scholarly journals Radio observations of the merging galaxy cluster Abell 520

2019 ◽  
Vol 622 ◽  
pp. A20 ◽  
Author(s):  
D. N. Hoang ◽  
T. W. Shimwell ◽  
R. J. van Weeren ◽  
G. Brunetti ◽  
H. J. A. Röttgering ◽  
...  
Keyword(s):  
Mach Number ◽  
Radio Emission ◽  
Low Frequency ◽  
Galaxy Cluster ◽  
Shock Acceleration ◽  
Relativistic Electrons ◽  
Diffusive Shock Acceleration ◽  
Cluster Emission ◽  
X Ray ◽  
Extended Radio

Context. Extended synchrotron radio sources are often observed in merging galaxy clusters. Studies of the extended emission help us to understand the mechanisms in which the radio emitting particles gain their relativistic energies. Aims. We examine the possible acceleration mechanisms of the relativistic particles that are responsible for the extended radio emission in the merging galaxy cluster Abell 520. Methods. We performed new 145 MHz observations with the LOw Frequency ARay (LOFAR) and combined these with archival Giant Metrewave Radio Telescope (GMRT) 323 MHz and Very Large Array (VLA) 1.5 GHz data to study the morphological and spectral properties of extended cluster emission. The observational properties are discussed in the framework of particle acceleration models associated with cluster merger turbulence and shocks. Results. In Abell 520, we confirm the presence of extended (760 × 950 kpc2) synchrotron radio emission that has been classified as a radio halo. The comparison between the radio and X-ray brightness suggests that the halo might originate in a cocoon rather than from the central X-ray bright regions of the cluster. The halo spectrum is roughly uniform on the scale of 66 kpc. There is a hint of spectral steepening from the SW edge towards the cluster centre. Assuming diffusive shock acceleration (DSA), the radio data are suggestive of a shock Mach number of ℳSW = 2.6−0.2+0.3 that is consistent with the X-ray derived estimates. This is in agreement with the scenario in which relativistic electrons in the SW radio edge gain their energies at the shock front via acceleration of either thermal or fossil electrons. We do not detect extended radio emission ahead of the SW shock that is predicted if the emission is the result of adiabatic compression. An X-ray surface brightness discontinuity is detected towards the NE region that may be a counter shock of Mach number ℳNEX = 1.52±0.05. This is lower than the value predicted from the radio emission which, assuming DSA, is consistent with ℳNE = 2.1 ± 0.2. Conclusions. Our observations indicate that the radio emission in the SW of Abell 520 is likely effected by the prominent X-ray detected shock in which radio emitting particles are (re-)accelerated through the Fermi-I mechanism. The NE X-ray discontinuity that is approximately collocated with an edge in the radio emission hints at the presence of a counter shock.

scholarly journals X-ray and Radio Observations of the Radio Relic Galaxy Clusters 1RXS J0603.3+4214 and RXC J1053.7+5453

Galaxies ◽  
2018 ◽  
Vol 7 (1) ◽  
pp. 2 ◽  
Author(s):  
Motokazu Takizawa
Keyword(s):  
Mach Number ◽  
Galaxy Clusters ◽  
Galaxy Cluster ◽  
Shock Acceleration ◽  
Temperature Structure ◽  
Radio Observations ◽  
Diffusive Shock Acceleration ◽  
X Ray ◽  
Acceleration Model ◽  
The Galaxy

We study two galaxy clusters with radio relics, 1RXS J0603.3+4214 and RXC J1053.7+5453, through X-ray and radio observations. Radio relics are diffuse non-thermal radio sources found in outskirts of galaxy clusters. Because of their shape and location, they are thought to be related to cluster merger shocks. The galaxy cluster 1RXS J0603.3+4214 has a well-known linear-shape “toothbrush” radio relic. We investigate the temperature structure across the relic to constrain the Mach number of the associated shock. The results are compared with radio spectral results, which suggest that a simple diffusive shock acceleration model does not hold for this relic. The RXC J1053.7+5453 harbors a standard arc-like relic. We also get the Mach number from the temperature profile. In addition, we found an edge-like structure in the X-ray image between the X-ray peak and relic. We investigate the density and temperature profiles across the edge and found that the structure is likely relevant to not a shock but a contact discontinuity.

scholarly journals Discovering the most elusive radio relic in the sky: diffuse shock acceleration caught in the act?

