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We report some highlights from the ARIES APEC workshop on ``Storage Rings and Gravitational Waves'' (SRGW2021), held in virtual space from 2 February to 18 March 2021, and sketch a tentative landscape for using accelerators and associated technologies for the detection or generation of gravitational waves., 17 pages, 9 figures, highlights from the ARIES APEC workshop on "Storage Rings and Gravitational Waves"(SRGW2021)

CIZA J2242.8+5301, a merging galaxy cluster at z=0.19, hosts a double-relic system and a faint radio halo. Radio observations at frequencies ranging from a few MHz to several GHz have shown that the radio spectral index at the outer edge of the N relic corresponds to a shock of Mach number 4.6+/-1.1, under the assumptions of diffusive shock acceleration of thermal particles in the test particle regime. Here, we present results from new Chandra observations of the cluster. The Chandra surface brightness profile across the N relic only hints to a surface brightness discontinuity (, 11 pages, 11 figures. Accepted for publication in MNRAS

Radio relics in galaxy clusters are associated with powerful shocks that (re)accelerate relativistic electrons. It is widely believed that the acceleration proceeds via diffusive shock acceleration. In the framework of thermal leakage, the ratio of the energy in relativistic electrons to the energy in relativistic protons should should be smaller than $K_{\rm e/p} \sim 10^{-2}$. The relativistic protons interact with the thermal gas to produce $\gamma$-rays in hadronic interactions. Combining observations of radio relics with upper limits from $\gamma$-ray observatories can constrain the ratio $K_{\rm e/p}$. In this work we selected 10 galaxy clusters that contain double radio relics, and derive new upper limits from the stacking of $\gamma$-ray observations by FERMI. We modelled the propagation of shocks using a semi-analytical model, where we assumed a simple geometry for shocks and that cosmic ray protons are trapped in the intracluster medium. Our analysis shows that diffusive shock acceleration has difficulties in matching simultaneously the observed radio emission and the constraints imposed by FERMI, unless the magnetic field in relics is unrealistically large ($\gg 10 ~\rm \mu G$). In all investigated cases (also including realistic variations of our basic model and the effect of re-acceleration) the mean emission of the sample is of the order of the stacking limit by FERMI, or larger. These findings put tension on the commonly adopted model for the powering of radio relics, and imply that the relative acceleration efficiency of electrons and protons is at odds with predictions of diffusive shock acceleration, requiring $K_{\rm e/p} \geq 10-10^{-2}$., Comment: 14 pages, 11 figures, MNRAS in press

Major astrophysical questions related to the formation and evolution of structures, and more specifically of galaxy groups and clusters, will still be open in the coming decade and beyond: what is the interplay of galaxy, supermassive black hole, and intergalactic gas evolution in the most massive objects in the Universe - galaxy groups and clusters? What are the processes driving the evolution of chemical enrichment of the hot diffuse gas in large-scale structures? How and when did the first galaxy groups in the Universe, massive enough to bind more than 10^7 K gas, form? Focussing on the period when groups and clusters assembled (0.5, Comment: Supporting paper for the science theme The Hot and Energetic Universe to be implemented by the Athena+ X-ray observatory (http://www.the-athena-x-ray-observatory.eu). 10 pages, 4 figures

