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Gas density map (top) and pressure ratio between cosmic ray protons and thermal gas (bottom) for thin slices through the center of two galaxy clusters simulated with the enzo grid code by Vazza et al. (2012a)
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Galaxy clusters grow by gas accretion, mostly from mergers of substructures, which release powerful shock waves into cosmic plasmas and convert a fraction of kinetic energy into thermal energy, amplification of magnetic fields and into the acceleration of energetic particles. The modeling of the radio signature of cosmic shocks, combined with the l...
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Citations
... Given the typical merger rate of galaxy clusters (e.g., [112]) and a combination of high spatial resolution and large effective area, AXIS is expected to detect a large number of such shocks. Together with radio surveys, we will be able to connect X-ray shocks with large-scale radio relics and understand how shocks with moderate Mach numbers accelerate electrons (see [113,114] for reviews). ...
Stellar and black hole feedback heat and disperse surrounding cold gas clouds, launching gas flows off circumnuclear and galactic disks, producing a dynamic interstellar medium. On large scales bordering the cosmic web, feedback drives enriched gas out of galaxies and groups, seeding the intergalactic medium with heavy elements. In this way, feedback shapes galaxy evolution by shutting down star formation and ultimately curtailing the growth of structure after the peak at redshift 2–3. To understand the complex interplay between gravity and feedback, we must resolve both the key physics within galaxies and map the impact of these processes over large scales, out into the cosmic web. The Advanced X-ray Imaging Satellite (AXIS) is a proposed X-ray probe mission for the 2030s with arcsecond spatial resolution, large effective area, and low background. AXIS will untangle the interactions of winds, radiation, jets, and supernovae with the surrounding interstellar medium across the wide range of mass scales and large volumes driving galaxy evolution and trace the establishment of feedback back to the main event at cosmic noon. This white paper is part of a series commissioned for the AXIS Probe mission concept; additional AXIS white papers can be found at the AXIS website.
... Collisionless shock waves are present in a large number of astrophysical systems, and they are pivotal for efficient energy conversion and particle acceleration in our Universe (e.g., Richardson 2011;Bykov et al. 2019). Generally speaking, shocks convert directed flow energy (upstream) into heat and magnetic energy (downstream), and in the collisionless case, a fraction of the available energy is channeled into the production of energetic particles. ...
The Parker Solar Probe (PSP) and Solar Orbiter (SolO) missions opened a new observational window in the inner heliosphere, which is finally accessible to direct measurements. On 2022 September 5, a coronal mass ejection (CME)-driven interplanetary (IP) shock was observed as close as 0.07 au by PSP. The CME then reached SolO, which was radially well-aligned at 0.7 au, thus providing us with the opportunity to study the shock properties at different heliocentric distances. We characterize the shock, investigate its typical parameters, and compare its small-scale features at both locations. Using the PSP observations, we investigate how magnetic switchbacks and ion cyclotron waves are processed upon shock crossing. We find that switchbacks preserve their V–B correlation while compressed upon the shock passage, and that the signature of ion cyclotron waves disappears downstream of the shock. By contrast, the SolO observations reveal a very structured shock transition, with a population of shock-accelerated protons of up to about 2 MeV, showing irregularities in the shock downstream, which we correlate with solar wind structures propagating across the shock. At SolO, we also report the presence of low-energy (∼100 eV) electrons scattering due to upstream shocklets. This study elucidates how the local features of IP shocks and their environments can be very different as they propagate through the heliosphere.
... Additionally, the inclusion of the soft X-ray data may tighten constraints on the measured parameters within our three models presented earlier. Another topic for exploration with these clusters is the detection of a possible nonthermal bremsstrahlung, corresponding with a population of suprathermal electrons (Bykov et al. 2019). Combining the NuSTAR data with LOFAR radio data could bridge the gap between the thermal and nonthermal electron distributions as well as provide insight into the nonthermal Sunyaev-Zeldovich effect (Blasi et al. 2000;Petrosian et al. 2008). ...
