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1. Magnetogram-matching Biot–Savart Law and Decomposition of Vector Magnetograms

   https://doi.org/10.3847/1538-4357/add895  

   Titov, Viacheslav S; Downs, Cooper; Török, Tibor; Linker, Jon A; Prazak, Michael; Qiu, Jiong A (July 2025, The Astrophysical Journal)

   Abstract We generalize a magnetogram-matching Biot–Savart law (BSl) from planar to spherical geometry. For a given coronal current densityJ, this law determines the magnetic field $\tilde{\mathit{B}}$ whose radial component vanishes at the surface. The superposition of $\tilde{\mathit{B}}$ with a potential field defined by a given surface radial field,B_r, provides the entire configuration whereB_rremains unchanged by the currents. Using this approach, we (1) upgrade our regularized BSls for constructing coronal magnetic flux ropes (MFRs) and (2) propose a new method for decomposing a measured photospheric magnetic field as $\mathit{B}={\mathit{B}}_{\mathrm{pot}}+{\mathit{B}}_T+{\mathit{B}}_{\tilde{S}}$ , where the potential,B_pot, toroidal,B_T, and poloidal, ${\mathit{B}}_{\tilde{S}}$ , fields are determined byB_r,J_r, and the surface divergence ofB–B_pot, respectively, all derived from magnetic data. OurB_Tis identical to the one in the alternative Gaussian decomposition by P. W. Schuck et al., whileB_potand ${\mathit{B}}_{\tilde{S}}$ are different from their poloidal fields ${\mathit{B}}_{\mathrm{P}}^{<}$ and ${\mathit{B}}_{\mathrm{P}}^{>}$ , which arepotentialin the infinitesimal proximity to the upper and lower side of the surface, respectively. In contrast, our ${\mathit{B}}_{\tilde{S}}$ has no such constraints and, asB_potandB_T, refers to thesameupper side of the surface. In spite of these differences, for a continuousJdistribution across the surface,B_potand ${\mathit{B}}_{\tilde{S}}$ are linear combinations of ${\mathit{B}}_{\mathrm{P}}^{<}$ and ${\mathit{B}}_{\mathrm{P}}^{>}$ . We demonstrate that, similar to the Gaussian method, our decomposition allows one to identify the footprints and projected surface-location of MFRs in the solar corona, as well as the direction and connectivity of their currents. 

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2. Ferrimagnetism and half-metallicity in Cr-substituted Mn _4–x Cr _x Al ₁₁

   https://doi.org/10.1088/1361-648X/add8d8  

   Noor, M_F B; Law, K-M; Yasmin, N; Hauser, A J; Besara, T (May 2025, Journal of Physics: Condensed Matter)

   Abstract Single crystals of disordered Mn_4–xCr_xAl₁₁have been synthesized via the flux method. EDS on several crystals of various sizes and shapes revealed an average molar ratio of 17:9:74 for Mn:Cr:Al, while x-ray diffraction on three different crystals yield compositions Mn_2.26Cr_1.74Al₁₁(Mn_4–xCr_xAl₁₁,x= 1.74), Mn_0.83Cr_3.17Al₁₁, and Mn_1.07Cr_2.93Al₁₁. This compound crystallizes in space group $P\overline{1}$ , isostructural with both Mn₄Al₁₁and Cr₄Al₁₁. Magnetic measurements on several crystals show that this disordered compound is ferrimagnetic with a low effective moment of ${\mu }_{\text{eff}}\approx 1.012\pm 0.004{\mu }_{\text{B}}/\text{f}.\text{u}.$ and a non-reachable transition temperature. Density functional theory calculations display opening of a bandgap in the spin-up channel near the Fermi level with increasing Cr content, an indication of half-metallicity. 

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3. Observation of $\text{γ}\text{γ}\to \text{τ}\text{τ}$ in proton–proton collisions and limits on the anomalous electromagnetic moments of the τ lepton

   https://doi.org/10.1088/1361-6633/ad6fcb  

   CMS_Collaboration, The (September 2024, Reports on Progress in Physics)

