Re-acceleration Of Electrons Research Articles

Energy depletion and re-acceleration of driver electrons in a plasma-wakefield accelerator

For plasma-wakefield accelerators to fulfill their potential for cost effectiveness, it is essential that their energy-transfer efficiency be maximized. A key aspect of this efficiency is the near-complete transfer of energy, or depletion, from the driver electrons to the plasma wake. Achieving full depletion is limited by the process of re-acceleration, which occurs when the driver electrons decelerate to nonrelativistic energies, slipping backward into the accelerating phase of the wakefield and being subsequently re-accelerated. Such re-acceleration is unambiguously observed here for the first time. At this re-acceleration limit, we measure a beam driver depositing (57 ± 3)% of its energy into a 195-mm-long plasma. This suggests that the energy-transfer efficiency of plasma accelerators could approach that of conventional accelerators. Published by the American Physical Society 2024

Abell 0399–Abell 0401 radio bridge spectral index: First multi-frequency detection

Aims. Recent low-frequency radio observations at 140 MHz discovered a bridge of diffuse emission with a length of 3 Mpc that connects the galaxy clusters Abell 0399 and Abell 0401. We present follow-up observations at 60 MHz to constrain the spectral index of the bridge, which has only been detected at 140 and 144 MHz so far. Methods. We analysed deep (∼18 h) LOw Frequency ARray (LOFAR) Low Band Antenna (LBA) data at 60 MHz to detect the bridge at very low frequencies. We then conducted a multi-frequency study with LOFAR HBA data at 144 MHz and uGMRT data at 400 MHz. Assuming second-order Fermi mechanisms for the re-acceleration of relativistic electrons driven by turbulence in the radio bridge regions, we compared the observed radio spectrum with theoretical synchrotron models. Results. The bridge is detected in the 75″ resolution LOFAR image at 60 MHz, and its emission fully connects the region between the two galaxy clusters. Between 60 MHz and 144 MHz, we found an integrated spectral index value of α60144 = −1.44 ± 0.16 for the bridge emission. For the first time, we produced spectral index and related uncertainties maps for a radio bridge. We produce a radio spectrum that shows a significant steepening between 144 and 400 MHz. Conclusions. This detection at low frequencies provides important information for models of particle acceleration and magnetic field structure on very extended scales. The spectral index gives important clues about the origin of inter-cluster diffuse emission. The steepening of the spectrum above 144 MHz can be explained in a turbulent re-acceleration framework, assuming that the acceleration timescales are longer than ∼200 Myr.

A multishock scenario for the formation of radio relics

ABSTRACT Radio relics are giant sources of diffuse synchrotron radio emission in the outskirts of galaxy clusters that are associated with shocks in the intracluster medium. Still, the origin of relativistic particles that make up relics is not fully understood. For most relics, diffusive shock acceleration (DSA) of thermal electrons is not efficient enough to explain observed radio fluxes. In this paper, we use a magnetohydrodynamic simulation of galaxy clusters in combination with Lagrangian tracers to simulate the formation of radio relics. Using a Fokker–Planck solver to compute the energy spectra of relativistic electrons, we determine the synchrotron emission of the relic. We find that re-acceleration of fossil electrons plays a major role in explaining the synchrotron emission of radio relics. Particles that pass through multiple shocks contribute significantly to the overall luminosity of a radio relic and greatly boost the effective acceleration efficiency. Furthermore, we find that the assumption that the luminosity of a radio relic can be explained with DSA of thermal electrons leads to an overestimate of the acceleration efficiency by a factor of more than 103.

