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1. The Origin of Power-law Spectra in Relativistic Magnetic Reconnection

   https://doi.org/10.3847/2041-8213/acfe7c  

   Zhang, Hao; Sironi, Lorenzo; Giannios, Dimitrios; Petropoulou, Maria (October 2023, The Astrophysical Journal Letters)

   Abstract Magnetic reconnection is often invoked as a source of high-energy particles, and in relativistic astrophysical systems it is regarded as a prime candidate for powering fast and bright flares. We present a novel analytical model—supported and benchmarked with large-scale three-dimensional kinetic particle-in-cell simulations in electron–positron plasmas—that elucidates the physics governing the generation of power-law energy spectra in relativistic reconnection. Particles with Lorentz factorγ≳ 3σ(here,σis the magnetization) gain most of their energy in the inflow region, while meandering between the two sides of the reconnection layer. Their acceleration time is $t_{\mathrm{acc}}\sim \gamma {\eta }_{\mathrm{rec}}^{-1}{\omega }_{\mathrm{c}}^{-1}\simeq 20\gamma {\omega }_{\mathrm{c}}^{-1}$, whereη_rec≃ 0.06 is the inflow speed in units of the speed of light andω_c=eB₀/mcis the gyrofrequency in the upstream magnetic field. They leave the region of active energization aftert_esc, when they get captured by one of the outflowing flux ropes of reconnected plasma. We directly measuret_escin our simulations and find thatt_esc∼t_accforσ≳ few. This leads to a universal (i.e.,σ-independent) power-law spectrum ${\mathit{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{-1}$for the particles undergoing active acceleration, and $\mathit{dN}/d\gamma \propto {\gamma }^{-2}$for the overall particle population. Our results help to shed light on the ubiquitous presence of power-law particle and photon spectra in astrophysical nonthermal sources. 

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2. Constraining Cosmological Parameters Using the Cluster Mass–Richness Relation

   https://doi.org/10.3847/1538-4357/ace773  

   Abdullah, Mohamed H.; Wilson, Gillian; Klypin, Anatoly; Ishiyama, Tomoaki (September 2023, The Astrophysical Journal)

   Abstract The cluster mass–richness relation (MRR) is an observationally efficient and potentially powerful cosmological tool for constraining the matter density Ω_mand the amplitude of fluctuationsσ₈using the cluster abundance technique. We derive the MRR relation usingGalWCat19, a publicly available galaxy cluster catalog we created from the Sloan Digital Sky Survey-DR13 spectroscopic data set. In the MRR, cluster mass scales with richness as $\log M_{200}=\alpha +\beta \log N_{200}$. We find that the MRR we derive is consistent with both the IllustrisTNG and mini-Uchuu cosmological numerical simulations, with a slope ofβ≈ 1. We use the MRR we derived to estimate cluster masses from theGalWCat19catalog, which we then use to set constraints on Ω_mandσ₈. Utilizing the all-member MRR, we obtain constraints of Ω_m= ${0.31}_{-0.03}^{+0.04}$andσ₈= ${0.82}_{-0.04}^{+0.05}$, and utilizing the red member MRR only, we obtain Ω_m= ${0.31}_{-0.03}^{+0.04}$andσ₈= ${0.81}_{-0.04}^{+0.05}$. Our constraints on Ω_mandσ₈are consistent and very competitive with the Planck 2018 results. 

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3. Evolution of Magnetohydrodynamic Turbulence in the Expanding Solar Wind: Residual Energy and Intermittency

   https://doi.org/10.3847/1538-4357/ad9f38  

   Shi_时, Chen_辰; Sioulas, Nikos; Huang_黄, Zesen_泽森; Velli, Marco; Tenerani, Anna; Réville, Victor (January 2025, The Astrophysical Journal)

   Abstract We conduct 3D magnetohydrodynamic simulations of decaying turbulence in the context of the solar wind. To account for the spherical expansion of the solar wind, we implement the expanding box model. The initial turbulence comprises uncorrelated counterpropagating Alfvén waves and exhibits an isotropic power spectrum. Our findings reveal the consistent generation of negative residual energy whenever nonlinear interactions are present, independent of the normalized cross helicityσ_cand compressibility. The spherical expansion facilitates this process. The resulting residual energy is primarily distributed in the perpendicular direction, withS₂(b) − S₂(u) ∝ l_⊥or equivalently $-E_r\propto k_{\perp }^{-2}$. HereS₂(b) andS₂(u) are second-order structure functions of magnetic field and velocity respectively. In most runs,S₂(b) develops a scaling relation $S_2\left(\mathit{b}\right)\propto l_{\perp }^{1/2}$( $E_b\propto k_{\perp }^{-3/2}$). In contrast,S₂(u) is consistently shallower thanS₂(b), which aligns with in situ observations of the solar wind. We observe that the higher-order statistics of the turbulence, which act as a proxy for intermittency, depend on the initialσ_cand are strongly affected by the expansion effect. Generally, the intermittency is more pronounced when the expansion effect is present. Finally, we find that in our simulations, although the negative residual energy and intermittency grow simultaneously as the turbulence evolves, the causal relation between them seems to be weak, possibly because they are generated on different scales. 

