On the Challenges of Cosmic-Ray Proton Shock Acceleration in the Intracluster Medium

Galaxy clusters host the largest particle accelerators in the Universe: Shock waves in the intracluster medium (ICM), a hot and ionised plasma, that accelerate particles to high energies. Radio observations pick up synchrotron emission in the ICM, proving the existence of accelerated cosmic-ray electrons. However, a sign of cosmic-ray protons, in form of $\gamma$-rays. remains undetected. This is know as the missing $\gamma$-ray problem and it directly challenges the shock acceleration mechanism at work in the ICM.Over the last decade, theoretical and numerical studies focused on improving our knowledge on the microphysics that govern the shock acceleration process in the ICM. These new models are able to predict a $\gamma$-ray signal, produced by shock accelerated cosmic-ray protons, below the detection limits set modern $\gamma$-ray observatories. In this review, we summarise the latest advances in solving the missing $\gamma$-ray problem.     

The Galactic Aberration and Its Impact on Astronomical Reference Frames

The Galactic aberration effect, also known as the secular aberration drift, is a consequence of the centripetal acceleration of the Solar System Barycenter in the circular orbit around the Galactic center. It causes distance-independent apparent proper motions (the amplitude is about 5 ${\mu \mathit{as}·\mathit{yr}}^{-1}$) for extragalactic sources which were regarded as motionless before 21th century. As the very long baseline interferometry (VLBI) has been greatly developed, and the ESA (European Space Agency) space mission Gaia has provided ultra high-precision astrometric data, the Galactic aberration effect has becoming important. It causes slow spin of the reference frame due to the non-uniform distribution of extragalactic sources. Therefore systematic corrections have to be applied to the Earth rotation parameters. For the precession rate, the correction is about 1 ${\mu \mathit{as}·\mathit{yr}}^{-1}$. For the very high accurate VLBI and Gaia reference frames, the Galactic aberration effect will introduce small distortion which is a crucial systematic effect for the link of the two reference frames.     

Turbulence Diffusion and Energy Spectrum Analysis of Large-scale Current Sheets in Flares

Magnetic reconnection (MR) is one of the most important physical processes for many dynamical phenomena in the universe. Magnetohydrodynamical (MHD) simulation is an effective way to study the MR process and the physical pictures related to the MR. With different parameter setups, we investigate the influences of the Magnetic Reynolds number and spatial resolution on the reconnection rate, numerical dissipation, and energy spectrum distribution in the MHD simulation. We have found that the magnetic Reynolds number $R_m$ has definite impact on the reconnection rate and energy spectrum distribution. The characteristic time for entering into the non-linear phase will be earlier as the Reynolds number increases. When it comes to the tearing phase, the reconnection rate will increase rapidly. On the other hand, the magnetic Reynolds number affects significantly the Kolmogorov microscopic scale $l_{\mathit{ko}}$, which becomes smaller as $R_m$ increases. An extra dissipation is defined as the combined effect of the numerical diffusion and turbulence dissipation. It is shown that the extra dissipation is dominated by the numerical diffusion before the tearing mode instability takes place. After the instability develops, the extra dissipation rises vastly, which indicates that turbulence caused by the instability can enhance the diffusion obviously. Furthermore, the energy spectrum analysis indicates that $l_{\mathit{ko}}$ of the large-scale current sheet may appear at a macroscopic MHD scale very possibly.     

LRS Bianchi I model with strange quark matter and Λ(t) in f(R,T) gravity

A spatially homogeneous and anisotropic locally-rotationally-symmetric Bianchi type-I spacetime model with strange quark matter (SQM) and a variable cosmological term $\Lambda \left(t\right)$ is studied in $f(R,T)$ theory. The exact solutions are obtained for a particular form of $f(R,T)=R+2\lambda T$ under the law of variable deceleration parameter proposed by Banerjee et al (2005). The model presents a cosmological scenario of transition from early deceleration to late time acceleration. In the absence of $f(R,T)$ gravity and SQM, $\Lambda \left(t\right)$ accelerates the universe. In a specific case when $\Lambda \left(t\right)$ vanishes at late times, the SQM accelerates the universe, even in the absence of $f(R,T)$ gravity. The model shows the possibility that SQM could be an alternative to dark energy. The physical viability of the model costs the observational inconsistency.