Radio relics are diffuse, extended synchrotron sources located at the outskirts of merging galaxy clusters. Their origin has been linked with shock waves injected into the intracluster medium, but the acceleration mechanism at the shock front is still under debate. Some clusters, like MACS J1752.0+4440, host a double relic system, with two relics found on opposite sides with respect to the cluster center. To investigate the acceleration mechanism that generates radio relics, we study the morphological and spectral properties of the double relic system in MACS J1752. We present new wideband radio continuum observations made with uGMRT and JVLA, and LOFAR data. We perform a detailed, high-resolution spectral analysis of the double relic system in MACS J1752, observing and characterizing substructures, particularly for the brighter relic. We find a double-peaked surface brightness and spectral index profile for the NE relic and identify a "bright bar" substructure. Moreover, we observed surprisingly flat integrated spectral indices for both relics, at $α_{\mathrm{int}}^{\mathrm{NE}} = -0.91 \pm 0.06$ and $α_{\mathrm{int}}^{\mathrm{SW}} = -0.83 \pm 0.05$. We study the spatial variation of the spectral index, observing a coherent trend with the observed substructures. We estimate an injection Mach number of $\mathcal{M}_{\mathrm{NE}} = 3.1^{+0.1}_{-0.1}$ and $\mathcal{M}_{\mathrm{SW}} = 3.2^{+0.1}_{-0.1}$. By performing a spectral curvature analysis for both relics, generating color-color plots and a spectral curvature maps, we observe two "concave" spectra represented by positive spectral curvature, in contrast with particle population ageing models. The observed properties of the NE relic are not consistent with a simple scenario with a single shock front. Multiple shock surfaces, re-acceleration, and projection effects likely play a role in shaping the morphology of the relic.
The investigation of merging galaxy clusters that exhibit radio relics is strengthening our understanding of the formation and evolution of galaxy clusters, the nature of dark matter, the intracluster medium, and astrophysical particle acceleration. Each merging cluster provides only a single view of the cluster formation process and the variety of merging clusters is vast. Clusters hosting double radio relics are rare and extremely important because they allow tight constraints on the merger scenario. We present a weak-lensing and X-ray analysis of MACSJ1752.0+4440 ($z$=0.365) and ZWCL1856.8+…
To investigate the relationship between thermal and non-thermal components in merger galaxy clusters, we present deep JVLA and Chandra observations of the HST Frontier Fields cluster MACS J0717.5+3745. The Chandra image shows a complex merger event, with at least four components belonging to different merging subclusters. NW of the cluster, $\sim 0.7$ Mpc from the center, there is a ram-pressure-stripped core that appears to have traversed the densest parts of the cluster after entering the ICM from the direction of a galaxy filament to the SE. We detect a density discontinuity NNE of this cor…
Studies have shown that mergers of massive galaxy clusters produce shocks and turbulence in the intra-cluster medium, the possible event that creates radio relics, as well as the radio halos. Here we present GMRT dual-band (235 and 610~MHz) radio observations of four such clusters from the MAssive Cluster Survey (MACS) catalogue. We report the discovery of a very faint, diffuse, elongated radio source with a projected size of about 0.5~Mpc in cluster MACSJ0152.5-2852. We also confirm the presence of a radio relic-like source (about 0.4~Mpc, previously reported at 325~MHz) in MACSJ0025.4-1222 c…
We present results from LOFAR and GMRT observations of the galaxy cluster MACS$\,$J0717.5$+$3745. The cluster is undergoing a violent merger involving at least four sub-clusters, and it is known to host a radio halo. LOFAR observations reveal new sources of radio emission in the Intra-Cluster Medium: (i) a radio bridge that connects the cluster to a head-tail radio galaxy located along a filament of galaxies falling into the main cluster, (ii) a 1.9 Mpc radio arc, that is located North West of the main mass component, (iii) radio emission along the X-ray bar, that traces the gas in the X-rays …
The HST Frontier Fields cluster MACS J1149.6+2223 is one of the most complex merging clusters, believed to consist of four dark matter halos. We present results from deep (365 ks) Chandra observations of the cluster, which reveal the most distant cold front (z=0.544) discovered to date. In the cluster outskirts, we also detect hints of a surface brightness edge that could be the bow shock preceding the cold front. The substructure analysis of the cluster identified several components with large relative radial velocities, thus indicating that at least some collisions occur almost along the lin…
ICM-SHOX III. The case of MACS J0018.5+1626, a radio relic that looks like a radio halo?
