Supernova Neutrino in a Strangeon Star Model

The neutrino burst detected during supernova SN1987A is explained in a strangeon star model, in which it is proposed that a pulsar-like compact object is composed of strangeons (strangeon: an abbreviation of "strange nucleon"). A nascent strangeon star's initial internal energy is calculated, with the inclusion of pion excitation (energy around 10^53 erg, comparable to the gravitational binding energy of a collapsed core). A liquid-solid phase transition at temperature ~ 1-2 MeV may occur only a few ten-seconds after core-collapse, and the thermal evolution of strangeon star is then modeled. It is found that the neutrino burst observed from SN 1987A could be re-produced in such a cooling model.Comment: 15 pages, 7 figures, 1 tabe

Autonomous synchronizing and frequency response control of multi-terminal DC systems with wind farm integration

Recent analyses have shown that the grid-integration of offshore wind farms through MTDC systems has brought low inertia and small-signal stability issues, in which the dynamics of phase-locked-loop (PLL) play a crucial role. To address this issue, this paper proposes a control strategy for the multi-terminal VSCs aiming at PLL-less synchronization and autonomous frequency response of the MTDC system. One of the significant features of the proposed control is that the deviation of the grid frequency can be instantaneously reflected on the deviation of the DC voltage without ancillary control. Based on this feature, a fast inertia response and primary frequency regulation among wind farms and AC systems interconnected by the MTDC system can be achieved. A small-signal model is established to evaluate the overall system stability using the proposed control. Finally, comparative studies of this proposed control with the conventional PLL-based vector control are conducted in PSCAD/EMTDC based on a practical MTDC system in China, the Zhangbei four-terminal HVDC transmission system. The analysis shows that the proposed control exhibits advantages in weak grid operation and autonomous frequency respons

Internal energy based grid-forming control for MMC-HVDC systems with wind farm integration

The virtual synchronous control is regarded as an effective solution for grid-tied converters to operate under weak grid conditions and provide inertia support. However, for those grid-tied converters which control the DC voltage, e.g., the receiving end converter (REC) in modular multilevel converter based high-voltage direct-current (MMC-HVDC) transmission systems with wind farm integration, the application of virtual synchronous control is difficult due to the cascade control loops. The dynamic performance of DC voltage control may also be degraded. To address these issues, a novel grid-forming control strategy with real-time inertia support and fast DC voltage control is proposed for the REC. The cascade control loops of the virtual synchronous control are simplified by utilizing the internal energy stored in sub-module capacitors to emulate synchronous generator rotors. Moreover, the DC voltage is decoupled with the submodule capacitors voltage, thus being robust to sudden power change. Simulation results in PSCAD/EMTDC show that the proposed control realizes fast inertia support and desired dynamic control of the DC voltage, especially in weak grid operation

On the Circular Polarisation of Repeating Fast Radio Bursts

Fast spinning (e.g., sub-second) neutron star with ultra-strong magnetic fields (or so-called magnetar) is one of the promising origins of repeating fast radio bursts (FRBs). Here we discuss circularly polarised emissions produced by propagation effects in the magnetosphere of fast spinning magnetars. We argue that the polarisation-limiting region is well beyond the light cylinder, suggesting that wave mode coupling effects are unlikely to produce strong circular polarisation for fast spinning magnetars. Cyclotron absorption could be significant if the secondary plasma density is high. However, high degrees of circular polarisation can only be produced with large asymmetries in electrons and positrons. We draw attention to the non-detection of circular polarisation in current observations of known repeating FRBs. We suggest that the circular polarisation of FRBs could provide key information on their origins and help distinguish different radiation mechanisms.Comment: ApJ accepte

Author Correction: Re-detection and a possible time variation of soft X-ray polarization from the Crab

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