Rotating Massive Strangeon Stars and X-Ray Plateau of Short GRBs

Strangeon stars,which are proposed to describe the nature of pulsar-like compact stars,have passed various observational tests.The maximum mass of a non-rotating strangeon star could be high,which implies that the remnants of binary strangeon star mergers could even be long-lived massive strangeon stars.We study rigidly rotating strangeon stars in the slowly rotating approximation,using the Lennard-Jones model for the equation of state.Rotation can significantly increase the maximum mass of strangeon stars with unchanged baryon numbers,enlarging the mass-range of long-lived strangeon stars.During spin-down after merger,the decrease of radius of the remnant will lead to the release of gravitational energy.Taking into account the efficiency of converting the gravitational energy luminosity to the observed X-ray luminosity,we find that the gravitational energy could provide an alternative energy source for the plateau emission of X-ray afterglow.The fitting results of X-ray plateau emission of some short gamma-ray bursts suggest that the magnetic dipole field strength of the remnants can be much smaller than that of expected when the plateau emission is powered only by spin-down luminosity of magnetars.

The FAST Galactic Plane Pulsar Snapshot Survey.II.Discovery of 76 Galactic Rotating Radio Transients and the Enigma of RRATs

We have carried out the Galactic Plane Pulsar Snapshot(GPPS)survey by using the Five-hundred-meter Aperture Spherical radio Telescope(FAST),the most sensitive systematic pulsar survey in the Galactic plane.In addition to more than 500 pulsars already discovered through normal periodical search,we report here the discovery of 76 new transient radio sources with sporadic strong pulses,detected by using the newly developed module for a sensitive single-pulse search.Their small DM values suggest that they all are Galactic rotating radio transients(RRATs).They show different properties in the follow-up observations.More radio pulses have been detected from 26 transient radio sources but no periods can be found due to a limited small number of pulses from all FAST observations.The followup observations show that 16 transient sources are newly identified as being the prototypes of RRATs with a period already determined from more detected sporadic pulses,and 10 sources are extremely nulling pulsars,and 24 sources are weak pulsars with sparse strong pulses.On the other hand,48 previously known RRATs have been detected by the FAST,either during verification observations for the GPPS survey or through targeted observations of applied normal FAST projects.Except for one RRAT with four pulses detected in a session of 5-minute observation and four RRATs with only one pulse detected in a session,sensitive FAST observations reveal that 43 RRATs are just generally weak pulsars with sporadic strong pulses or simply very nulling pulsars,so that the previously known RRATs always have an extreme emission state together with a normal hardly detectable weak emission state.This is echoed by the two normal pulsars J1938+2213 and J1946+1449 with occasional brightening pulses.Though strong pulses of RRATs are very outstanding in the energy distribution,their polarization angle variations follow the polarization angle curve of the averaged normal pulse profile,suggesting that the predominant sparse pulses of RRATs are emitted in the same region with the same geometry as normal weak pulsars.

Atlas of dynamic spectra of fast radio burst FRB 20201124A

Fast radio bursts(FRBs) are highly dispersed millisecond-duration radio bursts,[1,2]of which the physical origin is still not fully understood. FRB 20201124A is one of the most actively repeating FRBs. In this paper, we present the collection of 1863 burst dynamic spectra of FRB 20201124A measured with the Five-hundred-meter Aperture Spherical radio Telescope(FAST). The current collection, taken from the observation during the FRB active phase from April to June 2021, is the largest burst sample detected for any FRB so far. The standard PSRFITs format is adopted, including dynamic spectra of the burst, and the time information of the dynamic spectra, in addition, mask files help readers to identify the pulse positions are also provided. The dataset is available in Science Data Bank, with the link https://www.doi.org/10.57760/sciencedb.j00113.00076.

