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1. A Heating Mechanism via Magnetic Pumping in the Intracluster Medium

   https://doi.org/10.3847/1538-4357/acb3b1  

   Ley, Francisco; Zweibel, Ellen G.; Riquelme, Mario; Sironi, Lorenzo; Miller, Drake; Tran, Aaron (April 2023, The Astrophysical Journal)

   Abstract Turbulence driven by active galactic nuclei activity, cluster mergers, and galaxy motion constitutes an attractive energy source for heating the intracluster medium (ICM). How this energy dissipates into the ICM plasma remains unclear, given its low collisionality and high magnetization (precluding viscous heating by Coulomb processes). Kunz et al. proposed a viable heating mechanism based on the anisotropy of the plasma pressure under ICM conditions. The present paper builds upon that work and shows that particles can be heated by large-scale turbulent fluctuations via magnetic pumping. We study how the anisotropy evolves under a range of forcing frequencies, what waves and instabilities are generated, and demonstrate that the particle distribution function acquires a high-energy tail. For this, we perform particle-in-cell simulations where we periodically vary the mean magnetic fieldB(t). WhenB(t) grows (dwindles), a pressure anisotropyP_⊥>P_∥(P_⊥<P_∥) builds up (P_⊥andP_∥are, respectively, the pressures perpendicular and parallel toB(t)). These pressure anisotropies excite mirror (P_⊥>P_∥) and oblique firehose (P_∥>P_⊥) instabilities, which trap and scatter the particles, limiting the anisotropy, and providing a channel to heat the plasma. The efficiency of this mechanism depends on the frequency of the large-scale turbulent fluctuations and the efficiency of the scattering the instabilities provide in their nonlinear stage. We provide a simplified analytical heating model that captures the phenomenology involved. Our results show that this process can be relevant in dissipating and distributing turbulent energy at kinetic scales in the ICM. 

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2. Uncovering the invisible: A study of Gaia18ajz, a candidate black hole revealed by microlensing

   https://doi.org/10.1051/0004-6361/202451046  

   Howil, K; Wyrzykowski, Ł; Kruszyńska, K; Zieliński, P; Bachelet, E; Gromadzki, M; Mikołajczyk, P J; Kotysz, K; Jabłońska, M; Kaczmarek, Z; et al (February 2025, Astronomy & Astrophysics)

   Context.Identifying black holes is essential for our understanding of the development of stars and can reveal novel principles of physics. Gravitational microlensing provides an exceptional opportunity to examine an undetectable population of black holes in the Milky Way. In particular, long-lasting events are likely to be associated with massive lenses, including black holes. Aims.We present an analysis of the Gaia18ajz microlensing event reported by the Gaia Science Alerts system. Gaia18ajz is a long-timescale event exhibiting features indicative of the annual microlensing parallax effect. Our objective is to estimate its lens parameters based on the best-fitting model. Methods.We used photometric data obtained from the Gaia satellite and terrestrial observatories to investigate a variety of microlensing models and calculate the most probable mass and distance to the lens, taking into consideration a Galactic model as a prior. Subsequently, we applied a mass–brightness relation to evaluate the likelihood that the lens is a main sequence star. We also describe theDarkLensCode(DLC), an open-source routine that computes the distribution of probable lens mass, distance, and luminosity employing the Galaxy priors on stellar density and velocity for microlensing events with detected microlensing parallax. Results.We modelled the Gaia18ajz event and found its two possible models, the most probable Einstein timescales for which are 316₋₃₀⁺³⁶days and 299₋₂₂⁺²⁵days. Applying Galaxy priors for stellar density and motion, we calculated a most probable lens mass of 4.9_−2.3^+5.4 M_⊙located at 1.14_−0.57^+0.75 kpc, and a less probably mass of 11.1_−4.7^+10.3 M_⊙located at 1.31_−0.60^+0.80 kpc. Our analysis of the blended light suggests that the lens is likely a dark remnant of stellar evolution rather than a main sequence star. 

