高能暂现源 · 每日文献综述

Gamma-ray bursts (GRBs) are among the most energetic extragalactic transients, providing a unique laboratory to investigate the physical origin of the diverse morphologies observed in high-energy light curves and spectra. These morphologies, interpreted through synchrotron and synchrotron self-Compton (SSC) closure relations (CRs), may depend on the circumburst density profile, energy injection, and the dynamical evolution of the reverse shock in either the adiabatic or radiative regime. We derive SSC CRs produced in the reverse-shock region for both thick- and thin-shell cases, considering constant-density and stratified circumburst media. The analysis assumes two power-law (PL) distributions for the electron spectral index and includes scenarios with and without energy injection from the progenitor. We show that, depending on the physical parameters, SSC emission from the reverse shock can reproduce the initial steep-decay and plateau phases commonly observed in GRB afterglows. We then compare the predicted spectral and temporal indices with those derived from PL and broken power-law (BPL) fits reported in the Second Fermi-LAT Gamma-ray Burst Catalog (2FLGC). Our results indicate that PL (BPL) light-curve fits preferentially favor scenarios with (without) energy injection. In scenarios with energy injection, the data preferentially support a constant-density environment over a stellar-wind profile.

Flare ribbons form as a result of energy deposition associated with particles accelerated in low layers of the solar atmosphere. The fine-scale structures of flare ribbons, also called ribbon kernels, offer a potentially powerful diagnostic of the flare reconnection process, however, to date the dynamic evolution of ribbon kernels has not been fully characterized in statistical studies. Here, we checked the state-of-the-art observations (cadence ≤2.5 s) of solar flares in the ultraviolet from space by Interface Region Imaging Spectrograph over the past 12 yr. Our results showed the first statistical study of multiple spatially resolved flare kernel quasiperiodic pulsation events for 31 flares, with the period of 6─24 s. The ribbon kernels have a spatial scale of 480─1200 km and some kernels exhibit an unprecedented fast "winking" process, i.e., quasiperiodic pulsation-like flashing of individual kernels. The shortest heating time reaches about 2─3 s, implying that the energy is deposited only in a small localized region within flare ribbons, persisting for only a few seconds. Meanwhile, some ribbon kernels were observed to slip along the ribbon at speeds of 20─1800 km s<SUP>−1</SUP>. These observations strongly imply a joint picture for the dynamics and the bursty nature of ribbon kernels as being due to coupled effects of plasmoid formation and three-dimensional magnetic reconnection in the overlaying coronal current sheet. We suggest that the observed flare behaviors provide strong observational evidences of 3D bursty reconnection.

Fast radio bursts (FRBs) are luminous, millisecond-duration extragalactic radio transients that have emerged as a powerful, complementary cosmological probe for investigating the late-time cosmic evolution, offering unique advantages over conventional probes such as Type Ia supernovae, baryon acoustic oscillations, and cosmic microwave background radiation. This review systematically summarizes the cosmological applications of FRBs, focusing on their critical roles in measuring the Hubble constant (H<SUB>0</SUB>) and constraining dark energy properties. Benefiting from the precise dispersion measure—redshift relation of localized FRBs, the integrated electron density of the intergalactic medium (IGM) along the line of sight can be tightly modeled, enabling independent and low-redshift measurements of the cosmic expansion rate. Current FRB samples consisting of localized and non-localized events provide competitive H<SUB>0</SUB> constraints, offering an independent method to measure H<SUB>0</SUB>. FRBs also serve as effective tracers to constrain dark energy equation-of-state parameters. We comprehensively discuss key limiting factors for FRB cosmological precision, including uncertainties in Galactic and host galaxy electron density models, and IGM inhomogeneities. With the rapid growth of high-precision FRB surveys and localized FRB samples, FRBs are promising to provide stringent constraints on late-time cosmic acceleration, dark energy evolution and cosmic baryons.