Cahill Center for Astronomy and Astrophysics, California Institute of Technology（加州理工学院卡希尔天文与天体物理学中心）；Institute of Space Sciences (ICE-CSIC)（空间科学研究所（ICE-CSIC））；Istituto di Radio Astronomia, INAF（意大利国家天体物理研究所射电天文研究所）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）；Department of Astronomy, University of Massachusetts Amherst（马萨诸塞大学阿默斯特分校天文学系）
Exemplary Merging Clusters II: XMM-Newton and Chandra X-ray Observations of MACS J1752.0+4440 and ZwCl 1856.8+6616
Duke University（杜克大学）；California Institute of Technology（加州理工学院）；Max-Planck-Institut für Astronomie（马克斯·普朗克天文学研究所）；University of Maryland, Baltimore County (UMBC)（马里兰大学巴尔的摩分校）；NASA/Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）
Multi-Wavelength Signatures of a Giant Cometary Radio Halo in MACSJ0417-1154
National Centre for Radio Astrophysics, Tata Institute of Fundamental Research（塔塔基础研究所国家射电天体物理中心）；International Centre for Theoretical Sciences, Tata Institute of Fundamental Research（塔塔基础研究所国际理论科学中心）；Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna（博洛尼亚大学物理与天文系）；Istituto Nazionale di Astrofisica – Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF)（意大利国家天体物理研究所空间天体物理与宇宙物理学研究所）
Radio relics are diffuse radio sources observed in galaxy clusters, probably produced by shock acceleration during cluster-cluster mergers. Their large size, of the order of 1 Mpc, indicates that the emitting electrons need to be (re)accelerated locally. The usually invoked Diffusive Shock Acceleration models have been challenged by recent observations and theory. We report the discovery of complex radio emission in the Galaxy cluster PLCKG287.0 +32.9, which hosts two relics, a radio halo, and several radio filamentary emission. Optical observations suggest that the cluster is elongated, likel…
The galaxy cluster Abell 2256 hosts one of the most intriguing examples in the class of radio relics. It has been found that this radio relic has a rather flat integrated spectrum at low frequencies that would imply an injection spectral index for the electrons that is inconsistent with the flattest allowed by the test particle diffusive shock acceleration (DSA). We performed new high-frequency observations at 2273, 2640, and 4850 MHz. Combining these new observations with images available in the literature, we constrain the radio integrated spectrum of the radio relic in Abell 2256 over the w…
CIZA J2242.8+5301 is a post-core passage, binary merging cluster that hosts a large, thin, arc-like radio relic, nicknamed the `Sausage', tracing a relatively strong shock front. We perform spatially-resolved spectral fitting to the available radio data for this radio relic, using a variety of spectral ageing models, with the aim of finding a consistent set of parameters for the shock and radio plasma. We determine an injection index of $0.77^{+0.03}_{-0.02}$ for the relic plasma, significantly steeper than was found before. Standard particle acceleration at the shock front implies a Mach numb…
The results of long XMM-Newton X-ray observations of the NW radio relic of Abell 3667 are presented. A shock is detected at the sharp outer edge of the radio relic, both in the X-ray surface brightness and the temperature profiles. The Mach number is M = 2.54^+0.80_-0.43. The temperature jump at the shock is larger than expected from the density jump, which may indicate that a dynamically important magnetic field aligned primarily parallel to the shock front is present. The gas temperature rises gradually over several arc minutes within the shock region. This could indicate that the shock ener…
We present a multi-frequency study of radio relics associated with the galaxy clusters A4038, A1664 and A786. Radio images, integrated spectra, spectral index maps and fits to the integrated spectra in the framework of the adiabatic compression model are presented. Images of the relic in A4038 at 150, 240 and 606 MHz with the Giant Metrewave Radio Telescope have revealed extended ultra-steep spectrum (α~ -1.8 to -2.7) emission of an extent 210 x 80 kpc^2. The model of passively evolving radio lobes compressed by a shock best fits the integrated spectrum. The relic with a circular morphology at…
Abell 3667 is the archetype of a merging cluster with radio relics. The NW radio relic is the brightest cluster relic or halo known, and is believed to be due to a strong merger shock. We have observed the NW relic for 40 ksec of net XMM time. We observe a global decline of temperature across the relic from 6 to 1 keV, similar to the Suzaku results. Our new observations reveal a sharp change of both temperature and surface brightness near the position of the relic. The increased X-ray emission on the relic can be equivalently well described by either a thermal or nonthermal spectral model. The…