LOFAR, the Low Frequency Radio Array, is a new pan-European radio telescope that is almost fully operational. One of its main drivers is to make deep images of the low frequency radio sky. To be able to do this a number of challenges need to be addressed. These include the high data rates, removal of radio frequency interference, calibration of the beams and correcting for the corrupting influence of the ionosphere. One of the key science goals is to study merger shocks, particle acceleration mechanisms and the structure of magnetic fields in nearby and distant merging clusters. Recent studies with the GMRT and WSRT radio telescopes of the "Sausage" and the "Toothbrush" clusters have given a very good demonstration of the power of radio observations to study merging clusters. Recently we discovered that both clusters contain relic and halo sources, large diffuse regions of radio emission not associated with individual galaxies. The 2 Mpc northern relic in the Sausage cluster displays highly aligned magnetic fields and and exhibits a strong spectral index gradient that is a consequence of cooling of the synchrotron emitting particles in the post-shock region. We have argued that these observations provide strong evidence that shocks in merging clusters are capable of accelerating particles. For the Toothbrush cluster we observe a puzzling linear relic that extends over 2 Mpc. The proposed scenario is that a triplemerger can lead to such a structure. With LOFAR's sensitivity it will not only be possible to trace much weaker shocks, but also to study those shocks due to merging clusters up to redshifts of at least one. © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Aims: The aim of the LOFAR epoch of reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21 cm signal. This signal is weaker by several orders of magnitude than the astrophysical foreground signals and hence, in order to achieve this, very long integrations, accurate calibration for stations and ionosphere and reliable foreground removal are essential. Methods: One of the prospective observing windows for the LOFAR EoR project will be centered at the north celestial pole (NCP). We present results from observations of the NCP window using the LOFAR highband antenna (HBA) array in the frequency range 115 MHz to 163 MHz. The data were obtained in April 2011 during the commissioning phase of LOFAR. We used baselines up to about 30 km. The data was processed using a dedicated processing pipeline which is an enhanced version of the standard LOFAR processing pipeline. Results: With about 3 nights, of 6 h each, effective integration we have achieved a noise level of about 100 μJy/PSF in the NCP window. Close to the NCP, the noise level increases to about 180 μJy/PSF, mainly due to additional contamination from unsubtracted nearby sources. We estimate that in our best night, we have reached a noise level only a factor of 1.4 above the thermal limit set by the noise from our Galaxy and the receivers. Our continuum images are several times deeper than have been achieved previously using the WSRT and GMRT arrays. We derive an analytical explanation for the excess noise that we believe to be mainly due to sources at large angular separation from the NCP. We present some details of the data processing challenges and how we solved them. Conclusions: Although many LOFAR stations were, at the time of the observations, in a still poorly calibrated state we have seen no artefacts in our images which would prevent us from producing deeper images in much longer integrations on the NCP window which are about to commence. The limitations present in our current results are mainly due to sidelobe noise from the large number of distant sources, as well as errors related to station beam variations and rapid ionospheric phase fluctuations acting on bright sources. We are confident that we can improve our results with refined processing.

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 radio relics caused by shocks that propagate along the line-of-sight are rare events for simple geometrical reasons, and they have a low surface brightness making them elusive for current instruments. Our simulations show that the radial distribution of observed relics can be explained by the radial trend of dissipated kinetic energy in shocks, that increases with distance from the cluster centre up until half of the virial radius., 6 pages, 4 figures. MNRAS accepted

We use XMM-Newton observations of the galaxy cluster MaxBCG J217.95869+13.53470 to analyze its physical properties and dynamical state. MaxBCG J217.95869+13.53470 is found at a redshift of 0.16, has a mass of ~1x10^14 Msun, and a luminosity of 7.9x10^43 erg/s. The temperature map shows the presence of hot regions towards the north and west of the brightest cluster galaxy (BCG). From the entropy distribution, regions of high entropy match the location of the hot regions; more high entropy regions are found to the west, and ~165 kpc to the southwest of the central AGN. A second X-ray bright galaxy is visible ~90 kpc to the northeast of the BCG, at a redshift of 0.162. This galaxy is likely to be the BCG of a smaller, infalling galaxy cluster. The mass of the smaller cluster is ~10 percent the mass of MaxBCG J217.95869+13.53470, yielding an impact parameter of ~30-100 kpc. We compare the results of our X-ray observations with GMRT observations of the radio source VLSS J1431.8+1331, located at the center of the cluster. Two sources are visible in the radio: a central elongated source that bends at its northern and southern ends, and a southwestern source that coincides with a region of high entropy. The radio sources are connected by a bridge of faint radio emission. We speculate that the southwestern radio source is a radio relic produced by compression of old radio plasma by a merger shock., 9 pages, 8 figures, 1 table; accepted to MNRAS