2020 ◽  
Vol 496 (1) ◽  
pp. L48-L53 ◽  
Author(s):  
Nicola T Locatelli ◽  
Kamlesh Rajpurohit ◽  
Franco Vazza ◽  
Fabio Gastaldello ◽  
Daniele Dallacasa ◽  
...  
Keyword(s):  
Mach Number ◽  
Surface Brightness ◽  
Low Frequency ◽  
Galaxy Cluster ◽  
Total Power ◽  
Shock Acceleration ◽  
Diffusive Shock Acceleration ◽  
The Galaxy ◽  
Shock Mach Number ◽  
New Generation

ABSTRACT The origin of radio relics is usually explained via diffusive shock acceleration (DSA) or re-acceleration of electrons at/from merger shocks in galaxy clusters. The case of acceleration is challenged by the low predicted efficiency of low Mach number merger shocks, unable to explain the power observed in most radio relics. In this letter, we present the discovery of a new giant radio relic around the galaxy cluster Abell 2249 ($z$ = 0.0838) using Low-Frequency Array (LOFAR). It is special since it has the lowest surface brightness of all known radio relics. We study its radio and X-ray properties combining LOFAR data with uGMRT, JVLA, and XMM. This object has a total power of $L_{1.4\rm\, GHz}=4.1\pm 0.8 \times 10^{23}$ W Hz−1 and integrated spectral index α = 1.15 ± 0.23. We infer for this radio relic a lower bound on the magnetization of $B\ge 0.4\, \mu$G, a shock Mach number of $\mathcal {M}\approx 3.79$, and a low acceleration efficiency consistent with DSA. This result suggests that a missing population of relics may become visible, thanks to the unprecedented sensitivity of the new generation of radio telescopes.

scholarly journals Characterizing the radio emission from the binary galaxy cluster merger Abell 2146

2019 ◽  
Vol 622 ◽  
pp. A21 ◽  
Author(s):  
D. N. Hoang ◽  
T. W. Shimwell ◽  
R. J. van Weeren ◽  
H. J. A. Röttgering ◽  
A. Botteon ◽  
...  
Keyword(s):  
Particle Acceleration ◽  
Radio Emission ◽  
Low Frequency ◽  
Galaxy Cluster ◽  
Relativistic Electrons ◽  
Extended Source ◽  
Very Large Array ◽  
Radio Halo ◽  
Extended Radio ◽  
Relativistic Particles

Context. Collisions of galaxy clusters generate shocks and turbulence in the intra-cluster medium (ICM). The presence of relativistic particles and magnetic fields is inferred through the detection of extended synchrotron radio sources such as haloes and relics and implies that merger shocks and turbulence are capable of (re-)accelerating particles to relativistic energies. However, the precise relationship between merger shocks, turbulence, and extended radio emission is still unclear. Studies of the most simple binary cluster mergers are important to help understand the particle acceleration in the ICM. Aims. Our main aim is to study the properties of the extended radio emission and particle acceleration mechanism(s) associated with the generation of relativistic particles in the ICM. Methods. We measure the low-frequency radio emission from the merging galaxy cluster Abell 2146 with LOFAR at 144 MHz. We characterize the spectral properties of the radio emission by combining these data with data from archival Giant Metrewave Radio Telescope (GMRT) at 238 MHz and 612 MHz and Very Large Array (VLA) at 1.5 GHz. Results. We observe extended radio emission at 144 MHz behind the NW and SE shocks. Across the NW extended source, the spectral index steepens from −1.06 ± 0.06 to −1.29 ± 0.09 in the direction of the cluster centre. This spectral behaviour suggests that a relic is associated with the NW upstream shock. The precise nature of the SE extended emission is unclear. It may be a radio halo bounded by a shock or a superposition of a relic and halo. At 144 MHz, we detect a faint emission that was not seen with high-frequency observations, implying a steep (α <  −1.3) spectrum nature of the bridge emission. Conclusions. Our results imply that the extended radio emission in Abell 2146 is probably associated with shocks and turbulence during cluster merger. The relativistic electrons in the NW and SE may originate from fossil plasma and thermal electrons, respectively.