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4. Ultra-High-Energy Cosmic Rays Accelerated by Magnetically Dominated Turbulence

   https://doi.org/10.3847/2041-8213/ad955f  

   Comisso, Luca; Farrar, Glennys_R; Muzio, Marco_S (December 2024, The Astrophysical Journal Letters)

   Abstract Ultra-high-energy cosmic rays (UHECRs), particles characterized by energies exceeding 10¹⁸eV, are generally believed to be accelerated electromagnetically in high-energy astrophysical sources. One promising mechanism of UHECR acceleration is magnetized turbulence. We demonstrate from first principles, using fully kinetic particle-in-cell simulations, that magnetically dominated turbulence accelerates particles on a short timescale, producing a power-law energy distribution with a rigidity-dependent, sharply defined cutoff well approximated by the form $f_{\mathrm{cut}}\left(E,E_{\mathrm{cut}}\right)=\mathrm{sech}\left({\left(E/E_{\mathrm{cut}}\right)}^2\right)$. Particle escape from the turbulent accelerating region is energy dependent, witht_esc∝E^−δandδ∼ 1/3. The resulting particle flux from the accelerator follows $\mathit{dN}/\mathit{dEdt}\propto E^{-s}\mathrm{sech}\left({\left(E/E_{\mathrm{cut}}\right)}^2\right)$, withs∼ 2.1. We fit the Pierre Auger Observatory’s spectrum and composition measurements, taking into account particle interactions between acceleration and detection, and show that the turbulence-associated energy cutoff is well supported by the data, with the best-fitting spectral index being $s={2.1}_{-0.13}^{+0.06}$. Our first-principles results indicate that particle acceleration by magnetically dominated turbulence may constitute the physical mechanism responsible for UHECR acceleration. 

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5. The Dark Energy Survey: Cosmology Results with ∼1500 New High-redshift Type Ia Supernovae Using the Full 5 yr Data Set

   https://doi.org/10.3847/2041-8213/ad6f9f  

   Abbott, DES_Collaboration_T_M_C; Acevedo, M.; Aguena, M.; Alarcon, A.; Allam, S.; Alves, O.; Amon, A.; Andrade-Oliveira, F.; Annis, J.; Armstrong, P.; et al (October 2024, The Astrophysical Journal Letters)

   Abstract We present cosmological constraints from the sample of Type Ia supernovae (SNe Ia) discovered and measured during the full 5 yr of the Dark Energy Survey (DES) SN program. In contrast to most previous cosmological samples, in which SNe are classified based on their spectra, we classify the DES SNe using a machine learning algorithm applied to their light curves in four photometric bands. Spectroscopic redshifts are acquired from a dedicated follow-up survey of the host galaxies. After accounting for the likelihood of each SN being an SN Ia, we find 1635 DES SNe in the redshift range 0.10 <z< 1.13 that pass quality selection criteria sufficient to constrain cosmological parameters. This quintuples the number of high-qualityz> 0.5 SNe compared to the previous leading compilation of Pantheon+ and results in the tightest cosmological constraints achieved by any SN data set to date. To derive cosmological constraints, we combine the DES SN data with a high-quality external low-redshift sample consisting of 194 SNe Ia spanning 0.025 <z< 0.10. Using SN data alone and including systematic uncertainties, we find Ω_M= 0.352 ± 0.017 in flat ΛCDM. SN data alone now require acceleration (q₀< 0 in ΛCDM) with over 5σconfidence. We find $({\mathrm{\Omega }}_{\mathrm{M}},w)=({0.264}_{-0.096}^{+0.074},-{0.80}_{-0.16}^{+0.14})$in flatwCDM. For flatw₀w_aCDM, we find $({\mathrm{\Omega }}_{\mathrm{M}},w_0,w_a)=({0.495}_{-0.043}^{+0.033},-{0.36}_{-0.30}^{+0.36},-{8.8}_{-4.5}^{+3.7})$, consistent with a constant equation of state to within ∼2σ. Including Planck cosmic microwave background, Sloan Digital Sky Survey baryon acoustic oscillation, and DES 3 × 2pt data gives (Ω_M,w) = (0.321 ± 0.007, −0.941 ± 0.026). In all cases, dark energy is consistent with a cosmological constant to within ∼2σ. Systematic errors on cosmological parameters are subdominant compared to statistical errors; these results thus pave the way for future photometrically classified SN analyses. 

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