Cool cores of galaxy clusters are thought to be heated by low-power active galactic nuclei (AGN), whose accretion is regulated by feedback. However, the interaction between the hot gas ejected by the AGN and the ambient intracluster medium is extremely difficult to simulate as it involves a wide range of spatial scales and gas that is Rayleigh-Taylor (RT) unstable. Here we present a series of three-dimensional hydrodynamical simulations of a self-regulating AGN in a galaxy cluster. Our adaptive-mesh simulations include prescriptions for radiative cooling, AGN heating and a subgrid model for RT-driven turbulence, which is crucial to simulate this evolution. AGN heating is taken to be proportional to the rest-mass energy that is accreted onto the central region of the cluster. For a wide range of feedback efficiencies, the cluster regulates itself for at least several $10^9$ years. Heating balances cooling through a string of outbursts with typical recurrence times of around 80 Myrs, a timescale that depends only on global cluster properties. Under certain conditions we find central dips in the metallicity of the intracluster medium. Provided the sub-grid model used here captures all its key properties, turbulence plays an essential role in the AGN self-regulation in cluster cores., accepted by MNRAS

Ram pressure stripping can remove significant amounts of gas from galaxies that orbit in clusters and massive groups, and thus has a large impact on the evolution of cluster galaxies. In this paper, we reconstruct the present-day distribution of ram-pressure, and the ram pressure histories of cluster galaxies. To this aim, we combine the Millennium Simulation and an associated semi-analytic model of galaxy evolution with analytic models for the gas distribution in clusters. We find that about one quarter of galaxies in massive clusters are subject to strong ram-pressures that are likely to cause an expedient loss of all gas. Strong ram-pressures occur predominantly in the inner core of the cluster, where both the gas density and the galaxy velocity are higher. Since their accretion onto a massive system, more than 64 per cent of galaxies that reside in a cluster today have experienced strong ram-pressures of $>10^{-11}$ dyn cm$^{-2}$ which most likely led to a substantial loss of the gas., MNRAS accepted

We present 3D hydrodynamical simulations of ram pressure stripping of massive disc galaxies in clusters. Studies of galaxies that move face-on have predicted that in such a geometry the galaxy can lose a substantial amount of its interstellar medium. But only a small fraction of galaxies is moving face-on. Therefore, in this work we focus on a systematic study of the effect of the inclination angle between the direction of motion and the galaxy's rotation axis. In agreement with some previous works, we find that the inclination angle does not play a major role for the mass loss as long as the galaxy is not moving close to edge-on. We can predict this behaviour by extending Gunn & Gott's estimate of the stripping radius, which is valid for face-on geometries, to moderate inclinations. The inclination plays a role as long as the ram pressure is comparable to pressures in the galactic plane, which can span two orders of magnitude. For very strong ram pressures, the disc will be stripped completely, and for very weak ram pressures, mass loss is negligible independent of inclination. We show that in non-edge-on geometries the stripping proceeds remarkably similar. A major difference between different inclinations is the degree of asymmetry introduced in the remaining gas disc. We demonstrate that the tail of gas stripped from the galaxy does not necessarily point in a direction opposite to the galaxy's direction of motion. Therefore, the observation of a galaxy's gas tail may be misleading about the galaxy's direction of motion., 14 pages, 14 figures, submitted to MNRAS. pdf version with high resolution figures available at http://www.faculty.iu-bremen.de/eroediger/PLOTLINKS/eroediger_rps.pdf

One of the challenges that models of AGN heating of the intracluster medium (ICM) face, is the question how the mechanical luminosity of the AGN is tuned to the radiative losses of the ICM. Here we implement a simple 1D model of a feedback mechanism that links the luminosity of the AGN to the accretion rate. We demonstrate how this simple feedback mechanism leads to a quasi-steady state for a broad range of parameters. Moreover, within this feedback model, we investigate the effect of thermal conduction and find that its relative importance depends strongly on the cluster mass., 12 pages, 6 figures, submitted to ApJ