High-energy Neutrino Constraints on Cosmic-Ray Reacceleration in Radio Halos of Massive Galaxy Clusters

A fraction of merging galaxy clusters host diffuse radio emission in their central region, termed a giant radio halo (GRH). The most promising mechanism of GRHs is the reacceleration of nonthermal electrons and positrons by merger-induced turbulence. However, the origin of these seed leptons has been under debate, and either protons or electrons can be primarily accelerated particles. In this work, we demonstrate that neutrinos can be used as a probe of physical processes in galaxy clusters and discuss possible constraints on the number of relativistic protons in the intracluster medium with the existing upper limits by IceCube. We calculate radio and neutrino emission from massive (>1014 M ⊙) galaxy clusters using the cluster population model of Nishiwaki & Asano. This model is compatible with the observed statistics of GRHs, and we find that the contribution of GRHs to the isotropic radio background observed with the ARCADE-2 experiment should be subdominant. Our fiducial model predicts the all-sky neutrino flux that is consistent with IceCube's upper limit from the stacking analysis. We also show that the neutrino upper limit gives meaningful constraints on the parameter space of the reacceleration model, such as the electron-to-proton ratio of the primary cosmic rays and the magnetic field; in particular, the secondary scenario, where the seed electrons mostly originate from inelastic pp collisions, can be constrained even in the presence of reacceleration.

The Planck clusters in the LOFAR sky

Context. Diffuse cluster-scale synchrotron radio emission is discovered in an increasing number of galaxy clusters in the form of radio haloes, probing the presence of relativistic electrons and magnetic fields in the intra-cluster medium (ICM). The favoured scenario to explain their origin is that they trace turbulent regions that are generated during cluster-cluster mergers, where particles are re-accelerated. In this framework, radio haloes are expected to probe cluster dynamics and are predicted to be more frequent in massive systems, in which more energy becomes available for the re-acceleration of relativistic electrons. For these reasons, statistical studies of galaxy cluster samples have the power to derive fundamental information on the radio haloes populations and on their connection with cluster dynamics, and hence to constrain theoretical models. Furthermore, low-frequency cluster surveys have the potential to shed light on the existence of radio haloes with very steep radio spectra, which are a key prediction of turbulent models and are thought to be generated in less energetic merger events and thus be more common in the Universe. Aims. The main question we address is whether we can explain the observed properties of the radio halo population within the framework of current models. Methods. We study the occurrence and properties of radio haloes from clusters of the second catalogue of Planck Sunyaev Zel’dovich-detected sources that lie within the 5634 deg2 that are covered by the second data release (DR2) of the LOFAR Two-meter Sky Survey. We derive their integral number, flux density, and redshift distributions. We compare these observations with expectations of theoretical models. We also study the connection between radio haloes and cluster mergers by using cluster morphological parameters derived through Chandra and/or XMM-Newton data. Results. We find that the number of observed radio haloes, their radio flux density, and their redshift distributions agree with what is expected in the framework of the re-acceleration scenario. In line with model expectations, the fraction of clusters with radio haloes increases with the cluster mass, confirming the leading role of the gravitational process of cluster formation in the generation of radio haloes. These models predict a large fraction of radio haloes with very steep spectra in the DR2 Planck sample. This will be tested in future studies, but a comparison of the occurrence of haloes in GMRT and LOFAR samples indeed shows a more frequent occurrence of haloes at lower frequencies, suggesting the presence of a population of haloes with very steep spectra that is preferentially detected by LOFAR. Using morphological information, we confirm that radio haloes are preferentially located in merging systems, and that the fraction of newly LOFAR-discovered radio haloes is larger in less strongly disturbed systems.