scholarly journals On the influence of supra-thermal particle acceleration on the morphology of low-Mach, high-β shocks

2020 ◽  
Vol 496 (3) ◽  
pp. 3198-3208
Author(s):  
Allard Jan van Marle
Keyword(s):  
Mach Number ◽  
Galaxy Cluster ◽  
Cosmic Ray ◽  
Shock Acceleration ◽  
Diffusive Shock Acceleration ◽  
Particle Injection ◽  
Particle In Cell ◽  
Injection Process ◽  
The Magnetic Field ◽  
High Computational Efficiency

ABSTRACT When two galaxy clusters encounter each other, the interaction results in a collisionless shock that is characterized by a low (1–4) sonic Mach number, and a high-Alfvénic Mach number. Our goal is to determine if, and to what extent, such shocks can accelerate particles to sufficient velocities that they can contribute to the cosmic ray spectrum. We combine two different computational methods, magnetohydrodynamics (MHD) and particle-in-cell (PIC) into a single code that allows us to take advantage of the high computational efficiency of MHD while maintaining the ability to model the behaviour of individual non-thermal particles. Using this method, we perform a series of simulations covering the expected parameter space of galaxy cluster collision shocks. Our results show that for shocks with a sonic Mach number below 2.25 no diffusive shock acceleration can take place because of a lack of instabilities in the magnetic field, whereas for shocks with a sonic Mach number $\ge \, 3$ the acceleration is efficient and can accelerate particles to relativistic speeds. In the regime between these two extremes, diffusive shock acceleration can occur but is relatively inefficient because of the time- and space-dependent nature of the instabilities. For those shocks that show efficient acceleration, the instabilities in the upstream gas increase to the point where they change the nature of the shock, which, in turn, will influence the particle injection process.

scholarly journals Morphology of radio relics – I. What causes the substructure of synchrotron emission?

2020 ◽  
Vol 500 (1) ◽  
pp. 795-816
Author(s):  
P Domínguez-Fernández ◽  
M Brüggen ◽  
F Vazza ◽  
W E Banda-Barragán ◽  
K Rajpurohit ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Mach Number ◽  
Three Dimensional ◽  
Cosmic Ray ◽  
Shock Acceleration ◽  
Synchrotron Emission ◽  
Diffusive Shock Acceleration ◽  
X Ray ◽  
Set Up ◽  
Index Maps

ABSTRACT High-resolution radio observations of cluster radio relics often show complex spatial and spectral features. However, it is not clear what these features reveal about the underlying magnetic field properties. We performed three-dimensional magnetohydrodynamical simulations of merger shock waves propagating through a magnetized, turbulent intracluster medium. Our model includes the diffusive shock acceleration (DSA) of cosmic ray electrons, their spatial advection and energy losses at run-time. With this set-up we can investigate the relation between radio substructure and pre-shock plasma conditions in the host cluster. We find that upstream turbulence plays a major role in shaping the properties of radio relics produced downstream. Within the assumption of DSA, we can reproduce the observed discrepancy between the X-ray derived Mach number of shocks, and the Mach number inferred from radio spectra. Our simulated spectral index maps and profiles across the radio relic also suggest that the standard deviation of the upstream magnetic field must be relatively small ($\sigma _B\le 1 \, \mu$G) in order to reproduce observations and therefore radio relics can potentially constrain the distribution of magnetic fields in galaxy clusters outskirts.

