Context. A new class of distinct radio objects, commonly referred to as odd radio circles (ORCs), has been recently discovered. The origin of these features remains unclear because their peculiar properties challenge our current understanding of astrophysical sources for diffuse radio emission. Aims. We test the feasibility and limits of major mergers in galaxy groups as a possible formation channel for ORCs. Methods. By modelling the assembly of a massive galaxy group with a final virial mass of M₂₀₀ ∼ 10¹³ M_⊙ in a magnetohydrodynamic zoom-in simulation with on-the-fly cosmic ray treatment, we derive the X-ray and radio properties of the system self-consistently and compare them to observations. Results. We show that the X-ray properties of the simulated system agree with characteristics of observed galaxy groups in the relevant mass range, legitimating the comparison between the radio properties of the simulated halo and those of observed ORCs. A major merger between two galaxies in the simulation triggers a series of strong shocks in the circumgalactic medium, which in unison form a ring if the line of sight is perpendicular to the merger axis. The shock is rapidly expands radially and quickly reaches the virial radius of the halo. This formation channel thus readily explains the morphology and large extent of ORCs. However, the inferred radio luminosity of these features is lower than that of observed counterparts, while the degree of polarisation seems systematically over-predicted by the simulation. Conclusions. Fossil cosmic ray populations from active galactic nuclei and stellar feedback might be necessary to explain the full extent of the radio properties of ORCs, since diffusive shock acceleration was the only source term for non-thermal electrons considered in this work.

The Nuclear Stellar Disk (NSD), together with the Nuclear Stellar Cluster and the supermassive black hole Sgr A*, forms the central region of the Milky Way. Galactic X-ray background emission is known to be associated with the old stellar population, predominantly produced by accreting white dwarfs. In this work we characterize the X-ray emission of the Galactic Center (GC) region using wide-field observations with the ART-XC telescope on-board the SRG observatory in the 4−12 keV energy band. Our analysis demonstrates that the X-ray emission of the GC at a spatial scale of a few hundred parsecs is dominated by the regularly-shaped NSD aligned in the Galactic plane, and characterized by latitudinal and longitudinal scale heights of ∼20 pc and ∼100 pc, respectively. The measured flux (6.8−0.3+0.1)×10−10erg s⁻¹ cm⁻² in the 4−12 keV band corresponds to a luminosity of L4─12keV=(5.9−0.3+0.1)×1036erg s⁻¹, assuming the GC distance of 8.178 kpc. The average mass-normalized X-ray emissivity of the NSD, 〈L/M〉=(5.6−0.7+0.5)×1027erg s⁻¹ M⊙−1, exceeds the corresponding value for the Galactic ridge by a factor of 3.3−0.5+0.4, confirming other studies. We also perform a deprojection of the observed NSD surface brightness distribution in order to construct a three-dimensional X-ray luminosity density model, which can be directly compared to the existing 3D stellar mass models. Finally, we conclude that the spatial distribution of the X-ray emission from the NSD is consistent with the most recent stellar mass density distribution model within 30%, which suggests that this emission is dominated by unresolved point X-ray sources rather than by diffuse X-ray emission.