Statistical Properties of Radio Halos in Galaxy Clusters and the Origin of Seed Electrons for Reacceleration

One of the most promising mechanisms for producing radio halos (RHs) in galaxy clusters is the reacceleration of cosmic-ray electrons by turbulence. However, the origin of the seed electrons for reacceleration is still poorly constrained. In the secondary scenario, most of the seed electrons are injected via collision of proton cosmic-rays, while nonthermal electrons are directly injected in the primary scenario. In this paper, we examine the two scenarios for seed electrons with the observed statistical properties of RHs by combining two methods: by following the temporal evolutions of the electron energy and the radial distributions in a cluster, as well as the merger history of clusters. We find that the RH lifetime largely depends on the seed origin, as it could be longer than the cosmological timescale in the secondary scenario. We study the condition for the onset of RHs with the observed RH fraction and the RH lifetime we obtained and find that long-lived RHs in the secondary scenario should originate from major mergers with a mass ratio of ξ ∼ 0.1, while the short lifetime in the primary scenario requires more frequent onsets from minor mergers with ξ ∼ 0.01. Our simple model of the turbulence acceleration can reproduce the observed radio luminosity–mass relation. The RH luminosity functions we obtained suggest that the expected RH number count with the ASKAP survey will detect ≈103 RHs in both scenarios.

Particle re-acceleration and diffuse radio sources in the galaxy cluster Abell 1550

Context. Radio observations of galaxy clusters reveal a plethora of diffuse, steep-spectrum sources related to the re-acceleration of cosmic-ray electrons, such as halos, relics, and phoenices. In this context, the LOw Frequency ARray Low-Band Antenna (LOFAR-LBA) Sky Survey (LoLSS) provides the most sensitive images of the sky at 54 MHz to date, allowing us to investigate re-acceleration processes in a poorly explored frequency regime. Aims. We study diffuse radio emission in the galaxy cluster Abell 1550, with the aim of constraining particle re-acceleration in the intra-cluster medium. Methods. We exploited observations at four different radio frequencies: 54, 144, 400, and 1400 MHz. To complement our analysis, we made use of archival Chandra X-ray data. Results. At all frequencies we detect an ultra-steep spectrum radio halo (Sν ∝ ν−1.6) with an extent of ∼1.2 Mpc at 54 MHz. Its morphology follows the distribution of the thermal intra-cluster medium inferred from the Chandra observation. West of the centrally located head-tail radio galaxy, we detect a radio relic with a projected extent of ∼500 kpc. From the relic, a ∼600 kpc long bridge departs and connects with the halo. Between the relic and the radio galaxy, we observe what is most likely a radio phoenix, given its curved spectrum. The phoenix is connected to the tail of the radio galaxy through two arms, which show a nearly constant spectral index for ∼300 kpc. Conclusions. The halo could be produced by turbulence induced by a major merger, with the merger axis lying in the NE-SW direction. This is supported by the position of the relic, whose origin could be attributed to a shock propagating along the merger axis. It is possible that the same shock has also produced the phoenix through adiabatic compression, while we propose that the bridge could be generated by electrons which were pre-accelerated by the shock, and then re-accelerated by turbulence. Finally, we detect hints of gentle re-energisation in the two arms that depart from the tail of the radio galaxy.

Turbulent magnetic fields in the merging galaxy cluster MACS J0717.5+3745

We present wideband (1 − 6.5 GHz) polarimetric observations, obtained with the Karl G. Jansky Very Large Array, of the merging galaxy cluster MACS J0717.5+3745, which hosts one of the most complex known radio relic and halo systems. We used both rotation measure synthesis and QU-fitting to find a reasonable agreement of the results obtained with these methods, particularly when the Faraday distribution is simple and the depolarization is mild. The relic is highly polarized over its entire length (850 kpc), reaching a fractional polarization > 30% in some regions. We also observe a strong wavelength-dependent depolarization for some regions of the relic. The northern part of the relic shows a complex Faraday distribution, suggesting that this region is located in or behind the intracluster medium (ICM). Conversely, the southern part of the relic shows a rotation measure very close to the Galactic foreground, with a rather low Faraday dispersion, indicating very little magnetoionic material intervening along the line of sight. Based on a spatially resolved polarization analysis, we find that the scatter of Faraday depths is correlated with the depolarization, indicating that the tangled magnetic field in the ICM causes the depolarization. We conclude that the ICM magnetic field could be highly turbulent. At the position of a well known narrow-angle-tailed galaxy (NAT), we find evidence of two components that are clearly separated in the Faraday space. The high Faraday dispersion component seems to be associated with the NAT, suggesting the NAT is embedded in the ICM while the southern part of the relic lies in front of it. If true, this implies that the relic and this radio galaxy are not necessarily physically connected and, thus, the relic may, in fact, not be powered by the shock re-acceleration of fossil electrons from the NAT. The magnetic field orientation follows the relic structure indicating a well-ordered magnetic field. We also detected polarized emission in the halo region; however, the absence of significant Faraday rotation and a low value of Faraday dispersion suggests the polarized emission that was previously considered as the part of the halo does, in fact, originate from the shock(s).