scholarly journals Mystery solved: discovery of extended radio emission in the merging galaxy cluster Abell 2146

2017 ◽  
Vol 475 (2) ◽  
pp. 2743-2753 ◽  
Author(s):  
J Hlavacek-Larrondo ◽  
M-L Gendron-Marsolais ◽  
D Fecteau-Beaucage ◽  
R J van Weeren ◽  
H R Russell ◽  
...  
Keyword(s):  
Radio Emission ◽  
Galaxy Cluster ◽  
Extended Radio

scholarly journals The Intergalactic Medium

1982 ◽  
Vol 97 ◽  
pp. 453-459
Author(s):  
A. C. Fabian ◽  
A. K. Kembhavi
Keyword(s):  
Radio Emission ◽  
Intergalactic Medium ◽  
Radio Sources ◽  
Clusters Of Galaxies ◽  
Intergalactic Gas ◽  
X Ray ◽  
Extended Radio ◽  
X Ray Background ◽  
Ngc 1275 ◽  
Intracluster Gas

The density of intergalactic gas may be an important parameter in the formation of extended radio sources. It may range from ∼ 0.1 particle cm−3 in the centres of some rich clusters of galaxies down to 10−8cm−3 or less in intercluster space. The possible influence of the intracluster gas surrounding NGC 1275 on its radio emission is discussed, and the possibility that a significant fraction of the X-ray background is due to a hot intergalactic medium is explored in some detail.

scholarly journals Low-frequency radio emission in the massive galaxy cluster MACSJ0717.5 + 3745

2013 ◽  
Vol 557 ◽  
pp. A117 ◽  
Author(s):  
M. Pandey-Pommier ◽  
J. Richard ◽  
F. Combes ◽  
K. S. Dwarakanath ◽  
B. Guiderdoni ◽  
...  
Keyword(s):  
Radio Emission ◽  
Low Frequency ◽  
Galaxy Cluster ◽  
Massive Galaxy

scholarly journals Spectral and Polarization Signatures of Relativistic Shocks in Blazars

2016 ◽  
Author(s):  
Markus Boettcher
Keyword(s):  
Pitch Angle ◽  
High Energy ◽  
Shock Acceleration ◽  
Spectral Energy ◽  
Polarization Angle ◽  
Proton Synchrotron ◽  
Diffusive Shock Acceleration ◽  
X Ray ◽  
Relativistic Shocks ◽  
Multi Wavelength

Relativistic shocks are one of the most plausible sites of the emission of strongly variable, polarized multi-wavelength emission from relativistic jet sources such as blazars, via diffusive shock acceleration (DSA) of relativistic particles. This paper summarizes recent results on a self-consistent coupling of diffusive shock acceleration and radiation transfer in blazar jets. We demonstrate that the observed spectral energy distributions (SEDs) of blazars strongly constrain the nature of hydromagnetic turbulence responsible for pitch-angle scattering by requiring a strongly energy-dependent pitch-angle mean free path. The prominent soft X-ray excess (``Big Blue Bump'') in the SED of the BL Lac object AO 0235+164 can be modelled as the signature of bulk Compton scattering of external radiation fields by the thermal electron population, which places additional constraints on the level of hydromagnetic turbulence. It has further been demonstrated that internal shocks propagating in a jet pervaded by a helical magnetic field naturally produce polarization-angle swings by 180$^o$, in tandem with multi-wavelength flaring activity, without requiring any helical motion paths or other asymmetric jet structures. The specific application of this model to 3C279 presents the first consistent, simultaneous modeling of snap-shot SEDs, multi-wavelength light curves and time-dependent polarization signatures of a blazar during a polarization-angle (PA) rotation. This model has recently been generalized to a lepto-hadronic model, in which the high-energy emission is dominated by proton synchrotron radiation. It is shown that in this case, the high-energy (X-ray and $\gamma$-ray) polarization signatures are expected to be significantly more stable (not showing PA rotations) than the low-energy (electron-synchrotron) signatures.

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