Observations from past missions such as RXTE and Beppo-SAX suggested the presence of inverse Compton (IC) scattering at hard X-ray energies within the intracluster medium of some massive galaxy clusters. In subsequent years, observations by, e.g., Suzaku, and now NuSTAR, have not been able to confirm these detections. We report on NuSTAR hard X-ray searches for IC emission in two massive galaxy clusters, A665 and A2146. To constrain the global IC flux in these two clusters, we fit global NuSTAR spectra with three models: single (1T) and two-temperature (2T) models, and a 1T plus power-law component (T+IC). The temperature components are meant to characterize the thermal ICM emission, while the power law represents the IC emission. We find that the 3–30 keV A665 and 3–20 keV A2146 spectra are best described by thermal emission alone, with average global temperatures of kT = (9.15 ± 0.1) keV for A665 and kT = (8.29 ± 0.1) keV for A 2146. We constrain the IC flux to F NT <0.60 × 10 ⁻¹² erg s ⁻¹ cm ⁻² and F NT < 0.85 × 10 ⁻¹² erg s ⁻¹ cm ⁻² (20–80 keV) for A665 and A2146, respectively both at the 90% confidence level. When we couple the IC flux limits with 1.4 GHz diffuse radio data from the VLA, we set lower limits on the average magnetic field strengths of >0.14 μ G and >0.011 μ G for A665 and A2146, respectively.
... Before the discussion of the origin and evolution of magnetic fields, it is necessary to introduce some of the large-scale motions that are present in the ICM. These gas motions are, namely, turbulence, shock waves, and cold fronts [31][32][33][34][35]. All three of them are driven naturally in the ICM and they play crucial roles in the acceleration of cosmic rays. ...
... It is commonly accepted that radio relics are produced by the shock (re-)acceleration of cosmic-ray electrons (e.g., [21,[160][161][162][163][164][165][166] and references therein). While the details of the acceleration mechanism-especially regarding the micro physics-are still to be worked out, it is generally assumed that Diffusive Shock Acceleration (DSA) is the acceleration mechanism at work (e.g., [33,34,[167][168][169] and references therein). ...
... Extragalactic cosmic-ray electrons can be seen as the messengers that provide us with information about cosmic magnetic fields [33,34]. Hence, it is essential to study the physics of cosmic-ray acceleration in the Universe. ...
The discovery of diffuse radio emission in galaxy clusters proved the existence of energetic cosmic-ray electrons and cosmic magnetic fields on Mpc-scales in the Universe. Furthermore, both magnetic fields and cosmic-ray electrons are predicted to exist beyond galaxy clusters, namely, in the filaments and voids of the cosmic web. Recent detection of diffuse radio emission in intercluster bridges—the region between two merging clusters—strengthens the theory that both cosmic magnetic fields and cosmic-ray electrons exist on these large scales. Radio observations are our most powerful tool to study cosmic magnetic fields and cosmic-ray electrons in the Universe. The recent improvements in radio astronomy, including the exploration of the low-frequency radio sky, have led to the discovery of countless new radio sources, and hence a new understanding of the origin and evolution of cosmic magnetic fields and cosmic-ray electrons. In this contribution, we summarise the newest discoveries in the field. Furthermore, we discuss what these new radio observations teach us about cosmic magnetic fields and cosmic rays in galaxy clusters and beyond.
... In the collisionless case, a fraction of the available energy can be channeled into the production of energetic particles, a pivotal feature to understand many aspects of in-situ and remote observations (Burgess & Scholer 2015). Thus, collisionless shocks play a fundamental role in energy conversion in a variety of systems, ranging from solar flares (Woo & Armstrong 1981) to interacting galaxy clusters (Bykov et al. 2019). While some aspects of energy conversion at shock waves are not fully understood despite decades of research, a picture invoking a complex shock behaviour is emerging (e.g., Treumann 2009). ...
The complex interaction between shocks and plasma turbulence is extremely important to address crucial features of energy conversion in a broad range of astrophysical systems. We study the interaction between a supercritical, perpendicular shock and pre-existing, fully-developed plasma turbulence, employing a novel combination of magnetohydrodynamic (MHD) and small-scale, hybrid-kinetic simulations where a shock is propagating through a turbulent medium. The variability of the shock front in the unperturbed case and for two levels of upstream fluctuations is addressed.We find that the behaviour of shock ripples, i.e., shock surface fluctuations with short (a few ion skin depths, $d_i$) wavelengths, is modified by the presence of pre-existing turbulence, which also induces strong corrugations of the shock front at larger scales. We link this complex behaviour of the shock front and the shock downstream structuring with the proton temperature anisotropies produced in the shock-turbulence system. Finally, we put our modelling effort in the context of spacecraft observations, elucidating the role of novel cross-scale, multi-spacecraft measurements in resolving shock front irregularities at different scales. These results are relevant for a broad range of astrophysical systems characterised by the presence of shock waves interacting with plasma turbulence.
