOFAR
Extended radio sources in the sky require a dense sampling of short baselines
to be properly imaged by interferometers. This problem arises in many areas of
radio astronomy, such as in the study of galaxy clusters, which may host
Mpc-scale diffuse synchrotron sources in the form of radio halos. In clusters
where no radio halos are detected, owing to intrinsic absence of emission or
extrinsic (instrumental and/or observational) effects, it is possible to
determine upper limits. We consider a sample of Planck galaxy clusters from the
Second Data Release of the LOFAR Two Meter Sky Survey (LoTSS-DR2) where no
radio halos are detected. We use this sample to test the capabilities of LOFAR
to recover diffuse extended emission and derive upper limits. Through the
injection technique, we simulate radio halos with various surface brightness
profiles. We then predict the corresponding visibilities and image them along
with the real visibilities. This method allows us to test the fraction of flux
density losses owing to inadequate uv-coverage and obtain thresholds at which
the mock emission becomes undetectable by visual inspection. The dense
uv-coverage of LOFAR at short spacings allows to recover of the
flux density of targets with sizes up to . We find a relation that
provides upper limits based on the image noise and extent (in terms of number
of beams) of the mock halo. This relation can be safely adopted to obtain upper
limits without injecting when artifacts introduced by the subtraction of the
discrete sources are negligible in the central region of the cluster.
Otherwise, the injection process and visual inspection of the images are
necessary to determine more reliable limits. Through these methods, we obtain
upper limits for 75 clusters to be exploited in ongoing statistical studies.Comment: 20 pages (including Appendices). 21 Figures. 4 Tables. Accepted for
publication by A&A. For the project Webpage see
https://lofar-surveys.org/planck_dr2.htm
Evolutionary phases of merging clusters as seen by LOFAR
Massive, merging galaxy clusters often host giant, diffuse radio sources that arise from shocks and turbulence; hence, radio observations can be useful for determining the merger state of a cluster. In preparation for a larger study, we selected three clusters - Abell 1319, Abell 1314, and RXC J1501.3+4220 (Z7215) - making use of the new LOFAR Two-Metre Sky Survey (LoTSS) at 120-168 MHz, and together with archival data, show that these clusters appear to be in pre-merging, merging, and post-merging states, respectively. We argue that Abell 1319 is likely in its pre-merging phase, where three separate cluster components are about to merge. There are no radio haloes nor radio relics detected in this system. Abell 1314 is a highly disturbed, low-mass cluster that is likely in the process of merging. This low-mass system does not show a radio halo, however, we argue that the merger activates mechanisms that cause electron re-acceleration in the large 800 kpc radio tail associated with IC 711. In the cluster Z7215 we discover diffuse radio emission at the cluster centre, and we classify this emission as a radio halo, although it is dimmer and smaller than expected by the radio halo power versus cluster mass correlation. We suggest that the disturbed cluster Z7215 is in its post-merging phase. Systematic studies of this kind over a larger sample of clusters observed with LoTSS will help to constrain the timescales involved in turbulent re-acceleration and the subsequent energy losses of the underlying electrons
Reaching thermal noise at ultra-low radio frequencies: the Toothbrush radio relic downstream of the shock front
Ultra-low frequency observations (<100 MHz) 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 to study the emission from low-energy populations of
cosmic rays. We aim to obtain the first thermal-noise limited (~ 1.5 mJy/beam)
deep continuum radio map using the LOFAR Low Band Antenna (LBA) system. Our
demonstration observation targeted the galaxy cluster RX J0603.3+4214 (the
"Toothbrush" cluster). We used the resulting ultra-low frequency (58 MHz) image
to study cosmic-ray acceleration and evolution in the post shock region, as
well as their relation with the presence of a radio halo. We describe the data
reduction we have used to calibrate LOFAR LBA observations. The resulting image
is combined with observations at higher frequencies (LOFAR 150 MHz and VLA 1500
MHz) to extract spectral information. We obtained the first thermal-noise
limited image from an observation carried out with the LOFAR LBA system using
all Dutch stations at a central frequency of 58 MHz. With 8 hours of data, we
reached an rms noise of 1.3 mJy/beam at a resolution of 18" x 11". The
procedure we have developed is an important step forward towards routine
high-fidelity imaging with the LOFAR LBA. The analysis of the radio spectra
shows that the radio relic extends to distances of 800 kpc downstream from the
shock front, larger than what allowed by electron cooling time. Furthermore,
the shock wave started accelerating electrons already at a projected distance
of <300 kpc from the crossing point of the two clusters. These results can be
explained if electrons are reaccelerated downstream by background turbulence
possibly combined with projection effects.Comment: 15 pages, 19 figures, accepted A&
Discovery of a Radio Halo (and Relic) in a M 500< 2 × 1014M o˙Cluster
Radio halos are diffuse synchrotron sources observed in dynamically unrelaxed galaxy clusters. Current observations and models suggest that halos trace turbulent regions in the intracluster medium where mildly relativistic particles are reaccelerated during cluster mergers. Due to the higher luminosities and detection rates with increasing cluster mass, radio halos have been mainly observed in massive systems (M500 ≳ 5 × 1014M⊙). Here, we report the discovery of a radio halo with a largest linear scale of ≃750 kpc in PSZ2G145.92-12.53 (z = 0.03) using LOw Frequency ARray (LOFAR) observations at 120-168 MHz. With a mass of M500=(1.9 ± 0.2) × 1014M⊙ and a radio power at 150 MHz of P150=(3.5 ± 0.7) × 1023 W Hz-1, this is the least powerful radio halo in the least massive cluster discovered to date. Additionally, we discover a radio relic with a mildly convex morphology at ∼1.7 Mpc from the cluster center. Our results demonstrate that LOFAR has the potential to detect radio halos even in low-mass clusters, where the expectation to form them is very low (∼5%) based on turbulent reacceleration models. Together with the observation of large samples of clusters, this opens the possibility to constrain the low end of the power-mass relation of radio halos
Magnetic fields and relativistic electrons fill entire galaxy cluster
The hot plasma within merging galaxy clusters is predicted to be filled with
shocks and turbulence that may convert part of their kinetic energy into
relativistic electrons and magnetic fields generating synchrotron radiation.
Analyzing Low Frequency Array (LOFAR) observations of the galaxy cluster Abell
2255, we show evidence of radio synchrotron emission distributed over very
large scales of at least 5 megaparsec. The pervasive radio emission witnesses
that shocks and turbulence efficiently transfer kinetic energy into
relativistic particles and magnetic fields in a region that extends up to the
cluster outskirts. The strength of the emission requires a magnetic field
energy density at least 100 times higher than expected from a simple
compression of primordial fields, presumably implying that dynamo operates
efficiently also in the cluster periphery. It also suggests that nonthermal
components may contribute substantially to the pressure of the intracluster
medium in the cluster periphery.Comment: Published in Science Advances on Nov 2nd, 2022. 8 pages, 5 image
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