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While the precise mechanism of generating pulsed coherent radio emission from pulsars remains elusive, certain gap-invoking models (especially, the inner gap model) offer a comprehensive and plausible explanation for the genesis and termination of such emissions. However, the transition stage between the period of persistent radio emission and the period of radio quiet remains poorly understood, despite observations indicating that a radio pulsar in the pulse nulling state is undergoing the transition stage. In this study, we present a qualitative explanation for the elusive transition stage by modeling pulsar magnetospheres analytically as equivalent RC circuits based on the inner gap model. Our result indicates that, due to lengthy spin-down, older radio pulsars will gradually shift from the state of persistent radio emission to a certain type of pulse nulling state by delayed sparks within their inner gaps., Comment: 5 pages, 2 figures, ApJ accepted

Continuous gravitational waves (GWs) of neutrons stars haven't been detected directly until now. One possible way to indirectly identify their signatures is via the correlation between magnetars and gamma-ray bursts (GRBs), since, under this magnetar scenario of GRBs, GW radiation can affect the evolution of GRB X-ray light curves. Nevertheless, relevant studies lack essential details of this GRB magnetar scenario. For instance, the authors tend to avoid answering the questions like why GRB X-ray light curves can record the information of gravitational wave emissions, what are the reliable criteria for selecting samples that can reveal GW information, and what are the limitations of this GRB-magnetar scenario. In this paper, we elucidate these issues in detail., Comment: 7 pages, 1 figure, had been submitted to Astronomy Letters (but an additional comment is added in this version)

It seems that the wealth of information revealed by the multi-messenger observations of the binary neutron star (NS) merger event, GW170817/GRB 170817A/kilonova AT2017gfo, places irreconcilable constraints to models of the prompt emission of this gamma-ray burst (GRB). The observed time delay between the merger of the two NSs and the trigger of the GRB and the thermal tail of the prompt emission can hardly be reproduced by these models simultaneously. We argue that the merger remnant should be an NS (last for, at least, a large fraction of 1s), and that the difficulty can be alleviated by the delayed formation of the accretion disk due to the absorption of high-energy neutrinos emitted by the NS and the delayed emergence of an effective viscous in the disk. Further, we extend the consideration of the effect of the energy deposition of neutrinos emitted from the NS. If the NS is the central object of a GRB with a distance and duration similar to that of GRB 170817A, thermal emission of the thermal bubble inflated by the NS after the termination of accretion may be detectable. If our scenario is verified, it would be of interest to investigate the cooling of nascent NSs., Comment: 9 pages, 1 figure, RAA accepted

Glitches are sudden spin-up events of pulsars and are usually thought to be induced by unpinning of neutron superfluid vortices in pulsar crusts. Unpinning and repinning of superfluid vortices, and even thermoelectric effects induced by the deposited heat released during glitches, may vary the velocity fields in pulsars. We show that the generally invoked magnetic dipole fields of pulsars cannot remain stationary during the variation of the velocity fields, so that multipole components must be generated. We argue that the increase of the spark frequency of periodic radio pulses is the indicator for the emergence of the multipole components. Interpretations of pulsar nulling, rebrightening of radio-quiet magnetars, differences between Crab and Vela pulsars after glitches, and extra-galactic fast radio burst-like events from SGR 1935+2154 have been proposed based on the influence of the variation of the velocity field on the magnetic field., Comment: 6 pages, 1 figure, submitted to MNRAS

The central engine of gamma-ray bursts (GRBs) remains an open and forefront topic in the era of multimessenger astrophysics. The X-ray plateaus appear in some GRB afterglows, which are widely considered to originate from the spindown of magnetars. According to the stable magnetar scenario of GRBs, an X-ray plateau and a decay phase as $\sim t^{-2}$ should appear in X-ray afterglows. Meanwhile, the ``normal'' X-ray afterglow is produced by the external shock from GRB fireball. We analyze the Neil Gehrels \emph{Swift} GRB data, then find three gold samples, which have an X-ray plateau and a decay phase as $\sim t^{-2}$ superimposed on the jet-driven normal component. Based on these features of the lightcurves, we argue that the magnetars should be the central engines of these three GRBs. Future joint multimessenger observations might further test this possibility, then which can be beneficial to constrain GRB physics., Comment: 6 pages, 3 figures, 1 table, accepted for publication in ApJ