Particle Reacceleration by Turbulence and Radio Constraints on Multimessenger High-energy Emission from the Coma Cluster

Abstract Galaxy clusters are considered to be gigantic reservoirs of cosmic rays (CRs). Some of the clusters are found with extended radio emission, which provides evidence for the existence of magnetic fields and CR electrons in the intra-cluster medium. The mechanism of radio halo (RH) emission is still under debate, and it has been believed that turbulent reacceleration plays an important role. In this paper, we study the reacceleration of CR protons and electrons in detail by numerically solving the Fokker–Planck equation, and show how radio and gamma-ray observations can be used to constrain CR distributions and resulting high-energy emission for the Coma cluster. We take into account the radial diffusion of CRs and follow the time evolution of their one-dimensional distribution, by which we investigate the radial profile of the CR injection that is consistent with the observed RH surface brightness. We find that the required injection profile is nontrivial, depending on whether CR electrons have a primary or secondary origin. Although the secondary CR electron scenario predicts larger gamma-ray and neutrino fluxes, it is in tension with the observed RH spectrum for hard injection indexes, α < 2.45. This tension is relaxed if the turbulent diffusion of CRs is much less efficient than the fiducial model, or the reacceleration is more efficient for lower-energy CRs. In both the secondary and primary scenario, we find that galaxy clusters can make a sizable contribution to the all-sky neutrino intensity if the CR energy spectrum is nearly flat.

Two striking head–tail galaxies in the galaxy cluster IIZW108: insights into transition to turbulence, magnetic fields, and particle re-acceleration

ABSTRACT We present deep Jansky Very Large Array observations at 1.4 and 2.7 GHz (full polarization), as well as optical OmegaWINGS/WINGS and X-ray observations of two extended radio galaxies in the IIZW108 galaxy cluster at z = 0.04889. They show a bent tail morphology in agreement with a radio lobed galaxy falling into the cluster potential. Both galaxies are found to possess properties comparable with narrow-angle tail galaxies in the literature even though they are part of a low mass cluster. We find a spectral index steepening and an increase in fractional polarization through the galaxy jets and an ordered magnetic field component mostly aligned with the jet direction. This is likely caused by either shear due to the velocity difference of the intracluster medium and the jet fluid and/or magnetic draping of the intracluster medium across the galaxy jets. We find clear evidence that one source is showing two active galactic nuclei (AGN) outbursts from which we expect the AGN has never turned off completely. We show that pure standard electron cooling cannot explain the jet length. We demonstrate therefore that these galaxies can be used as a laboratory to study gentle re-acceleration of relativistic electrons in galaxy jets via transition from laminar to turbulent motion.

K-α x-ray measurements and their applicability for fast electron generation and transport studies in ultrashort intense laser foil interaction

K-α x-ray measurements using a high-resolution highly annealed pyrolytic graphite crystal spectrograph have been performed for ultrashort intense laser foil interaction. Experiments were performed using Ti:sapphire laser of 25 fs duration, intensity of 1 × 1019 W cm−2 and 7 µm Cu foil target. Using the observed laser-to-x-ray conversion, the fast electron conversion efficiency was estimated to be ∼1%. Intensity scaling of x-ray conversion was used to establish the role of the J × B mechanism of fast electron generation. Furthermore, through parametric studies, the role of refluxing and reacceleration of electrons in x-ray generation is also established. Finally, the role of polarization (p vs. s) is also observed and briefly discussed.