   Abstract The production of a pair of τ leptons via photon–photon fusion, $\text{γ}\text{γ}\to \text{τ}\text{τ}$ , is observed for the first time in proton–proton collisions, with a significance of 5.3 standard deviations. This observation is based on a data set recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb⁻¹. Events with a pair of τ leptons produced via photon–photon fusion are selected by requiring them to be back-to-back in the azimuthal direction and to have a minimum number of charged hadrons associated with their production vertex. The τ leptons are reconstructed in their leptonic and hadronic decay modes. The measured fiducial cross section of $\text{γ}\text{γ}\to \text{τ}\text{τ}$ is ${\sigma }_{\text{obs}}^{\text{fid}}={12.4}_{-3.1}^{+3.8}\text{fb}$ . Constraints are set on the contributions to the anomalous magnetic moment ( $a_{\text{τ}}$ ) and electric dipole moments ( $d_{\text{τ}}$ ) of the τ lepton originating from potential effects of new physics on the $\text{γ}\text{τ}\text{τ}$ vertex: $a_{\text{τ}}={0.0009}_{-0.0031}^{+0.0032}$ and $|d_{\text{τ}}|<2.9\times {10}^{-17}e\text{cm}$ (95% confidence level), consistent with the standard model. 

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4. Searching for GEMS: Discovery and Characterization of Two Brown Dwarfs Around M Dwarfs*

   https://doi.org/10.3847/1538-3881/adbb54  

   Larsen, Alexander; Swaby, Tera N; Kobulnicky, Henry A; Cañas, Caleb I; Kanodia, Shubham; Libby-Roberts, Jessica; Monson, Andrew; Gupta, Arvind F; Cochran, William; Mahadevan, Suvrath; et al (April 2025, The Astronomical Journal)

   Abstract Brown dwarfs bridge the gap between stars and planets, providing valuable insight into both planetary and stellar-formation mechanisms. Yet the census of transiting brown-dwarf companions, in particular around M-dwarf stars, remains incomplete. We report the discovery of two transiting brown dwarfs around low-mass hosts using a combination of space- and ground-based photometry along with near-infrared radial velocities. We characterize TOI-5389Ab ( $68.0_{-2.2}^{+2.2}{M}_{\mathrm{J}}$ ) and TOI-5610b ( $40.4_{-1.0}^{+1.0}{M}_{\mathrm{J}}$ ), two moderately massive brown dwarfs orbiting early M-dwarf hosts (T_eff = 3569 ± 59 K and 3618 ± 59 K, respectively). For TOI-5389Ab, the best fitting parameters are periodP = 10.40046 ± 0.00002 days, radius $R_{\mathrm{BD}}=0.824_{-0.031}^{+0.033}$ R_J, and low eccentricity $e=0.0962_{-0.0046}^{+0.0027}$ . In particular, this constitutes one of the most extreme substellar-stellar companion-to-host mass ratios ofq= 0.150. For TOI-5610b, the best-fitting parameters are periodP = 7.95346 ± 0.00002 days, radius $R_{\mathrm{BD}}=0.887_{-0.031}^{+0.031}$ R_J, and moderate eccentricity $e=0.354_{-0.012}^{+0.011}$ . Both targets are expected to have shallow, but potentially observable, occultations: ≲500 ppm in the JohnsonKband. A statistical analysis of M-dwarf/BD systems reveals for the first time that those at short orbital periods (P < 13 days) exhibit a dearth of 13M_J < M_BD < 40M_Jcompanions (q < 0.1) compared to those at slightly wider separations. 

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5. Suppression of Collisionless Magnetic Reconnection in the High Ion β, Strong Guide Field Limit

   https://doi.org/10.3847/1538-4357/ad9274  

   Giai, Carlos A; Haggerty, Colby C; Shay, Michael A; Cassak, Paul A; Davis, Zachary K (December 2024, The Astrophysical Journal)

   Abstract In magnetic reconnection, the ion bulk outflow speed and ion heating have been shown to be set by the available reconnecting magnetic energy, i.e., the energy stored in the reconnecting magnetic field (B_r). However, recent simulations, observations, and theoretical works have shown that the released magnetic energy is inhibited by upstream ion plasma betaβ_i—the relative ion thermal pressure normalized to magnetic pressure based on the reconnecting field—for antiparallel magnetic field configurations. Using kinetic theory and hybrid particle-in-cell simulations, we investigate the effects ofβ_ion guide field reconnection. While previous works have suggested that guide field reconnection is uninfluenced byβ_i, we demonstrate that the reconnection process is modified and the outflow is reduced for sufficiently large ${\beta }_i>(B_r^2+B_g^2)/B_r^2$ . We develop a theoretical framework that shows that this reduction is consistent with an enhanced exhaust pressure gradient, which reduces the outflow speed as $v_{\mathrm{out}}\propto 1/\sqrt{{\beta }_i}$ . These results apply to systems in which guide field reconnection is embedded in hot plasmas, such as reconnection at the boundary of eddies in fully developed turbulence like the solar wind or the magnetosheath as well as downstream of shocks such as the heliosheath or the mergers of galaxy clusters. 

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