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3. Instability of the Octahedral Symmetry in Si8O12H8 and Ge8O12H8: A Consequence of the Pseudo-Jahn–Teller Effect

   https://doi.org/10.1007/s10904-025-03970-7  

   Muya, Jules Tshishimbi; Ceulemans, Arnout; Parish, Carol (August 2025, Journal of Inorganic and Organometallic Polymers and Materials)

   Abstract The symmetry breaking in octahedral silsesquioxane and its germanium analogues (Si₈O₁₂H₈and Ge₈O₁₂H₈) has been investigated using the M06-2X/6-31++G(3df, 3pd) method and group theory. Both structures undergo$${O}_{h}\downarrow {T}_{h}$$symmetry breaking, characterized by pseudo-Jahn−Teller stabilization energies of 0.22 kcal/mol for Si-POSS and 9.82 kcal/mol for Ge-POSS. Under the influence of the pseudo-Jahn–Teller effect, the distortion vector involves the vibrational a_2gmode with imaginary frequency. The distortion forces in O_h-POSS are predominantly localized on the oxygen atoms and driven by the coupling between the lowest unoccupied molecular orbital (a_1g) and the highest occupied molecular orbital (a_2g). The symmetry breaking is attributed to a pseudo-Jahn–Teller mechanism of type (a_2gx a_1g) = a_2g. The symmetrical substitution of oxygen atoms by X (where X = C, N, P) results in viable T_h-Si₈X₁₂H₈and T_h-Ge₈X₁₂H₈structures. The observed pseudo-Jahn–Teller distortion and substitutional symmetry breaking caused by X indicates a consistent electronic relaxation mechanism, characterized by the formation of C=C, N=N and P=P bonds on the POSS cubic faces, which serves as hallmarks of stability. Additionally, we find that the volume of substituted T_h-symmetrical POSS is sufficiently large to accommodate small ions. 

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4. The Increasing Efficiency of the Poleward Energy Transport Into the Arctic in a Warming Climate

   https://doi.org/10.1029/2022GL100834  

   Cardinale, Christopher_J; Rose, Brian_E_J (January 2023, Geophysical Research Letters)

   Abstract This study quantifies the contribution to Arctic winter surface warming from changes in the tropospheric energy transport (F_trop) and the efficiency with whichF_tropheats the surface in the RCP8.5 warming scenario of the Community Earth System Model Large Ensemble. A metric for this efficiency,E_trop, measures the fraction of anomalousF_tropthat is balanced by an anomalous net surface flux (NSF). Drivers ofE_tropare identified in synoptic‐scale events during whichF_tropis the dominant driver of NSF.E_tropis sensitive to the vertical structure ofF_tropand pre‐existing Arctic lower‐tropospheric stability (LTS). In RCP8.5, winter‐meanF_tropdecreases from 95.1 to 85.4 W m⁻², whileE_tropincreases by 5.7%, likely driven by decreased Arctic LTS, indicating an increased coupling betweenF_tropand the surface energy budget. The net impact of decreasingF_tropand increasing efficiency is a positive 0.7 W m⁻²contribution to winter‐season surface heating. 

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5. Hölder‐Conditioned Hypsometry: A Refinement to a Classical Approach for the Characterization of Topography

   https://doi.org/10.1029/2019WR025412  

   Keylock, Christopher J.; Singh, Arvind; Passalacqua, Paola; Foufoula‐Georgiou, Efi (May 2020, Water Resources Research)

   Abstract The effective characterization of topographic surfaces is a central tenet of geomorphology. Differences in land surface properties reveal variations in structural controls and the nature and efficacy of Earth‐shaping processes. In this paper, we employ the Hölder exponents,α, characterizing the local scaling behavior of topography and commonly used in the study of the (multi)fractal properties of landscapes and show that the joint probability distribution of the area of the terrain with a given elevation andαcontains a wealth of information on topographic structure. The conditional distributions of the hypsometric integrals as a function ofα, that is,I_hyp|α, are shown to capture this structure. A multivariate analysis reveals three metrics that summarize these conditional distributions: Strahler's original hypsometric integral, the standard deviation of theI_hyp|α, and the nature of any trend of theI_hyp|αagainstα. An analysis of five digital elevation models (DEMs) from different regions of the United States shows that only one is truly described by the hypsometric integral (Mettman Ridge from central Oregon). In the other cases, the new metrics clearly discriminate between instances where topographic roughness is more clearly a function of elevation, as captured by the conditional variables. In a final example, we artificially sharpen the ridges and valleys of one DEM to show that while the hypsometric integral and standard deviation ofI_hyp|αare invariant to the change, the trend ofI_hyp|αagainstαcaptures the changes in topography. 

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