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Renxin Xu

Publications and source records attributed to Renxin Xu.

At least 91 records · Page 5Linked to original sources
arXiv2022

The radio signal of PSR B0950+08 is detected over the whole pulse phase

Pulsars are the "lighthouses" in the universe. Periodic pulses with the duty-cycle ~10% are detected when the radio beam of the rotating pulsar sweeps across the telescope. In this report, the 160 min-data of a nearby pulsar, PSR B0950+08, observed with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) is analysed. Thanks to the extremely high sensitivity of FAST, it is found that the radiation of PSR B0950+08 could be detected over the entire pulse period. To investigate the radiative characteristics of the pulsar' "bridge emission", a function, $Θ$(n), is defined to reveal the weak radiation there. It is suggested that the narrow peaks of both the main and the inter pulses could be radiated at low altitude, while other weak emission (e.g., the "bridges") from upper magnetosphere though its radiative mechanism is still a matter of debate. The measured mean pulse behaviors are consistent with previous results in the phase of strong emission of this pulsar, and both the frequency-independent separation between the inter-pulse and main pulse and the narrow pulse width may support a double-pole model. In order to understand the magnetospheric geometry of this pulsar, further polarization calibrated observation with FAST and a proper determination of the baseline emission, especially during the weak emission phase, are surely required.

astro-ph.HE↗
arXiv2022

Investigating magnetically-induced distortions of neutron stars through gamma-ray burst X-ray plateaus

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, $ε_{\rm B,eff}$, and the effective dipole magnetic field strength on NS surfaces, $B_{\rm eff}$, namely $ε_{\rm B,eff}\sim 10^{-3}(B_{\rm eff}/10^{15}\rm G)^{2}$. 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.

astro-ph.HE↗
arXiv2022

Repeating Fast Radio Burst: Coherent Circular Polarization By Bunches

Fast radio bursts (FRBs) are millisecond-duration signals that are highly dispersed at distant galaxies. However, the physical origin of FRBs is still unknown. Coherent curvature emission by bunches, e.g., powered by starquakes, has already been proposed for repeating FRBs. It has the nature of understanding narrowband radiation exhibiting time--frequency drifting. Recently, a highly active FRB source, i.e., FRB 20201124A, was reported to enter a newly active episode and emit at least some highly circular-polarized bursts. In this study, we revisit the polarized FRB emission, particularly investigating the production mechanisms of a highly circular polarization (CP) by deriving the intrinsic mechanism and propagative effect. The intrinsic mechanisms of invoking charged bunches are approached with radiative coherence. Consequently, a highly CP could naturally be explained by the coherent summation of outcome waves, generated or scattered by bunches, with different phases and electric vectors. Different kinds of evolutionary trajectories are found on the Poincare sphere for the bunch-coherent polarization, and this behavior could be tested through future observations. Cyclotron resonance can result in the absorption of R-mode photons at a low altitude region of the magnetosphere, and an FRB should then be emitted from a high-altitude region if the waves have strong linear polarization. Circularly polarized components could be produced from Faraday conversion exhibiting a λ^3-oscillation, but the average CP fraction depends only on the income wave, indicating a possibility of a highly circular-polarized income wave.The analysis could be welcome if extremely high CP from repeating FRBs is detected in the future. Finally, the production of a bulk of energetic bunches in the pulsar-like magnetosphere is discussed, which is relevant to the nature of the FRB central engine.

astro-ph.HE↗
arXiv2022

Radio pulsations from a neutron star within the gamma-ray binary LS I +61$^{\circ}$ 303