The discovery of fast radio burst (FRB) 200428 from galactic SGR J1935+2154 makes it possible to measure rotational changes accompanied by FRBs and to test several FRB models which may be simultaneously associated with glitches. Inspired by this idea, we present order of magnitude calculations to the scenarios proposed. FRB models such as global starquakes, crust fractures and collisions between pulsars and asteroids/comets are discussed. For each mechanism, the maximum glitch sizes are constrained by the isotropic energy release during the X-ray burst and/or the SGR J1935+2154-like radio burst rate. Brief calculations show that, the maximum glitch sizes for different mechanisms differ by order(s) of magnitude. If glitches are detected to be coincident with FRBs from galactic magnetars in the future, glitch behaviors (such as glitch size, rise timescale, the recovery coefficient and spin down rate offset) are promising to serve as criterions to distinguish glitch mechanisms and in turn to constrain FRB models., Comment: 13 pages, 1 figure, 3 tables. Accepted for publication in MNRAS

Magnetic field may distort neutron stars (NSs), but its effect has not been robustly tested through gravitational-wave observation yet due to the absence of a fast rotating Galactic magnetar. Part of gamma-ray bursts (GRBs) are potential to investigate the magnetically-induced distortion since their central objects may be millisecond magnetars. In this paper, we propose a method to estimate the distortions of these possible magnetars under GRB magnetar scenario. According to the case study of GRB 070521, we find a relation between the effective magnetically-induced ellipticity, $\epsilon_{\rm B,eff}$, and the effective dipole magnetic field strength on NS surfaces, $B_{\rm eff}$, namely $\epsilon_{\rm B,eff}\sim 10^{-3}(B_{\rm eff}/10^{15}\rm G)^{2}$. Furthermore, we constrain the internal magnetic-field structure of the magnetar to be $B_{\rm eff}\sim 0.02 $ and $B_{\rm eff}\sim 0.1B_{\rm t}$, where $$ is the volume-averaged internal toroidal field. The constraint may be used as the initial condition in modeling the structure of NS magnetospheres. At last, the possibility of testing the method shown in this paper through gravitational-wave observations is discussed., Comment: 6 pages, 4 figures, A&A accepted

The two radio pulses (TRPs) from SGR 1935+2154 detected by \cite{BKRMHB} and \cite{2020arXiv200510324T} have similar features to that of cosmological fast radio bursts (FRBs). Many authors directly call the TRPs as FRB 200428 without consider two questions carefully. (1) Are the TRPs just two brighter subpulses of a normal radio pulse liking the normal radio pulses seen in other magnetars during their outburst? (2) If the TRPs are two bursts of an FRB, does this make other FRBs difficult to understand? In this paper, we try to clarify these two questions. First, we compare these TRPs with previous observations of normal radio pulses from pulsars and magnetars, and find that the TRPs should be produced by different mechanism from that of normal radio pulses. We then investigate the second question by assuming the TRPs have the same origin as that of periodically repeating FRBs and find that the origin of the periodicity of periodically repeating FRBs should not be induced by precession. Otherwise, we should expect that the TRPs are not an FRB, at least the TRPs don't have the same origin as that of periodically repeating FRBs., Comment: 8 pages, 3 figure

By assuming the formation of a black hole soon after the merger event of GW170817, Shibata et al. updated the constraints on the maximum mass ($M_\textrm{max}$) of a stable neutron star within $\lesssim$ 2.3 $M_{\odot}$, but there is no solid evidence to rule out $M_\textrm{max}>2.3~M_{\odot}$ from the point of both microphysical and astrophysical views. In order to explain massive pulsars, it is naturally expected that the equation of state (EOS) would become stiffer beyond a specific density. In this paper, we consider the possibility of EOSs with $M_\textrm{max}>2.3~M_{\odot}$, investigating the stiffness and the transition density in a polytropic model. Two kinds of neutron stars are considered, i.e., normal neutron stars (the density vanishes on gravity-bound surface) and strange stars (a sharp density discontinuity on self-bound surface). The polytropic model has only two parameter inputs in both cases: ($\rho_{\rm t}$, $\gamma$) for gravity-bound objects, while ($\rho_{\rm s}$, $\gamma$) for self-bound ones, with $\rho_{\rm t}$ the transition density, $\rho_{\rm s}$ the surface density and $\gamma$ the polytropic exponent. In the matter of $M_\textrm{max}>2.3~M_{\odot}$, it is found that the smallest $\rho_{\rm t}$ and $\gamma$ should be $\sim 0.50~\rho_0$ and $\sim 2.65$ for normal neutron stars, respectively, whereas for strange star, we have $\gamma > 1.40$ if $\rho_{\rm s} > 1.0~\rho_0$ and $\rho_{\rm s} < 1.58~\rho_0$ if $\gamma <2.0$ ($\rho_0$ is the nuclear saturation density). These parametric results could guide further research of the real EOS with any foundation of microphysics if a pulsar mass higher than $2.3~M_{\odot}$ is measured in the future. We also derive rough results of common neutron star radius range, which is $9.8~\rm{km} < R_{1.4} < 13.8~\rm{km}$ for normal neutron stars and $10.5~\rm{km} < R_{1.4} < 12.5~\rm{km}$ for strange stars., Comment: 7 pages, 6 figures