... Shocks are ubiquitous, and they are fundamental for a broad range of astrophysical systems (e.g., Kivelson & Russell 1995;Bykov et al. 2019). Generally speaking, shocks convert directed flow energy (upstream) into heat and magnetic energy (downstream) and, in the collisionless case, in energetic particles (e.g., Burgess & Scholer 2015). ...
Interplanetary (IP) shocks are fundamental building blocks of the heliosphere, and the possibility to observe them in-situ is crucial to address important aspects of energy conversion for a variety of astrophysical systems. Steepened waves known as shocklets are known to be important structures of planetary bow shocks, but they are very rarely observed related to IP shocks. We present here the first multi-spacecraft observations of shocklets observed by upstream of an unusually strong IP shock observed on November 3rd 2021 by several spacecraft at L1 and near-Earth solar wind. The same shock was detected also by radially aligned Solar Orbiter at 0.8 AU from the Sun, but no shocklets were identified from its data, introducing the possibility to study the environment in which shocklets developed. The Wind spacecraft has been used to characterise the shocklets, associated with pre-conditioning of the shock upstream by decelerating incoming plasma in the shock normal direction. Finally, using the Wind observations together with ACE and DSCOVR spacecraft at L1, as well as THEMIS B and THEMIS C in the near-Earth solar wind, the portion of interplanetary space filled with shocklets is addressed, and a lower limit for its extent is estimated to be of about 110 RE in the shock normal direction and 25 RE in the directions transverse to the shock normal. Using multiple spacecraft also reveals that for this strong IP shock, shocklets are observed for a large range of local obliquity estimates (9-64 degrees).
... Such processes continually shape the baryonic components of clusters and can inject up to 10 64 ergs of gravitational potential energy during one cluster crossing time (∼ Gyr), primarily dissipated by shocks into heating of the intra-cluster gas to high (X-ray emitting) temperatures (Markevitch & Vikhlinin 2007), but also through large-scale ICM motions generating cluster-wide turbulence (Hitomi Collaboration et al. 2016Li et al. 2020). A fraction of this energy can also be channeled into non-thermal plasma components such as cosmic rays (Brunetti & Jones 2014;Bykov et al. 2019) and magnetic field amplification (Donnert et al. 2018) as revealed by the presence of extended radio emission (Ferrari et al. 2008;Feretti et al. 2012;van Weeren et al. 2019). These energetic processes are expected to contribute to the deviation of cluster properties from self-similar predictions, which only account for gravitational evolution in scale-free cluster evolution (Kaiser 1986). ...
We present the Rhapsody-C simulations that extend the Rhapsody-G suite of massive galaxy clusters at the $M_{\rm vir}\sim10^{15}\thinspace{\rm M}_{\odot}$ scale with cosmological magneto-hydrodynamic zoom-in simulations that include anisotropic thermal conduction, modified supermassive black hole (SMBH) feedback, new SMBH seeding and SMBH orbital decay model. These modelling improvements have a dramatic effect on the SMBH growth, star formation and gas depletion in the proto-clusters. We explore the parameter space of the models and report their effect on both star formation and the thermodynamics of the intra-cluster medium (ICM) as observed in X-ray and SZ observations. We report that the star formation in proto-clusters is strongly impacted by the choice of the SMBH seeding as well as the orbital decay of SMBHs. Feedback from AGNs is substantially boosted by the SMBH decay, its time evolution and impact range differ noticeably depending on the AGN energy injection scheme used. Compared to a mass-weighted injection whose energy remains confined close to the central SMBHs, a volume-weighted thermal energy deposition allows to heat the ICM out to large radii which severely quenches the star formation in proto-clusters. By flattening out temperature gradients in the ICM, anisotropic thermal conduction can reduce star formation early on but weakens and delays the AGN activity. Despite the dissimilarities found in the stellar and gaseous content of our haloes, the cluster scaling relations we report are surprisingly insensitive to the subresolution models used and are in good agreement with recent observational and numerical studies.
... Given the long lifetimes of relativistic protons in the intercluster medium (Völk et al. 1996;Berezinsky et al. 1997), they may experience acceleration by multiple weak shocks and large-scale MHD plasma motions at the cluster scale r 200 which can form a power-law distribution of GeV-TeV regime protons with indices of p = 2.4−3 (see e.g., Bykov et al. 2019). Then the diffuse electrons of the spectral index about 2.4−2.6 can be re-accelerated by the relic shock of M X 2, providing the possibility to relax the apparent M X −M R discrepancy. ...
