The observed hardening of the spectra of cosmic ray protons and helium nuclei is studied within the model of nonlinear diffusive shock acceleration of supernova remnants (SNRs). In this model, the injected particles with energies below the spectral " knee” are assumed to be described by two populations with different spectral indexes around 200 GeV. The high-energy population is dominated by the particles with energies above 200 GeV released upstream of the shock of SNR, and the low-energy population is attributed to the particles with energies below 200 GeV released downstream of the shock of SNR. In this scenario, the spectral hardening of cosmic ray protons and helium nuclei observed by PAMELA, AMS-02, and CREAM experiments can be reproduced.

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Two-dimensional cylindrical magnetohydrodynamic (MHD) simulations are implemented to investigate the dynamical properties of young type Ia supernova remnants (SNRs) undergoing shock acceleration in a turbulent medium. In our simulations, an MHD code is coupled with a semianalytical kinetic treatment of shock acceleration by means of a time-dependent effective adiabatic index. Large-scale density and magnetic field fluctuations are calculated and mapped into the computational domain before simulations. The above configurations allow us to study the time-dependent dynamical properties and magnetic field structure of a benchmark SNR undergoing shock acceleration in a turbulent medium, along with the relative positions of the contact discontinuity. Our simulation results reveal that there is a rippled forward shock, a thinner shocked ejecta layer and a denser, narrower intershock region. The resulting net effect is a higher density difference between the shocked ejecta and the shocked interstellar medium, leading to a growth of the Rayleigh–Taylor instability. The amplified magnetic field occurs not only at the contact discontinuity but also near the immediate downstream of the shock. The spatial location of the maximum magnetic field is in the vicinity of immediate downstream, which is different with Guo et al. Our derived profiles of the relative contact discontinuity positions are compatible with the results of two typical young type Ia SNRs: SN 1006 and Tycho, with the lowest value reaching ∼1.02 for both cases. Moreover, we find no obvious ejecta protrusions beyond the main forward shock.

Blazar CTA 102 experienced an intense multiwavelength activity phase from 2015 to 2018; in particular, an unprecedented outburst was observed from 2016 October to 2017 February. In this work, we extract a 7 day binned γ-ray light curve from 2008 August to 2018 March in the energy range 0.1–300 GeV and identify three main outbursts. We study in detail the short-timescale variability of these three outbursts via an exponential function with parameterized rise and decay timescales. The obtained shortest rise and decay timescales are 0.70 ± 0.05 hr and 0.79 ± 0.27 hr, respectively. Based on these variability timescales, the physical parameters of the flaring region (e.g., the minimum Doppler factor and the emission region size) are constrained. The short-timescale flares exhibit a symmetric temporal profile within the error bars, implying that the rise and decay timescales are dominated by the light-crossing timescale or by disturbances caused by dense plasma blobs passing through the standing shock front in the jet region. We also find that the best-fitting form of the γ-ray spectra during the flare period is a power law with an exponential cutoff. The derived jet parameters from the spectral behavior and the temporal characteristics of the individual flares suggest that the γ-ray emission region is located upstream of the radio core. The extreme γ-ray flare of CTA 102 is likely to have been caused by magnetic reconnection.

We study the polarized radio emission from young Type Ia supernova remnants by means of three-dimensional numerical MHD simulations and the assumption of relativistic electron distribution related to magnetic field energy density. In our simulations, the turbulent component of background plasma is taken into account by introducing a 3D Kolmogorov-like power spectrum. The simulation results indicate (i) the different orientations of the interstellar magnetic field around SNRs and lines of sight could produce different polarized radio emission shells, (ii) the fraction of polarization may be decreased through integrating the synchrotron emissivities along the line of sight, which is mainly due to the emission from the strong turbulent and disordered region of the magnetic field, and (iii) the total intensity is strong in some regions where the polarization degree is low.