Relativistic leptons in galaxy clusters lose their energy via radiation (synchrotron and inverse Compton losses) and interactions with the ambient plasma. At z ∼ 0, pure radiative losses limit the lifetime of electrons emitting at ∼GHz frequencies to t_r ≲ 100 Myr. Adiabatic losses can further lower Lorentz factors of electrons trapped in an expanding medium. If the propagation speed of electrons relative to the ambient weakly magnetized (plasma β ∼ 10²) intracluster medium (ICM) is limited by the Alfvén speed (v_a,ICM = c_s,ICM/β^1/2 ∼ 10⁷ cm s⁻¹), GHz-emitting electrons can travel only 1 ∼ v_a,ICMt_r ∼ 10 kpc relative to the underlying plasma. However, elongated structures spanning hundreds of kiloparsecs (or even a megaparsec) have been observed, requiring either a re-acceleration mechanism or another form of synchronization (e.g., via a large-scale shock). We argue that filaments with ordered magnetic fields supported by nonthermal pressure are characterized by v_a ≫ v_a,ICM and, thus, they can provide such a synchronization even without re-acceleration or shocks. In particular, along quasi-stationary filaments, electrons can propagate without experiencing adiabatic losses, and their velocity is not limited by the Alfvén or sound speeds of the ambient thermal plasma. This model predicts that along filaments that span significant pressure gradients (e.g., in the cores of galaxy clusters), the synchrotron break frequency, ν_b ∝ B, scales with the ambient gas pressure as P^1/2. The emission from such filaments should be strongly polarized due to the magnetic field being ordered along them. While some of these structures can be observed as "filaments" (i.e., long and narrow bright structures), others can be unresolved and appear as a diffuse emission. They could also be too faint to be detected, while continuing to provide channels for electron propagation. We examine several cases of filamentary structures in tailed radio galaxies and in a cluster relic, where "lossless" propagation provides an attractive alternative to other mechanisms for explaining the observed spectral behaviors.

To examine how active galactic nucleus (AGN) feedback shapes the intracluster medium (ICM) and fuels black hole accretion in the cool-core galaxy cluster A2597, we present deep (∼600 ks) Chandra X-ray observations complemented by archival GMRT radio and SINFONI near-infrared data. Radio-mode AGN activity has inflated seven X-ray cavities and driven one to three potential weak shocks ( M∼1.05−1.14 ) extending to ∼150 kpc, suggesting recurrent outbursts occurring on ∼10⁷ yr timescales. We also detect a narrow, ∼57 kpc X-ray surface brightness deficit—a potential plasma depletion layer—likely shaped by residual sloshing motions that amplified magnetic fields and/or displaced gas within the cluster core. Although the AGN injects ∼10⁴⁴ erg s⁻¹ of energy, comparable to the cluster's cooling luminosity, radiative cooling persists at ∼15 M_⊙ yr⁻¹, replenishing the billion-solar-mass cold gas reservoir at the heart of the brightest cluster galaxy. Sustaining this level of activity requires a continuous fuel supply, yet the estimated Bondi accretion power (∼2 × 10⁴³ erg s⁻¹) falls an order of magnitude short of the observed cavity power, suggesting that "hot" gas fueling is insufficient. Instead, archival Atacama Large Millimeter/submillimeter Array observations continue to support a chaotic cold accretion scenario, where turbulence-driven condensation fuels the AGN at rates exceeding Bondi accretion, sustaining a self-regulated feedback cycle that repeatedly shapes the core of A2597.

Searches for the putative large-scale X-ray halo around the Geminga pulsar have been extensively performed using various narrow field-of-view X-ray telescopes. In this paper, we present wide-field scanning observation of Geminga with SRG/ART-XC. Our X-ray analysis provides, for the first time, direct imaging of a 3.5° ×3.5° region in the 4−12 keV energy band, comparable in extent to the expected Geminga emission. The ART-XC observation provides a highly uniform sky coverage without strong vignetting effects. The synchrotron X-ray halo flux was predicted using a physical model based on particle injection, diffusion, and cooling over the pulsar's lifetime, as well as the spectral and spatial properties of the synchrotron X-ray and inverse-Compton gamma-ray emissions. The model is tuned to reproduce existing multiwavelength data from X-ray upper limits and GeV to TeV gamma-ray observations. After accounting for the high particle background and its uncertainties, no significant emission is found in the assumed source region, and X-ray flux upper limits are derived. These limits are less constraining by up to a factor of three with respect to existing results obtained with narrow field-of-view telescopes and longer exposure times. Nonetheless, we place direct and independent constraints on Geminga's ambient magnetic field strength, which are compatible with other studies. Our methodology, including simulation for longer observation times, is applied for the first time to the wide field-of-view search for pulsar halos. Using extensive simulations, we also show that a 68% probability of detecting the Geminga pulsar halo can be achieved with a 20-day SRG/ART-XC exposure for a 3 μG magnetic field.