This is the second paper in a series of studies of the Coma cluster using the SRG/eROSITA X-ray data obtained during the calibration and performance verification phase of the mission. Here, we focus on the region adjacent to the radio source 1253+275 (radio relic, RR, hereafter). We show that the X-ray surface brightness exhibits its steepest gradient at ∼79′ (∼2.2 Mpc ≈ R 200c ), which is almost co-spatial to the outer edge of the RR. As in the case of several other relics, the Mach number of the shock derived from the X-ray surface brightness profile ( M X ≈ 1.9) appears to be lower than needed to explain the slope of the integrated radio spectrum in the diffusive shock acceleration model ( M R ≈ 3.5) if the magnetic field is uniform and the radiative losses are fast. However, the shock geometry is plausibly much more complicated than a spherical wedge centered on the cluster, given the non-trivial correlation between radio, X-ray, and SZ images. While the complicated shock geometry alone might cause a negative bias in M X , we speculate on a few other possibilities that may affect the M X − M R relation, including the shock substructure that might be modified by the presence of non-thermal filaments stretching across the shock and the propagation of relativistic electrons along the non-thermal filaments with a strong magnetic field. We also discuss the “history” of the radio galaxy NGC 4789, which is located ahead of the relic in the context of the Coma-NGC 4839 merger scenario.
... Shocks are ubiquitous, and they are fundamental for a broad range of astrophysical systems (e.g., Kivelson & Russell 1995;Bykov et al. 2019). Generally speaking, shocks convert directed flow energy (upstream) into heat and magnetic energy (downstream) and, in the collisionless case, in energetic particles (e.g., Burgess & Scholer 2015). ...
Interplanetary (IP) shocks are fundamental building blocks of the heliosphere, and the possibility to observe them \emph{in-situ} is crucial to address important aspects of energy conversion for a variety of astrophysical systems. Steepened waves known as shocklets are known to be important structures of planetary bow shocks, but they are very rarely observed related to IP shocks. We present here the first multi-spacecraft observations of shocklets observed by upstream of an unusually strong IP shock observed on November 3rd 2021 by several spacecraft at L1 and near-Earth solar wind. The same shock was detected also by radially aligned Solar Orbiter at 0.8 AU from the Sun, but no shocklets were identified from its data, introducing the possibility to study the environment in which shocklets developed. The Wind spacecraft has been used to characterise the shocklets, associated with pre-conditioning of the shock upstream by decelerating incoming plasma in the shock normal direction. Finally, using the Wind observations together with ACE and DSCOVR spacecraft at L1, as well as THEMIS B and THEMIS C in the near-Earth solar wind, the portion of interplanetary space filled with shocklets is addressed, and a lower limit for its extent is estimated to be of about 110 $R_E$ in the shock normal direction and 25 $R_E$ in the directions transverse to the shock normal. Using multiple spacecraft also reveals that for this strong IP shock, shocklets are observed for a large range of local obliquity estimates (9-64 degrees).
... Within DSA particles repeatedly cross a shock front and are reflected up-and downstream by scattering off electromagnetic turbulence (see e.g. Drury 1983;Bykov et al. 2019, for reviews). As this process is self-similar the particle distribution naturally approaches a power-law in energy or momentum. ...
Non-thermal emission from relativistic Cosmic Ray (CR) electrons gives insight into the strength and morphology of intra-cluster magnetic fields, as well as providing powerful tracers of structure formation shocks. Emission caused by CR protons on the other hand still challenges current observations and is therefore testing models of proton acceleration at intra-cluster shocks. Large-scale simulations including the effects of CRs have been difficult to achieve and have been mainly reduced to simulating an overall energy budget, or tracing CR populations in post-processing of simulation output and has often been done for either protons or electrons. We introduce CRESCENDO: Cosmic Ray Evolution with SpeCtral Electrons aND prOtons, an efficient on-the-fly Fokker-Planck solver to evolve distributions of CR protons and electrons within every resolution element of our simulation. The solver accounts for CR (re-)acceleration at intra-cluster shocks, based on results of recent PIC simulations, adiabatic changes and radiative losses of electrons. We show its performance in test cases as well as idealized galaxy cluster (GC) simulations. We apply the model to an idealized GC merger following best-fit parameters for CIZA J2242.4+5301-1 and study CR injection, radio relic morphology, spectral steepening and synchrotron emission.