Radio relics in galaxy clusters are giant diffuse synchrotron sources powered in cluster outskirts by merger shocks. Although the relic-shock connection has been consolidated in recent years by a number of observations, the details of the mechanisms leading to the formation of relativistic particles in this environment are still not well understood. The diffusive shock acceleration (DSA) theory is a commonly adopted scenario to explain the origin of cosmic rays at astrophysical shocks, including those in radio relics in galaxy clusters. However, in a few specific cases it has been shown that t…
In this paper we address the question why cluster radio relics that are connected to shock acceleration, so-called radio gischt, have preferentially been found in the outskirts of galaxy clusters. By identifying merger shock waves in cosmological grid simulations, we explore several prescriptions for relating the energy dissipated in shocks to the energy emitted in the radio band. None of the investigated models produce detectable radio relics within 100-200 kpc from the cluster centre. All models cause > 50 per cent of the detectable relic emission at projected distances > 800 kpc. Central ra…
Radio relics, elongated, non-thermal, structures located at the edges of galaxy clusters, are the result of synchrotron radiation from cosmic-ray electrons accelerated by merger-driven shocks at the cluster outskirts. However, X-ray observations of such shocks in some clusters suggest that they are too weak to efficiently accelerate electrons via diffusive shock acceleration to energies required to produce the observed radio power. We examine this issue in the merging galaxy cluster Abell 3667 (A3667), which hosts a pair of radio relics. While the Northwest relic in A3667 has been well studied…
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 LOFAR. It is special since it has the lowest surface brightness of all known radio relics. We study its radio and X-ray properties combinig LOFAR data w…
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 …
Mergers between galaxy clusters often drive shocks into the intra cluster medium (ICM), the effects of which are sometimes visible via temperature and density jumps in the X-ray, and via radio emission from relativistic particles energized by the shock's passage. Abell2108 was selected as a likely merger system through comparing the X-ray luminosity to the Planck Sunyaev-Zeldovich signal, where this cluster appeared highly X-ray underluminous. Follow up observations confirmed it to be a merging low mass cluster featuring two distinct subclusters, both with a highly disturbed X-ray morphology. …
Recent radio observations have identified a class of structures, so-called radio relics, in clusters of galaxies. The radio emission from these sources is interpreted as synchrotron radiation from GeV electrons gyrating in microG-level magnetic fields. Radio relics, located mostly in the outskirts of clusters, seem to associate with shock waves, especially those developed during mergers. In fact, they seem to be good structures to identify and probe such shocks in intracluster media (ICMs), provided we understand the electron acceleration and re-acceleration at those shocks. In this paper, we …
We present an X-ray and radio study of the famous `El Gordo', a massive and distant ($z=0.87$) galaxy cluster. In the deep (340 ks) Chandra observation, the cluster appears with an elongated and cometary morphology, a sign of its current merging state. The GMRT radio observations at 610 MHz reveal the presence of a radio halo which remarkably overlaps the X-ray cluster emission and connects a couple of radio relics. We detect a strong shock ($\mathcal{M}\gtrsim3$) in the NW periphery of the cluster, co-spatially located with the radio relic. This is the most distant ($z=0.87$) and one of the s…
Radio relics in galaxy clusters are extended synchrotron sources produced by cosmic-ray electrons in the $μ$G magnetic field. Many relics are found in the cluster periphery and have a cluster-centric, narrow arc-like shape, which suggests that the electrons are accelerated or re-accelerated by merger shock fronts propagating outward in the intracluster plasma. In the X-ray, some relics do exhibit such shocks at the location of the relic, but many do not. We explore the possibility that radio relics trace not the shock fronts but the shape of the underlying distribution of seed relativistic ele…
With Australia Telescope Compact Array observations, we detect a highly elongated Mpc-scale diffuse radio source on the eastern periphery of the Bullet cluster 1E0657-55.8, which we argue has the positional, spectral and polarimetric characteristics of a radio relic. This powerful relic (2.3+/-0.1 x 10^25 W Hz^-1) consists of a bright northern bulb and a faint linear tail. The bulb emits 94% of the observed radio flux and has the highest surface brightness of any known relic. Exactly coincident with the linear tail we find a sharp X-ray surface brightness edge in the deep Chandra image of the …