LS I +61$^{\circ}$ 303 is one of the rare gamma-ray binaries, emitting most of their luminosity in photons with energies beyond 100 MeV. The $\sim$26.5 d orbital period is clearly detected at many wavelengths. Additional aspects of its multi-frequency behavior make it the most interesting example of the class. The morphology of high-resolution radio images changes with orbital phase displaying a cometary tail pointing away from the high-mass star. LS I +61$^{\circ}$ 303 also shows superorbital variability. A couple of energetic ($\sim 10^{37}$ erg s$^{-1}$), short, magnetar-like bursts have been plausibly ascribed to it. LS I +61$^{\circ}$ 303's phenomenology has been put under theoretical scrutiny for decades, but the lack of certainty regarding the nature of the compact object in the binary has prevented advancing our understanding of the source. Here, using observations done with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we report on the existence of transient radio pulsations from the direction of LS I +61$^{\circ}$ 303. We find a period $P=269.15508 \pm 0.00016$ ms at a significance of $> 20σ$. This is the first evidence for pulsations from this source at any frequency, and strongly argues for the existence of a rotating neutron star in LS I +61$^{\circ}$ 303.

astro-ph.HE↗
arXiv2021

Magnetospheric curvature radiation by bunches as emission mechanism for repeating fast radio bursts

Coherent curvature radiation as the radiation mechanism for fast radio bursts (FRBs) has been discussed since FRBs were discovered. We study the spectral and polarization properties of repeating FRBs within the framework of coherent curvature radiation by charged bunches in the magnetosphere of a highly magnetized neutron star. The spectra can be generally characterized by multisegment broken power laws, and evolve as bunches move and the line of sight sweeps. Emitted waves are highly linear polarized if the line of sight is confined to the beam within an angle of $1/γ$, while circular polarized degree becomes strong for off-beam cases. The spectro-temporal pulse-to-pulse properties can be a natural consequence due to the magnetospheric geometry. We investigate the relationship between drift rate, central frequency and their temporal duration. The radius-to-frequency mapping is derived and simulated within the assumptions of both dipolar and quadrupolar magnetic configurations. The geometric results show that FRBs are emitted in field lines more curved than open field lines for a dipolar geometry. This suggests that there are most likely existing multipolar magnetic configurations in the emission region.

astro-ph.HE↗
arXiv2021

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.

astro-ph.HE↗
arXiv2021

X-ray polarimetry of the Crab nebula with PolarLight: polarization recovery after the glitch and a secular position angle variation

We report follow-up observations of the Crab nebula with the PolarLight X-ray polarimeter, which revealed a possible variation in polarization associated with a pulsar glitch in 2019. The new observations confirm that the polarization has recovered roughly 100 days after the glitch. With the new observations, we find that the polarization angle (PA) measured with PolarLight from the total nebular emission has a difference of 18.0 +- 4.6 (deg) from that measured 42 years ago with OSO-8, indicating a secular evolution of polarization with either the Crab nebula or pulsar. The long-term variation in PA could be a result of multiple glitches in the history, magnetic reconnection or movement of synchrotron emitting structures in the nebula, or secular evolution of the pulsar magnetic geometry.

astro-ph.HE↗
arXiv2021

Symmetry energy effect on the secondary component of GW190814 as a neutron star

The secondary component of GW190814 with a mass of 2.50-2.67 $M_{\odot}$ may be the lightest black hole or the heaviest neutron star ever observed in a binary compact object system. To explore the possible equation of state (EOS), which can support such massive neutron star, we apply the relativistic mean-field model with a density-dependent isovector coupling constant to describe the neutron-star matter. The acceptable EOS should satisfy some constraints: the EOS model can provide a satisfactory description of the nuclei; the maximum mass $M_\textrm{TOV}$ is above 2.6 $M_{\odot}$; the tidal deformability of a canonical 1.4 $M_{\odot}$ neutron star $Λ_{1.4}$ should lie in the constrained range from GW170817. In this paper, we find that the nuclear symmetry energy and its density dependence play a crucial role in determining the EOS of neutron-star matter. The constraints from the mass of 2.6 $M_{\odot}$ and the tidal deformability $Λ_{1.4}=616_{-158}^{+273}$ (based on the assumption that GW190814 is a neutron star-black hole binary) can be satisfied as the slope of symmetry energy $L \leq 50$ MeV. Even including the constraint of $Λ_{1.4}=190_{-120}^{+390}$ from GW170817 which suppresses the EOS stiffness at low density, the possibility that the secondary component of GW190814 is a massive neutron star cannot be excluded in this study.