The LOFAR and JVLA view of the distant steep spectrum radio halo in MACS J1149.5+2223

Context.Radio halos and relics are Mpc-scale diffuse radio sources in galaxy clusters, which have a steep spectral indexα > 1 (defined asS ∝ ν−α). It has been proposed that halos and relics arise from particle acceleration induced by turbulence and weak shocks that are injected into the intracluster medium (ICM) during mergers.Aims.MACS J1149.5+2223 is a high-redshift (z = 0.544) galaxy cluster possibly hosting a radio halo and a relic. We analysed LOw Frequency Array (LOFAR), Giant Metrewave Radio Telescope, andKarl G. JanskyVery Large Array (JVLA) radio data at 144, 323, and 1500 MHz, respectively. In addition, we analysed archivalChandraX-ray data to characterise the thermal and non-thermal properties of the cluster.Methods.We obtained radio images at different frequencies to investigate the spectral properties of the radio halo. We usedChandraX-ray images to constrain the thermal properties of the cluster and to search for discontinuities (due to cold fronts or shock fronts) in the surface brightness of the ICM. By combining radio and X-ray images, we carried out a point-to-point analysis to study the connection between the thermal and non-thermal emission.Results.We measured a steep spectrum of the halo, which can be described by a power-law withα = 1.49 ± 0.12 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. The derived correlation shows a sub-linear slope in the range 0.4–0.6. 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.Conclusions.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 (∼1 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 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.

γ-ray detection toward the Coma cluster withFermi-LAT: Implications for the cosmic ray content in the hadronic scenario

The presence of relativistic electrons within the diffuse gas phase of galaxy clusters is now well established, thanks to deep radio observations obtained over the last decade, but their detailed origin remains unclear. Cosmic ray protons are also expected to accumulate during the formation of clusters. They may explain part of the radio signal and would lead toγ-ray emission through hadronic interactions within the thermal gas. Recently, the detection ofγ-ray emission has been reported toward the Coma cluster withFermi-LAT. Assuming that thisγ-ray emission arises essentially from pion decay produced in proton-proton collisions within the intracluster medium (ICM), we aim at exploring the implication of this signal on the cosmic ray content of the Coma cluster and comparing it to observations at other wavelengths. We use theMINOTsoftware to build a physical model of the Coma cluster, which includes the thermal target gas, the magnetic field strength, and the cosmic rays, to compute the corresponding expectedγ-ray signal. We apply this model to theFermi-LAT data using a binned likelihood approach, together with constraints from X-ray and Sunyaev-Zel’dovich observations. We also consider contamination from compact sources and the impact of various systematic effects on the results. We confirm that a significantγ-ray signal is observed within the characteristic radiusθ500of the Coma cluster, with a test statistic TS ≃ 27 for our baseline model. The presence of a possible point source (4FGL J1256.9+2736) may account for most of the observed signal. However, this source could also correspond to the peak of the diffuse emission of the cluster itself as it is strongly degenerate with the expected ICM emission, and extended models match the data better. Given theFermi-LAT angular resolution and the faintness of the signal, it is not possible to strongly constrain the shape of the cosmic ray proton spatial distribution when assuming an ICM origin of the signal, but preference is found in a relatively flat distribution elongated toward the southwest, which, based on data at other wavelengths, matches the spatial distribution of the other cluster components well. Assuming that the wholeγ-ray signal is associated with hadronic interactions in the ICM, we constrain the cosmic ray to thermal energy ratio withinR500toXCRp= 1.79−0.30+1.11% and the slope of the energy spectrum of cosmic rays toα= 2.80−0.13+0.67(XCRp= 1.06−0.22+0.96% andα= 2.58−0.09+1.12when including both the cluster and 4FGL J1256.9+2736 in our model). Finally, we compute the synchrotron emission associated with the secondary electrons produced in hadronic interactions assuming steady state. This emission is about four times lower than the overall observed radio signal (six times lower when including 4FGL J1256.9+2736), so that primary cosmic ray electrons or reacceleration of secondary electrons is necessary to explain the total emission. We constrain the amplitude of the primary to secondary electrons, or the required boost from reacceleration with respect to the steady state hadronic case, depending on the scenario, as a function of radius. Our results confirm thatγ-ray emission is detected in the direction of the Coma cluster. Assuming that the emission is due to hadronic interactions in the intracluster gas, they provide the first quantitative measurement of the cosmic ray proton content in a galaxy cluster and its implication for the cosmic ray electron populations.