Diffuse radio emission in the form of radio halos and relics has been found in a number of merging galaxy clusters. These structures indicate that shock and turbulence associated with the merger accelerate electrons to relativistic energies. We report the discovery of a radio relic + radio halo system in PSZ1 G108.18-11.53 (z=0.335). This cluster hosts the second most powerful double radio relic system ever discovered. We observed PSZ1 G108.18-11.53 with the Giant Meterwave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT). We obtained radio maps at 147, 323, 607 and 1…
It has often been thought that the northern radio relic in the galaxy cluster CIZA J2242.8+5301 (the "Sausage" Cluster) is associated with cosmic ray (CR) electrons that are accelerated at a shock through the diffusive shock acceleration (DSA) mechanism. However, recent radio observations have shown that the radio spectrum is curved, which is inconsistent with the prediction of a simple DSA model. Moreover, the CR electron spectrum before being affected by radiative cooling seems to be too hard for DSA. In this study, we show that these facts are natural consequences if the electrons are reacc…
We report the discovery of a putative radio relic, 830 kpc in length and found toward the outskirts of galaxy cluster Abell 1697 ($z=0.181$), using the LOFAR Two Meter Sky Survey (LoTSS) at 144 MHz. With an X-ray-inferred mass of $M_{500}^{X-ray}=2.9^{+0.8}_{-0.7}\times10^{14}~\rm{M_{\odot}}$, this places Abell 1697 among the least massive relic hosts. The relic is also detected at 325 MHz in the Westerbork Northern Sky Survey (WENSS) and at 1.4 GHz in the NRAO VLA Sky Survey (NVSS) with an average spectral index of $α(144,325,1400~\rm{MHz})=-0.98\pm0.01$ and magnetic field of $B_{eq}\sim0.6~μ…
Giant radio relics are the arc-shaped diffuse radio emission regions observed in the outskirts of some merging galaxy clusters. They are believed to trace shock-waves in the intra-cluster medium. Recent observations demonstrated that some prominent radio relics exhibit a steepening above 2 GHz in their radio spectrum. This challenges standard theoretical models because shock acceleration is expected to accelerate electrons to very high energies with a power-law distribution in momentum. In this work we attempt to reconcile these data with the shock-acceleration scenario. We propose that the sp…
Radio relics are Mpc-scale diffuse radio sources at the peripheries of galaxy clusters which are thought to trace outgoing merger shocks. We present XMM-Newton and Suzaku observations of the galaxy cluster Abell 2744 (z=0.306), which reveal the presence of a shock front 1.5 Mpc East of the cluster core. The surface-brightness jump coincides with the position of a known radio relic. Although the surface-brightness jump indicates a weak shock with a Mach number $\mathcal{M}=1.7_{-0.3}^{+0.5}$, the plasma in the post-shock region has been heated to a very high temperature ($\sim13$ keV) by the pa…
Double radio relics in galaxy clusters are rare phenomena that trace shocks in the outskirts of merging galaxy clusters. We have carried out a spectral and polarization study of the spectacular double relics in the galaxy cluster A3376 using the Giant Metrewave Radio Telescope at 150 and 325 MHz and the Very Large Array at 1400 MHz. The polarization study at 1400 MHz reveals a high degree of polarization (~30%) and aligned magnetic field vectors (not corrected for Faraday rotation) in the eastern relic. A highly polarized (>60%) filamentary radio source of size ~300 kpc near the eastern relic …
Zooming in on radio relics -- II. How relic morphology probes density fluctuations at the edge of galaxy clusters
Leibniz-Institut für Astrophysik Potsdam (AIP)（波茨坦莱布尼茨天体物理研究所）；Max-Planck-Institut für Astrophysik（马克斯·普朗克天体物理研究所）；Universität Potsdam（波茨坦大学）；Universität Heidelberg（海德堡大学）
Some galaxy clusters show diffuse radio emission in the form of peripheral relics (so far attributed to primary, shock-(re)accelerated electrons) or central halos. Analysing radio and X-ray data from the literature, we find new connections between halos and relics, such as a universal linear relation between their peak radio brightness and the gas column density. Our results indicate that halos, relics, and halo-relic bridges in a cluster, all arise from the same, homogeneous cosmic ray (CR) ion (CRI) distribution. We analytically derive the signature of synchrotron emission from secondary ele…
Mega-parsec scale radio sources in the form of halos and relics are often detected in dynamically disturbed galaxy clusters. Although they are associated with merger-induced turbulence and shocks, respectively, their formation is not fully understood. We aim to identify the mechanisms responsible for particle acceleration and magnetic field amplification in the halo and relics of the galaxy cluster ClG 0217+70. We observed ClG 0217+70 with LOFAR at 141 MHz and VLA at 1.5 GHz, and combine these observations with VLA 1.4 GHz archival data to study the morphological and spectral properties of the…