nucl-th↗
arXiv2020

A roadmap to strange star

What if normal baryonic matter is compressed so tightly that atomic nuclei come into close contact? This question has been asked since 1930s. The fist answer was presented by Lev Landau whose speculation has been developed, and the concept of neutron star is then popularized. However, another answer is related to strange star, which becomes worthy of attention especially after the establishment of the standard model of particle physics in 1960s. The basic ideas of this study are introduced pedagogically. We must point out emphatically that flavour symmetry of and strong coupling between quarks would be essential in seeking true answer to the question. The final answer is expected to appear in the era of multimessenger astronomy. It is emphasized too that, besides the differences of global properties (e.g., mass-radius relation, maximum mass, tidal deformability), the strong-bound surface of strange star (rather than the gravity-bound one for conventional neutron star) could play an important role in identifying a strange star by astronomical observations.

astro-ph.HE↗
arXiv2020

Re-detection and a Possible Time Variation of Soft X-ray Polarisation from the Crab

The Crab nebula is so far the only celestial object with a statistically significant detection in soft x-ray polarimetry, a window that has not been explored in astronomy since the 1970s. However, soft x-ray polarimetry is expected to be a sensitive probe of magnetic fields in high energy astrophysical objects including rotation-powered pulsars and pulsar wind nebulae. Here we report the re-detection of soft x-ray polarisation after 40 years from the Crab nebula and pulsar with PolarLight, a miniature polarimeter utilising a novel technique onboard a CubeSat. The polarisation fraction of the Crab in the on-pulse phases was observed to decrease after a glitch of the Crab pulsar on July 23, 2019, while that of the pure nebular emission remained constant within uncertainty. The phenomenon may have lasted about 100 days. If the association between the glitch and polarisation change can be confirmed with future observations, it will place strong constraints on the physical mechanism of the high energy emission and glitch of pulsars.

astro-ph.HE↗
arXiv2020

The geometry and environment of repeating FRBs

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 $r_{\rm min}<r_{\rm c}<r_{\rm max}$. However, if $r_{\rm max}<r_{\rm c}$, the modulation of orbital motion will not work due to persistent interaction, and this kind of repeating FRBs should be nonperiodic. We test relevant NS-companion binary scenarios on the basis of FRB 180916.J0158+65 and FRB 121102 under this geometrical frame. It is found that the pulsar-asteroid belt impact model is more suitable to explain these two FRBs since this model is compatible with different companions (e.g., massive stars and black holes). At last, we point out that FRB 121102-like samples are potential objects which can reveal the evolution of star-forming region.

astro-ph.HE↗
arXiv2020

Pulsar glitches in a strangeon star model. II. The activity

Glitch is supposed to be a useful probe into pulsar's interior, but the underlying physics remains puzzling. The glitch activity may reflect a lower limit of the crustal moment of inertia in conventional neutron star models. Nevertheless, its statistical feature could also be reproduced in the strangeon star model, which is focused here. We formulate the glitch activity of normal radio pulsars under the framework of starquake of solid strangeon star model, the shear modulus of strangeon matter is constrained to be $μ\simeq 3\times10^{34}~\rm erg/cm^{3}$, consistent with previous work. Nevertheless, about ten times the shift in oblateness accumulated during glitch interval is needed to fulfill the statistical observations. The fact that typical glitch sizes of two rapidly evolving pulsars (the Crab pulsar and PSR B0540-69) are about two orders of magnitude lower than that of the Vela pulsar, significantly lower than the oblateness change they can supply, indicates probably that only a part of oblateness change is relieved when a pulsar is young. The unreleased oblateness and stress may relax as compensation in the following evolution. The small glitch sizes and low glitch activity of the Crab pulsar can be explained simultaneously in this phenomenological model. Finally, we obtain energy release to be $ΔE\sim 2.4\times 10^{40}~\rm erg$ and $ΔE\sim 4.2\times 10^{41}~\rm erg$ for typical glitch size of $Δν/ν\sim 10^{-6}$ (Vela-like) and $\sim 10^{-8}$ (Crab-like). The upcoming SKA may test this model through the energy release and the power-law relation between the reduced recovery coefficient $Q/|\dotν|^{1/2}$ and $Δν/ν$.