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

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.

Second-order Fermi Reacceleration Mechanisms and Large-Scale Synchrotron Radio Emission in Intracluster Bridges.

Radio observations at low frequencies with the low frequency array (LOFAR) start discovering gigantic radio bridges connecting pairs of massive galaxy clusters. These observations probe unexplored mechanisms of insitu particle acceleration that operate on volumes of several Mpc^{3}. Numerical simulations suggest that such bridges are dynamically complex and that weak shocks and super-Alfvénic turbulence can be driven across the entire volume of these regions. In this Letter, we explore, for the first time, the role of second-order Fermi mechanisms for the reacceleration of relativistic electrons interacting with turbulence in these peculiar regions. We assume the turbulent energy flux measured in simulations and adopt a scenario in which relativistic particles scatter with magnetic field lines diffusing in super-Alfvénic turbulence and magnetic fields are amplified by the same turbulence. We show that steep spectrum and volume filling synchrotron emission can be generated in the entire intracluster bridge region, thus providing a natural explanation for radio bridges. Consequently, radio observations have the potential to probe the dissipation of energy on scales larger than galaxy clusters and second-order Fermi mechanisms operating in physical regimes that are still poorly explored. This has a potential impact on several branches of astrophysics and cosmology.

Chandra Observations of the Spectacular A3411–12 Merger Event

Abstract We present deep Chandra observations of A3411–12, a remarkable merging cluster that hosts the most compelling evidence for electron reacceleration at cluster shocks to date. Using the scaling relation, we find r 500 ∼ 1.3 Mpc, , , and a gas mass of . The gas mass fraction within r 500 is . We compute the shock strength using density jumps to conclude that the Mach number of the merging subcluster is small ( ). We also present density, temperature, pseudo-pressure, and pseudo-entropy maps. Based on the pseudo-entropy map, we conclude that the cluster is undergoing a mild merger, consistent with the small Mach number. On the other hand, radio relics extend over Mpc scale in the A3411–12 system, which strongly suggests that a population of energetic electrons already existed over extended regions of the cluster.

Particle re-acceleration and Faraday-complex structures in the RXC J1314.4-2515 galaxy cluster

Abstract Radio relics are sites of electron (re)acceleration in merging galaxy clusters but the mechanism of acceleration and the topology of the magnetic field in and near relics are yet to be understood. We are carrying out an observational campaign on double relic galaxy clusters starting with RXC J1314.4-2515. With Jansky Very Large Array multi-configuration observations in the frequency range 1-4 GHz, we perform both spectral and polarization analyses, using the Rotation Measure synthesis technique. We use archival XMM-Newton observations to constrain the properties of the shocked region. We discover a possible connection between the activity of a radio galaxy and the emission of the eastern radio relic. In the northern elongated arc of the western radio relic, we detect polarized emission with an average polarization fraction of 31 % at 3 GHz and we derive the Mach number of the underlying X-ray shock. Our observations reveal low levels of fractional polarization and Faraday-complex structures in the southern region of the relic, which point to the presence of thermal gas and filamentary magnetic field morphology inside the radio emitting volume. We measured largely different Rotation Measure dispersion from the two relics. Finally, we use cosmological magneto-hydrodynamical simulations to constrain the magnetic field, viewing angle, and to derive the acceleration efficiency of the shock. We find that the polarization properties of RXC J1314.4-2515 are consistent with a radio relic observed at 70○ with respect to the line of sight and that efficient re-acceleration of fossil electrons has taken place.