We present the results of Suzaku observations of the galaxy cluster 1RXS J0603.3+4214 with "toothbrush" radio relic. Although a shock with Mach number $M \simeq 4$ is expected at the outer edge of the relic from the radio observation, our temperature measurements of the intracluster medium indicate a weaker temperature difference than what is expected. The Mach number estimated from the temperature difference at the outer edge of the relic is $M \simeq 1.5$, which is significantly lower than the value estimated from the radio data even considering both statistical and systematic errors. This s…
We present low-frequency observations with the Giant Metrewave Radio Telescope (GMRT) of three giant radio sources (GRSs, J0139+3957, J0200+4049 and J0807+7400) with relaxed diffuse lobes which show no hotspots and no evidence of jets. The largest of these three, J0200+4049, exhibits a depression in the centre of the western lobe, while J0139+3957 and J0807+7400 have been suggested earlier by Klein et al. and Lara et al. respectively to be relic radio sources. We estimate the ages of the lobes. We also present Very Large Array (VLA) observations of the core of J0807+7400, and determine the cor…
We present an X-ray spectral analysis of the nearby double radio relic merging cluster Abell 3376 ($z$ = 0.046), observed with the $Suzaku$ XIS instrument. These deep ($\sim$360 ks) observations cover the entire double relic region in the outskirts of the cluster. These diffuse radio structures are amongst the largest and arc-shaped relics observed in combination with large-scale X-ray shocks in a merging cluster. We confirm the presence of a stronger shock (${\cal M}_{\rm{W}}$ = 2.8 $\pm~0.4$) in the western direction at $r\sim26$', derived from a temperature and surface brightness discontinu…
We search for extended regions of radio emission not associated with Active Galactic Nuclei, known as 'relics', 'halos' and 'mini halo's, in a sample of 70 Abell clusters for which we have radio, optical and X-ray data. AGN can produce particle bubbles of non-thermal emission, which can restrict cosmic rays. Hence radio relics and (mini) halos could be forming as a result of the confinement of cosmic rays by these bubbles. We are probing the role that intracluster mag- netic fields (using Faraday rotation measure and inverse compton arguments), mergers (through radio/X-ray interactions), cooli…
Radio relics in galaxy clusters are thought to be associated with powerful shock waves that accelerate particles via diffusive shock acceleration (DSA). Among the particles accelerated by DSA, relativistic protons should outnumber electrons by a large factor. While the relativistic electrons emit synchrotron emission detectable in the radio band, the protons interact with the thermal gas to produce gamma-rays in hadronic interactions. Using simple models for the propagation of shock waves through clusters, the distribution of thermal gas and the efficiency of DSA, we find that the resulting ha…
In the past few years, some studies claimed that annihilating dark matter with mass $\sim 10-100$ GeV can explain the GeV gamma-ray excess in our Galaxy. However, recent analyses of the Fermi-LAT and radio observational data rule out the possibility of the thermal relic annihilating dark matter with mass $m \le 100$ GeV for some popular annihilation channels. By using the new observed radio data of the Andromeda galaxy, we rule out the existence of $\sim 100-300$ GeV thermal relic annihilating dark matter for ten annihilation channels. The lower limits of annihilating dark matter mass are impr…
Radio relics are vast synchrotron sources that sit on the outskirts of merging galaxy clusters. In this work we model their formation using a Press-Schechter formalism to simulate merger rates, analytical models for the intracluster medium and the shock dynamics, as well as a simple model for the cosmic-ray electrons at the merger shocks. We show that the statistical properties of the population of radio relics are strongly dependent on key physical parameters, such as the acceleration efficiency, the magnetic field strength at the relic, the geometry of the relic and the duration of the elect…
We report the discovery of large scale diffuse non-thermal radio emission in PLCK G287.0+32.9, an exceptionally hot (T ~ 13 keV), massive and luminous galaxy cluster, strongly detected by the PLANCK satellite in a recent, all-sky blind search for new clusters through Sunyaev-Zel'dovich effect. Giant Metrewave Radio telescope 150 MHz and Very Large Array 1.4 GHz radio data reveal a pair of giant (>1 Mpc) "arc" shaped peripheral radio-relics (signatures of shock waves) of unprecedented scale (linear separation ~4.4 Mpc at redshift 0.39), located at distances from the cluster center which are abo…