astro-ph.HE↗
arXiv2020

On the magnetoionic environments of fast radio bursts

Observations of the Faraday rotation measure, combined with the dispersion measure, can be used to infer the magnetoionic environment of a radio source. We investigate the magnetoionic environments of FRBs by deriving their estimated average magnetic field strengths along the line of sight $\langle B_{\parallel}\rangle$ in their host galaxies and comparing them with those of Galactic pulsars and magnetars. We find that for those FRBs with RM measurements, the mean $\langle B_{\parallel}\rangle$ is $1.77^{+9.01}_{-1.48}\,\rmμG$ and $1.74^{+14.82}_{-1.55}\,\rmμG$ using two different methods, which is slightly larger but not inconsistent with the distribution of Galactic pulsars, $1.00^{+1.51}_{-0.60}\,\rmμG$. Only six Galactic magnetars have estimated $\langle B_{\parallel}\rangle$. Excluding PSR J1745--2900 that has an anomalously high value due to its proximity with the Galactic Centre, the other three sources have a mean value of $1.70\,\rmμG$, which is statistically consistent with the $\langle B_{\parallel}\rangle$ distributions of both Galactic pulsars and FRBs. There is no apparent trend of evolution of magnetar $\langle B_{\parallel}\rangle$ as a function of age or surface magnetic field strength. Galactic pulsars and magnetars close to the Galactic Centre have relatively larger $\langle B_{\parallel}\rangle$ values than other pulsars/magnetars. We discuss the implications of these results for the magnetoionic environments of FRB 121102 within the context of magnetar model and the model invoking a supermassive black hole, and for the origin of FRBs in general.

astro-ph.HE↗
arXiv2020

On the magnetospheric origin of Repeating Fast Radio Bursts

A bright radio burst was newly discovered in SGR 1935+2154, which exhibit some FRB-like temporal- and frequency-properties, suggesting a neutron star (NS)/magnetar magnetospheric origin of FRBs. We propose an explanation of the temporal- and frequency-properties of sub-pulses of repeating FRBs based on the generic geometry within the framework of charged-bunching coherent curvature radiation in the magnetosphere of an NS. The sub-pulses in a radio burst come from bunches of charged particles moving along different magnetic field lines. Their radiation beam sweep across the line of sight at slightly different time, and those radiating at the more curved part tend to be seen earlier and at higher frequency. However, by considering bunches generated at slightly different times, we find there is also a small probability that the emission from the less curved part be seen earlier. We simulate the time--frequency structures by deriving various forms of the electric acceleration field in the magnetosphere. Such structure of sub-pulses is a natural consequence of coherent curvature radiation from an NS magnetosphere with suddenly and violently triggered sparks. We apply this model to explain the time--frequency structure within specific dipolar configuration by invoking the transient pulsar-like sparking from the inner gap of a slowly rotating NS, and have also developed in more generic configurations.

astro-ph.HE↗
arXiv2020

What if the neutron star maximum mass is beyond $\sim2.3 M_{\odot}$?

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: ($ρ_{\rm t}$, $γ$) for gravity-bound objects, while ($ρ_{\rm s}$, $γ$) for self-bound ones, with $ρ_{\rm t}$ the transition density, $ρ_{\rm s}$ the surface density and $γ$ the polytropic exponent. In the matter of $M_\textrm{max}>2.3~M_{\odot}$, it is found that the smallest $ρ_{\rm t}$ and $γ$ should be $\sim 0.50~ρ_0$ and $\sim 2.65$ for normal neutron stars, respectively, whereas for strange star, we have $γ> 1.40$ if $ρ_{\rm s} > 1.0~ρ_0$ and $ρ_{\rm s} < 1.58~ρ_0$ if $γ<2.0$ ($ρ_0$ 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.

astro-ph.HE↗
arXiv2020

Light-quark flavours in a triangle

All of the light flavors seem to work well for bulk strong matter at pressure free, which could be manifested diversely in the form of compact star and strange nugget. Observational tests are discussed.

hep-ph↗
arXiv2019

FRB 171019: An event of binary neutron star merger?

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.

astro-ph.HE↗