Merging strangeon stars

The state of supranuclear matter in compact stars remains puzzling, and it is argued that pulsars could be strangeon stars. What would happen if binary strangeon stars merge? This kind of merger could result in the formation of a hyper-massive strangeon star, accompanied by bursts of gravitational waves and electromagnetic radiation(and even a strangeon kilonova explained in the paper). The tidal polarizability of binary strangeon stars is different from that of binary neutron stars, because a strangeon star is self-bound on the surface by the fundamental strong force while a neutron star by the gravity, and their equations of state are different. Our calculation shows that the tidal polarizability of merging binary strangeon stars is favored by GW170817. Three kinds of kilonovae(i.e., of neutron, quark and strangeon) are discussed, and the light curve of the kilonova AT 2017 gfo following GW170817 could be explained by considering the decaying strangeon nuggets and remnant star spin-down. Additionally,the energy ejected to the fireball around the nascent remnant strangeon star, being manifested as a gamma-ray burst, is calculated. It is found that, after a prompt burst, an X-ray plateau could follow in a timescale of 10~2-10~3 s. Certainly, the results could be tested also by further observational synergies between gravitational wave detectors(e.g., Advanced LIGO) and X-ray telescopes(e.g., the Chinese HXMT satellite and e XTP mission), and especially if the detected gravitational wave form is checked by peculiar equations of state provided by the numerical relativistical simulation.

The role of FAST in pulsar timing arrays

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) will become one of the world-leading telescopes for pulsar timing array(PTA) research. The primary goals for PTAs are to detect(and subsequently study) ultra-low-frequency gravitational waves, to develop a pulsar-based time standard and to improve solar system planetary ephemerides. FAST will have the sensitivity to observe known pulsars with significantly improved signal-to-noise ratios and will discover a large number of currently unknown pulsars. We describe how FAST will contribute to PTA research and show that jitter-and timing-noise will be the limiting noise processes for FAST data sets. Jitter noise will limit the timing precision achievable over data spans of a few years while timing noise will limit the precision achievable over many years.

Resonant cyclotron scattering in pulsar magnetospheres and its application to isolated neutron stars

Resonant cyclotron scattering(RCS) in pulsar magnetospheres is considered.The photon diffusion equation(Kompaneets equation) for RCS is derived.The photon system is modeled three dimensionally.Numerical calculations show that there exist not only up scattering but also down scattering of RCS,depending on the parameter space.RCS's possible applications to spectral energy distributions of magnetar candidates and radio quiet isolated neutron stars(INSs) are pointed out.The optical/UV excess of INSs may be caused by the down scattering of RCS.The calculations for RX J1856.5-3754 and RX J0720.4-3125 are presented and compared with their observational data.In our model,the INSs are proposed to be normal neutron stars,although the quark star hypothesis is still possible.The low pulsation amplitude of INSs is a natural consequence in the RCS model.

Braking PSR J1734–3333 with a possible fall-back disk

The very small braking index of PSR J1734–3333,n=0.9±0.2,challenges the current theories of braking mechanisms in pulsars.We present a possible interpretation that this pulsar is surrounded by a fall-back disk and braked by it.A modified braking torque is proposed based on the competition between the magnetic energy density of the pulsar and the kinetic energy density of the fall-back disk.With this torque,a self-similar disk can fit all the observed parameters of PSR J1734–3333with natural initial values of parameters.In this regime,the star will evolve to the region having anomalous X-ray pulsars and soft gamma repeaters in the P˙P diagram in about 20 000 years and stay there for a very long time.The mass of the disk around PSR J1734–3333 in our model is about 10M⊕,similar to the observed mass of the disk around AXP 4U 0142+61.

The FAST Galactic Plane Pulsar Snapshot survey:I.Project design and pulsar discoveries

Discovery of pulsars is one of the main goals for large radio telescopes.The Five-hundredmeter Aperture Spherical radio Telescope(FAST),that incorporates an L-band 19-beam receiver with a system temperature of about 20 K,is the most sensitive radio telescope utilized for discovering pulsars.We designed the snapshot observation mode for a FAST key science project,the Galactic Plane Pulsar Snapshot(GPPS)survey,in which every four nearby pointings can observe a cover of a sky patch of 0.1575 square degrees through beam-switching of the L-band 19-beam receiver.The integration time for each pointing is 300 seconds so that the GPPS observations for a cover can be made in 21 minutes.The goal of the GPPS survey is to discover pulsars within the Galactic latitude of±10∘from the Galactic plane,and the highest priority is given to the inner Galaxy within±5∘.Up to now,the GPPS survey has discovered 201 pulsars,including currently the faintest pulsars which cannot be detected by other telescopes,pulsars with extremely high dispersion measures(DMs)which challenge the currently widely used models for the Galactic electron density distribution,pulsars coincident with supernova remnants,40 millisecond pulsars,16 binary pulsars,some nulling and mode-changing pulsars and rotating radio transients(RRATs).The follow-up observations for confirmation of new pulsars have polarization-signals recorded for polarization profiles of the pulsars.Re-detection of previously known pulsars in the survey data also leads to significant improvements in parameters for 64 pulsars.The GPPS survey discoveries are published and will be updated at http://zmtt.bao.ac.cn/GPPS/.