Radio relics are arc-like synchrotron sources at the periphery of galaxy clusters, produced by cosmic-ray electrons in a $μ$G magnetic field which are believed to have been (re-)accelerated by merger shock fronts. However, not all relics appear at the same location as shocks as seen in the X-ray. In a previous work, we suggested that the shape of some relics may result from the pre-existing spatial distribution of cosmic-ray electrons, and tested this hypothesis using simulations by launching AGN jets into a cluster atmosphere with sloshing gas motions generated by a previous merger event. We …
We analyzed a deep Chandra observation (334 ks) of the galaxy cluster Abell 115 and detected a shock co-spatial with the radio relic. The X-ray surface brightness profile across the shock region presents a discontinuity, corresponding to a density compression factor $\mathcal{C}=2.0\pm0.1$ leading to a Mach number $\mathcal{M}=1.7\pm0.1$ ($\mathcal{M}=1.4-2$ including systematics). Temperatures measured in the upstream and downstream regions are consistent with what expected for such a shock: $T_u=4.3^{+1.0}_{-0.6}\:\rm{keV}$ and $T_d=7.9^{+1.4}_{-1.1}\:\rm{keV}$ respectively, implying a Mach …
We present LOw Frequency ARray observations of the Coma cluster field at 144\,MHz. The cluster hosts one of the most famous radio halos, a relic, and a low surface-brightness bridge. We detect new features that allow us to make a step forward in the understanding of particle acceleration in clusters. The radio halo extends for more than 2 Mpc, which is the largest extent ever reported. To the North-East of the cluster, beyond the Coma virial radius, we discover an arc-like radio source that could trace particles accelerated by an accretion shock. To the West of the halo, coincident with a shoc…
We discuss a new technique to constrain models for the origin of radio relics in galaxy clusters using the correlation between the shock Mach number and the radio power of relics. This analysis is carried out using a sample of relics with information on both the Mach numbers derived from X-ray observation, $\mathcal{M}_X$, and using spectral information from radio observations of the peak and the average values of the spectral index along the relic, $\mathcal{M}_R$. We find that there is a lack of correlation between $\mathcal{M}_X$ and $\mathcal{M}_R$; this result is an indication that the sp…
We explore the shock acceleration model for giant radio relics, in which relativistic electrons are accelerated via diffusive shock acceleration (DSA) by merger-driven shocks in the outskirts of galaxy clusters. In addition to DSA, turbulent acceleration by compressive MHD mode downstream of the shock is included as well as energy losses of postshock electrons by Coulomb scattering, synchrotron emission, and inverse Compton scattering off the cosmic background radiation. Considering that only a small fraction of merging clusters host radio relics, we favor the reacceleration scenario in which …
We present 610 MHz and 2.1 GHz imaging of the massive SZE-selected z=0.870 cluster merger ACT-CL J0102-4915 (El Gordo), obtained with the GMRT and the ATCA, respectively. We detect two complexes of radio relics separated by 3.4' (1.6 Mpc) along the system's NW-to-SE collision axis that have high integrated polarizations (33%) and steep spectral indices, consistent with creation via Fermi acceleration by shocks in the ICM. From the spectral index of the relics, we compute a Mach number of 2.5^{+0.7}_{-0.3} and shock speed of 2500^{+400}_{-300} km/s. With our ATCA data, we compute the Faraday de…
Radio relics are giant ($\sim$Mpc) synchrotron sources that are believed to be produced by the (re)acceleration of cosmic-ray electrons (CRe) by shocks in the intracluster medium. In this numerical study, we focus on the possibility that some radio relics may arise when electrons undergo diffusive shock acceleration at multiple shocks in the outskirts of merging galaxy clusters. This multi-shock (MS) scenario appears viable to produce CRe that emit visible synchrotron emission. We show that electrons that have been shocked multiple times develop an energy spectrum that significantly differs fr…
The Toothbrush radio relic associated the merging cluster 1RXS J060303.3 is presumed to be produced by relativistic electrons accelerated at merger-driven shocks. Since the shock Mach number inferred from the observed radio spectral index, $M_{radio}\approx 2.8$, is larger than that estimated from X-ray observations, $M_{X-ray}\lesssim 1.5$, we consider the re-acceleration model in which a weak shock of $M_s\approx 1.2-1.5$ sweeps through the intracluster plasma with a preshock population of relativistic electrons. We find the models with a power-law momentum spectrum with the slope, $s\approx…
