tokens: ACKNOWLEDGEMENTS The authors acknowledge support from the PRIN-MIUR project Multi-scale Simulations of High-Energy Astrophysical Plasmas (Prot. 2015L5EE2Y). We would also grateful to the referee for his/her constructive comments which helped to improve the quality of this manuscript. We wish to thank L. Pareschi for valuable comments on the development of the Riemann solver in the context of IMEX schemes.
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