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1. Coupling between magnetic reconnection, energy release, and particle acceleration in the X17.2 2003 October 28 solar flare

   https://doi.org/10.1051/0004-6361/202349130  

   Kurt, Victoria G; Veronig, Astrid M; Fleishman, Gregory D; Hinterreiter, Jürgen; Tschernitz, Johannes; Lysenko, Alexandra L (June 2024, Astronomy & Astrophysics)

   Context.The 2003 October 28 (X17.2) eruptive flare was a unique event. The coronal electric field and theπ-decayγ-ray emission flux displayed the highest values ever inferred for solar flares. Aims.Our aim is to reveal physical links between the magnetic reconnection process, energy release, and acceleration of electrons and ions to high energies in the chain of the magnetic energy transformations in the impulsive phase of the solar flare. Methods.The global reconnection rate,φ̇(t), and the local reconnection rate (coronal electric field strength),E_c(r, t), were calculated from flare ribbon separation in Hαfiltergrams and photospheric magnetic field maps. Then, HXRs measured by CORONAS-F/SPR-N and the derivative of the GOES SXR flux,İ_SXR(t) were used as proxies of the flare energy release evolution. The flare early rise phase, main raise phase, and main energy release phase were defined based on temporal profiles of the above proxies. The available results of INTEGRAL and CORONAS-F/SONG observations were combined with Konus-Wind data to quantify the time behavior of electron and proton acceleration. Promptγ-ray lines and delayed 2.2 MeV line temporal profiles observed with Konus-Wind and INTEGRAL/SPI were used to detect and quantify the nuclei with energies of 10−70 MeV. Results.The magnetic-reconnection rates,φ̇(t) andE_c(r, t), follow a common evolutionary pattern with the proxies of the flare energy released into high-energy electrons. The global and local reconnection rates reach their peaks at the end of the main rise phase of the flare. The spectral analysis of the high-energyγ-ray emission revealed a close association between the acceleration process efficiency and the reconnection rates. High-energy bremsstrahlung continuum and narrowγ-ray lines were observed in the main rise phase whenE_c(r, t) of the positive (negative) polarity reached values of ∼120 V cm⁻¹(∼80 V cm⁻¹). In the main energy release phase, the upper energy of the bremsstrahlung spectrum was significantly reduced and the pion-decayγ-ray emission appeared abruptly. We discuss the reasons why the change of the acceleration regime occurred along with the large-scale magnetic field restructuration of this flare. Conclusions.The similarities between the proxies of the flare energy release withφ̇(t) andE_c(r, t) in the flare’s main rise phase are in accordance with the reconnection models. We argue that the main energy release and proton acceleration up to subrelativistic energies began just when the reconnection rate was going through the maximum, that is, following a major change of the flare topology. 

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2. Investigating Turbulence Effects on Magnetic Reconnection Rates through 3D Resistive Magnetohydrodynamic Simulations

   https://doi.org/10.3847/1538-4357/addc62  

   Vicentin, Giovani H; Kowal, Grzegorz; de_Gouveia_Dal_Pino, Elisabete M; Lazarian, Alex (July 2025, The Astrophysical Journal)

   Abstract We investigate the impact of turbulence on magnetic reconnection through high-resolution 3D magnetohydrodynamic (MHD) simulations, spanning Lundquist numbers fromS= 10³to 10⁶. Building on the A. Lazarian & E. T. Vishniac theory, which asserts reconnection rate independence from ohmic resistivity, we introduce small-scale perturbations untilt= 0.1t_A. Even after the perturbations cease, turbulence persists, resulting in sustained high reconnection rates ofV_rec/V_A∼ 0.03–0.08. These rates exceed those generated by resistive tearing modes (plasmoid chain) in 2D and 3D MHD simulations by factors of 5–6. Our findings match observations in solar phenomena and previous 3D MHD global simulations of solar flares, accretion flows, and relativistic jets. The simulations show a steady-state fast reconnection rate compatible with the full development of turbulence in the system, demonstrating the robustness of the process in turbulent environments. We confirm reconnection rate independence from the Lundquist number, supporting Lazarian and Vishniac’s theory of fast turbulent reconnection. Additionally, we find a mild dependence ofV_recon the plasma–βparameter, decreasing from 0.036 to 0.028 (in Alfvén units) asβincreases from 2.0 to 64.0 for simulations with a Lundquist number of 10⁵. Lastly, we explore the magnetic Prandtl number’s (Pr_m=ν/η) influence on the reconnection rate and find it negligible during the turbulent regime across the range tested, from Pr_m= 1 to 60. 