Expanding the Sample of Radio Minihalos in Galaxy Clusters

Radio minihalos are diffuse synchrotron sources of unknown origin found in the cool cores of some galaxy clusters. We use GMRT and VLA data to expand the sample of minihalos by reporting three new minihalo detections (A 2667, A 907 and PSZ1 G139.61+24.20) and confirming minihalos in five clusters (MACS J0159.8-0849, MACS J0329.6-0211, RXC J2129.6+0005, AS 780 and A 3444). With these new detections and confirmations, the sample now stands at 23, the largest sample to date. For consistency, we also reanalyze archival VLA 1.4 GHz observations of 7 known minihalos. We revisit possible correlations between the non-thermal emission and the thermal properties of their cluster hosts. Consistently with our earlier findings from a smaller sample, we find no strong relation between the minihalo radio luminosity and the total cluster mass. Instead, we find a strong positive correlation between the minihalo radio power and X-ray bolometric luminosity of the cool core (r<70 kpc). This supplements our earlier result that most if not all cool cores in massive clusters contain a minihalo. Comparison of radio and Chandra X-ray images indicates that the minihalo emission is typically confined by concentric sloshing cold fronts in the cores of most of our clusters, supporting the hypothesis that minihalos arise from electron reacceleration by turbulence caused by core gas sloshing. Taken together, our findings suggest that the origin of minihalos should be closely related to the properties of thermal plasma in cluster cool cores.

The effect of adiabatic losses on spectra of stationary jets and the origin of soft radio spectra of accreting black hole sources

It has been suggested that adiabatic energy losses are not effective in stationary jets, where the jet expansion is not associated with net work. Here, we study jet solutions without them, assuming that adiabatic losses are balanced by electron reacceleration. The absence of effective adiabatic losses makes electron advection along the jet an important process, and we solve the electron kinetic equation including that process. We find analytical solutions for the case of conical jets with advection and synchrotron losses. We show that accounting for adiabatic losses in the case of sources showing soft partially self-absorbed spectra with the spectral index of $\alpha<0$ in the radio-to-IR regime requires deposition of large amounts of energy at large distances in the jet. On the other hand, such spectra can be accounted for by advection of electrons in the jet. We compare our results to the quiescent spectrum of the blazar Mrk 421. We find its soft radio-IR spectrum can be fitted either by a model without adiabatic losses and advection of electrons or by one with adiabatic losses, but the latter requires injection of a very large power at large distances.

On the Broadband Synchrotron Spectra of Pulsar Wind Nebulae

Abstract As shown by broadband observations, pulsar wind nebulae (PWNe) are characterized by a broken power-law spectrum of synchrotron emission. Based on modern magnetohydrodynamic (MHD) turbulence theories, we investigate the reacceleration of electrons in the PWN through the adiabatic stochastic acceleration (ASA), which arises from fundamental dynamics of MHD turbulence. The ASA acts to flatten the injected energy spectrum of electrons at low energies, while synchrotron cooling results in a steep spectrum of electrons at high energies. Their dominance in different energy ranges leads to a flat radio spectrum (F ν ) and a steep X-ray spectrum. Our analytical spectral shapes generally agree well with the observed synchrotron spectra of radio- and X-ray-bright PWNe. The spectral break corresponding to the balance between the ASA and synchrotron losses provides a constraint on the acceleration timescale of the ASA and the magnetic field strength in the PWN.