The groundbreaking discovery of the optical transient AT2017gfo associated with GW170817 opens a unique opportunity to study the physics of double neutron star (NS) mergers. We argue that the standard interpretation of AT2017gfo as being powered by radioactive decays of r-process elements faces the challenge of simultaneously accounting for the peak luminosity and peak time of the event, as it is not easy to achieve the required high mass, and especially the low opacity of the ejecta required to fit the data. A plausible solution would be to invoke an additional energy source, which is probably provided by the merger product. We consider energy injection from two types of the merger products: (1) a post-merger black hole powered by fallback accretion; and (2) a long-lived NS remnant. The former case can only account for the early emission of AT2017gfo, with the late emission still powered by radioactive decay. In the latter case, both early-and late-emission components can be well interpreted as due to energy injection from a spinning-down NS, with the required mass and opacity of the ejecta components well consistent with known numerical simulation results. We suggest that there is a strong indication that the merger product of GW170817 is a long-lived (supramassive or even permanently stable), low magnetic field NS. The result provides a stringent constraint on the equations of state of NSs.

By fitting the bolometric light curves of 31 super-luminous supernovae (SLSNe) with the magnetar engine model, we derive the ejecta masses and magnetar parameters for these SLSNe. The lower boundary of magnetic field strengths of SLSN magnetars can be set just around the critical field strength B c of electron Landau quantization. In more details, SLSN magnetars can further be divided into two subclasses of magnetic fields of ∼ (1 − 5)B c and ∼ (5 − 10)B c , respectively. It is revealed that these two subclasses of magnetars are just associated with the slow-evolving and fast-evolving bolometric light curves of SLSNe. In comparison, the magnetars harbored in gamma-ray bursts (GRBs) and associated hypernovae are usually inferred to have much higher magnetic fields with a lower boundary about ∼ 10B c . This robustly suggests that it is the magnetic fields that play the crucial role in distinguishing SLSNe from GRBs/hypernovae. The rotational energy of SLSN magnetars are found to be correlated with the masses of supernova ejecta, which provides a clue to explore the nature of their progenitors. Moreover, the distribution of ejecta masses of SLSNe is basically intermediate between those of normal core-collapse supernovae and hypernovae. This could indicate an intrinsic connection among these different stellar explosions.

We provide a general analysis onthe properties of the emitting material of some rapidly evolving and luminous transients discovered recently with the Pan-STARRS1 Medium Deep Survey. It was found that these transients are probably produced by a low-mass non-relativistic outflow that is continuously powered by a newly born, rapidly spinning, and highly magnetized neutron star (NS). Such a system could originate from an accretion-induced collapse of a white dwarf or a merger of an NS-NS binary. Therefore, observations of these transients would be helpful for constraining white dwarf and NS physics and/or for searching and identifying gravitational wave signals from the mergers.

A supra-massive neutron star (NS) spinning extremely rapidly could survive from a merger of NS-NS binary. The spin-down of this remnant NS that is highly magnetized would power the isotropic merger ejecta to produce a bright mergernova emission in ultraviolet/optical bands. Before the mergernova, the early interaction between the NS wind and the ejecta can drive a forward shock propagating outwards into the ejecta. As a result, a remarkable amount of heat can be accumulated behind the shock front and the final escaping of this heat can produce a shock breakout emission. We describe the dynamics and thermal emission of this shock with a semi-analytical model. It is found that sharp and luminous breakout emission appears mainly in soft X-rays with a luminosity of ∼ 10 45 erg s −1 at a few hours after the merger, by leading the mergernova emission as a precursor. Therefore, detection of such an X-ray precursor would provide a smoking-gun evidence for identifying NS-powered mergernovae and distinguishing them from the radioactive-powered ones (i.e., kilonovae or macronovae). The discovery of NS-powered mergernovae would finally help to confirm the gravitational wave signals due to the mergers and the existence of supra-massive NSs.

A rapidly rotating and highly magnetized neutron star (NS) could be formed from explosive phenomena such as superluminous supernovae and gamma-ray bursts. This newborn NS can substantially influence the emission of these explosive transients through its spin-down. The spin-down evolution of the NS can sometimes be affected by fallback accretion, although it is usually regulated by the magnetic dipole radiation and gravitational wave radiation of the NS. Under appropriate conditions, the accreting material can be first ejected and subsequently recycled back, so that the accretion disk can remain in a quasi-steady state for a long time. Here we describe the interaction of the NS with such a propeller-recycling disk and their coevolution. Our result shows that the spin-down of the NS can be initially dominated by the propeller, which prevents the disk material from falling onto the NS until hundreds or thousands of seconds later. It is suggested that the abrupt fall of the disk material onto the NS could significantly suppress the magnetic dipole radiation and then convert the NS from a normal magnetar to a low-field magnetar. This evolution behavior of the newborn NS can help us understand the very different influence of the NS on the early GRB afterglows and the late supernova/kilonova emission.

Significant undulations appear in the light curve of a recently discovered superluminous supernova (SLSN) SN 2015bn after the first peak, while the underlying profile of the light curve can be well explained by a continuous energy supply from a central engine, possibly the spin-down of a millisecond magnetar. We propose that these undulations are caused by an intermittent pulsed energy supply, indicating an energetic flare activity of the central engine of the SLSN. Many post-burst flares were discovered during X-ray afterglow observations of Gamma-Ray Bursts (GRBs). We find that the SLSN flares described here approximately obey the empirical correlation between the luminosity and time scale of GRB flares, extrapolated to the relevant longer time scales of SLSN flares. This confirms the possible connection between these two different phenomena as previously suggested.

We have constrained the charge-mass ($\varepsilon-m$) phase space of millicharged particles through the simulation of the rotational evolution of neutron stars, where an extra slow-down effect due to the accretions of millicharged dark matter particles is considered. For a canonical neutron star of $M=1.4~M_{\odot}$ and $R=10~{\rm km}$ with typical magnetic field strength $B_{0}=10^{12}$ G, we have shown an upper limit of millicharged particles, which is compatible with recently experimental and observational bounds. Meanwhile, we have also explored the influences on the $\varepsilon-m$ phase space of millicharged particles for different magnetic fields $B_{0}$ and dark matter density $\rho_{\rm{DM}}$ in the vicinity of the neutron star.Comment: 5 pages, 3 figure

The famous ancient supernova SN 1054 could have been too bright to be explained in the "standard" radioactive-powered supernova scenario. As an alternative attempt, we demonstrate that the spin-down of the newly born Crab pulsar could provide a sufficient energy supply to make SN 1054 visible at daytime for 23 days and at night for 653 days, where a one-zone semi-analytical model is employed. Our results indicate that SN 1054 could be a "normal" cousin of magnetar-powered superluminous supernovae. Therefore, SN 1054-like supernovae could be a probe to uncover the properties of newly born neutron stars, which provide initial conditions for studies on neutron star evolutions.