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3. A 7 Day Multiwavelength Flare Campaign on AU Mic. I. High-time-resolution Light Curves and the Thermal Empirical Neupert Effect

   https://doi.org/10.3847/1538-4357/acc94f  

   Tristan, Isaiah I; Notsu, Yuta; Kowalski, Adam F; Brown, Alexander; Wisniewski, John P; Osten, Rachel A; Vrijmoet, Eliot H; White, Graeme L; Carter, Brad D; Grady, Carol A; et al (June 2023, The Astrophysical Journal)

   Abstract We present light curves and flares from a 7 day, multiwavelength observational campaign of AU Mic, a young and active dM1e star with exoplanets and a debris disk. We report on 73 unique flares between the X-ray to optical data. We use high-time-resolution near-UV (NUV) photometry and soft X-ray (SXR) data from the X-ray Multi-Mirror Mission to study the empirical Neupert effect, which correlates the gradual and impulsive phase flaring emissions. We find that 65% (30 of 46) flares do not follow the Neupert effect, which is 3 times more excursions than seen in solar flares, and propose a four-part Neupert effect classification (Neupert, quasi-Neupert, non-Neupert types I and II) to explain the multiwavelength responses. While the SXR emission generally lags behind the NUV as expected from the chromospheric evaporation flare models, the Neupert effect is more prevalent in larger, more impulsive flares. Preliminary flaring rate analysis with X-ray andU-band data suggests that previously estimated energy ratios hold for a collection of flares observed over the same time period, but not necessarily for an individual, multiwavelength flare. These results imply that one model cannot explain all stellar flares and care should be taken when extrapolating between wavelength regimes. Future work will expand wavelength coverage using radio data to constrain the nonthermal empirical and theoretical Neupert effects to better refine models and bridge the gap between stellar and solar flare physics. 

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4. Scaling of Ion Bulk Heating in Magnetic Reconnection Outflows for the High-Alfvén-speed and Low-β Regime in Earth’s Magnetotail

   https://doi.org/10.3847/1538-4357/ad6151  

   Øieroset, M; Phan, T D; Drake, J F; Starkey, M; Fuselier, S A; Cohen, I J; Haggerty, C C; Shay, M A; Oka, M; Gershman, D J; et al (August 2024, The Astrophysical Journal)

   Abstract We survey 20 reconnection outflow events observed by Magnetospheric MultiScale in the low-βand high-Alfvén-speed regime of the Earth’s magnetotail to investigate the scaling of ion bulk heating produced by reconnection. The range of inflow Alfvén speeds (800–4000 km s⁻¹) and inflow ionβ(0.002–1) covered by this study is in a plasma regime that could be applicable to the solar corona and flare environments. We find that the observed ion heating increases with increasing inflow (upstream) Alfvén speed,V_A, based on the reconnecting magnetic field and the upstream plasma density. However, ion heating does not increase linearly as a function of available magnetic energy per particle, ${{\mathit{m}}_{\mathrm{i}}{\mathit{V}}_{\mathrm{A}}}^2$. Instead, the heating increases progressively less as ${{\mathit{m}}_{\mathrm{i}}{\mathit{V}}_{\mathrm{A}}}^2$rises. This is in contrast to a previous study using the same data set, which found that electron heating in this high-Alfvén-speed and low-βregime scales linearly with ${{\mathit{m}}_{\mathrm{i}}{\mathit{V}}_{\mathrm{A}}}^2$, with a scaling factor nearly identical to that found for the low-V_Aand high-βmagnetopause. Consequently, the ion-to-electron heating ratio in reconnection exhausts decreases with increasing upstreamV_A, suggesting that the energy partition between ions and electrons in reconnection exhausts could be a function of the available magnetic energy per particle. Finally, we find that the observed difference in ion and electron heating scaling may be consistent with the predicted effects of a trapping potential in the exhaust, which enhances electron heating, but reduces ion heating. 

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5. No X-Rays or Radio from the Nearest Black Holes and Implications for Future Searches

   https://doi.org/10.1088/1538-3873/ad228e  

   Rodriguez, Antonio C.; Cendes, Yvette; El-Badry, Kareem; Berger, Edo (February 2024, Publications of the Astronomical Society of the Pacific)

   Abstract Astrometry from the Gaia mission was recently used to discover the two nearest known stellar-mass black holes (BHs), Gaia BH1 and Gaia BH2. These objects are among the first stellar-mass BHs not discovered via X-rays or gravitational waves. Both systems contain ∼1M_⊙stars in wide orbits (a≈ 1.4 au, 4.96 au) around ∼9M_⊙BHs, with both stars (solar-type main sequence star, red giant) well within their Roche lobes in Gaia BH1 and BH2, respectively. However, the BHs are still expected to accrete stellar winds, leading to potentially detectable X-ray or radio emission. Here, we report observations of both systems with the Chandra X-ray Observatory, the Very Large Array (for Gaia BH1) and MeerKAT (for Gaia BH2). We did not detect either system, leading to X-ray upper limits ofL_X< 9.4 × 10²⁸andL_X< 4.0 × 10²⁹erg s⁻¹and radio upper limits ofL_r< 1.6 × 10²⁵andL_r< 1.0 × 10²⁶erg s⁻¹for Gaia BH1 and BH2, respectively. For Gaia BH2, the non-detection implies that the accretion rate near the horizon is much lower than the Bondi rate, consistent with recent models for hot accretion flows. We discuss implications of these non-detections for broader BH searches, concluding that it is unlikely that isolated BHs will be detected via interstellar medium accretion in the near future. We also calculate evolutionary models for the binaries’ future evolution using Modules for Experiments in Stellar Astrophysics, and find that Gaia BH1 will be visible as a symbiotic BH X-ray binary for 5–50 Myr. Since no symbiotic BH X-ray binaries are known, this implies either that fewer than ∼10⁴Gaia BH1-like binaries exist in the Milky Way, or that they are common but have evaded detection. 

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