The origin of gamma-ray burst (GRB) X-ray plateau, especially the internal plateau, is still unclear, but it could be related to GRB's central engine of magnetar. It is generally believed that the spin-down power of the magnetar is injected into forward external shock, however we propose here that most of the power will be dissipated behind the GRB jet through larger amplitude electromagnetic wave (LAEMW). Based on this proposal, the relevant physical conditions and observational implications are analyzed and discussed, and various kinds of X-ray light curves could be reproduced. Although it is still a matter of debate about the chromatic multi-band afterglow in the standard external afterglow fireball model, we can explain naturally this feature under this proposal, i.e., the electrons generating the X-ray plateau and emitting the optical afterglow are accelerated by different mechanisms. %Furthermore, we predict that the X-ray emission of spin-down wind could possibly precede the prompt emission of GRB jet if the energy of LAEMW is dissipated first but shock-induced radiation in the jet is produced later. It is emphasized that both the GRB jet and the spin-down wind should have significant observational consequences in the magnetar scenario, and should be focused equally in GRB physics., Comment: 8 pages, 2 figures, ApJ submitted on 16-Mar-2020, ApJ accepted

We propose a geometrical explanation for periodically and nonperiodically repeating fast radio bursts (FRBs) under neutron star (NS)-companion systems. We suggest a constant critical binary separation, $r_{\rm c}$, within which the interaction between the NS and companion can trigger FRB bursts. For an elliptic orbit with the minimum and maximum binary separations, $r_{\rm min}$ and $r_{\rm max}$, a periodically repeating FRB with an active period could be reproduced if $r_{\rm min}

The fast radio burst, FRB 171019, was relatively bright when discovered first by ASKAP, but was identified as a repeater with three faint bursts detected later by GBT and CHIME. These observations lead to the discussion of whether the first bright burst shares the same mechanism with the following repeating bursts. A model of binary neutron star merger is proposed for FRB 171019, in which the first bright burst occurred during the merger event, while the subsequent repeating bursts are starquake-induced, and generally fainter, as the energy release rate for the starquakes can hardly exceed that of the catastrophic merger event. This scenario is consistent with the observation that no burst detected is as bright as the first one., Comment: 10 pages, 2 figures. RAA accepted

The unknown equation of state (EoS) of neutron stars (NSs) is puzzling because of rich non-perturbative effects of strong interaction there. A method to constrain the EoS by using the detected X-ray plateaus of gamma-ray bursts (GRBs) is proposed in this paper. Observations show some GRB X-ray plateaus may be powered by strongly magnetized millisecond NSs. The properties of these NSs should then satisfy: (i) the spin-down luminosity of these NSs should be brighter than the observed luminosity of the X-ray plateaus; (ii) the total rotational energy of these NSs should be larger than the total energy of the X-ray plateaus. Through the case study of GRB 170714A, the moment of inertia of NSs is constrained as $I>1.0\times 10^{45}\left ( \frac{P_{\rm cri}}{1\;\rm ms} \right )^{2} \;\rm g\cdot cm^{2}$, where $P_{\rm cri}$ is the critical rotational period that an NS can achieve. The constraint of the radii of NSs according to GRB 080607 is shown in Table 1., Comment: 6 pages, 2 figute, The Astrophysical Journal, 886:87, 2019 December 1, https://doi.org/10.3847/1538-4357/ab4901

Strict measurement of the speed of gravitational wave (GW) is very important for fundamental physics. In this paper, taking cosmological effect into account, we derive a more precise formula for calculating the speed of GW based on GW 170817-like events. We find that in the case of high redshift, the usual luminosity distance needs to be replaced by co-moving distance. We consider some possible electromagnetic signals which can be significantly reduce the intrinsic uncertainty between the emission times of the gravitational and electromagnetic signals. Hopefully, the measurement accuracy will be improved to $\sim 10^{-18}\;\rm m\; s^{-1}$., Comment: 10 pages, 1 figure