How can FAST improve study of the pulsar emission mechanism and magnetospheric dynamics?

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) has the potential to discover many new pulsars and new phenomena. In this paper we mainly concentrate on how FAST can impact study of the pulsar emission mechanism and magnetospheric dynamics. Several observational programs heading to this direction are reviewed. To make full use of the superior performance of FAST and maximize the scientific outcome, these programs can be arranged in different phases of FAST according to their demands for observational conditions. We suggest that programs can be performed following the test phase, which are observations of multifrequency mean pulse profiles, anomalous X-ray pulsars(AXPs)/soft gamma-ray repeaters(SGRs), mode changing, drifting subpulse and nulling. The long-term monitoring can be carried out for mode changing, AXPs/SGRs and precessional pulsars. Others programs, including polarization observations of radio and γ-ray pulsars, searching for weak pulse components, and multifrequency observations of subpulse drifting, microstructure and giant pulses, can be conducted in all the normal operating phases(the first and second phases). These programs will push forward the frontier in this field in different respects. The search for sub-millisecond pulsars and follow-up observations of their emission properties are very important projects for FAST, but they may be covered by other papers in this mini-volume; therefore,they are not discussed here.

Flux density measurements for 32 pulsars in the 20 cm observing band

Flux densities are fundamental observational parameters that describe a pulsar.In the current pulsar catalogue,27% of the listed radio pulsars have no flux density measurement in the 20 cm observing band.Here,we present the first such measurements for 32 pulsars observed employing the Parkes radio telescope.We have used both archival and new observations to make these measurements.Various schemes exist for measuring flux densities and we show how the measured flux densities vary between these methods and how the presence of radio-frequency interference will bias the flux density measurements.

A solution to the puzzling symbiotic X-ray system 4U 1700+24

A circumstellar corona is proposed to explain a strange quark-cluster star during an accretion phase, which could be essential for understanding the observations of the puzzling symbiotic X-ray system 4U 1700+24. The state of cold matter at supranuclear density is still an important matter of debate, and one of the consequences of a strange star acting as a pulsar is the self-bound phenomenon on the surface, which makes extremely low-mass compact objects unavoidable. In principle,both the redshifted O VIII Ly-α emission line and the change in the blackbody radiation area could naturally be understood if 4U 1700+24 is a low-mass quark-cluster star which exhibits wind accretion.

Supernova neutrinos in a strangeon star model

The neutrino burst detected during supernova SN 1987A 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 for "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 ~ 1-2MeV may occur only a few tens of seconds after core collapse, and the thermal evolution of a strangeon star is then modeled.It is found that the neutrino burst observed from SN 1987A can be reproduced in such a cooling model.

Spindown of magnetars:quantum vacuum friction?

Magnetars are proposed to be peculiar neutron stars which could power their X-ray radiation by super-strong magnetic fields as high as > 10^(14) G.However,no direct evidence for such strong fields has been obtained till now,and the recent discovery of low magnetic field magnetars even indicates that some more efficient radiation mechanism than magnetic dipole radiation should be included.In this paper,quantum vacuum friction(QVF) is suggested to be a direct consequence of super-strong surface fields,therefore the magnetar model could then be tested further through QVF braking.The high surface magnetic field of a pulsar interacting with the quantum vacuum results in a significantly high spindown rate(P).It is found that a QVF dominates the energy loss of pulsars when the pulsar's rotation period and its first derivative satisfy the relationship P^3P > 0.63 ×10^(-16)ξ^(-4) s^2,whereξ is the ratio of the surface magnetic field over the dipole magnetic field.In the "QVF + magnetodipole" joint braking scenario,the spindown behavior of magnetars should be quite different from that in the pure magnetodipole model.We are expecting these results could be tested by magnetar candidates,especially low magnetic field cases,in the future.

Small glitches: the role of strange nuggets?

Pulsar glitches, i.e. the sudden spin-ups of pulsars, have been detected for most known pulsars.The mechanism giving rise to this kind of phenomenon is uncertain, although a large data set has been built.In the framework of the starquake model, based on Baym & Pines, the glitch sizes(the relative increases of spin-frequencies during glitches) ??/? depend on the released energies during glitches, with less released energies corresponding to smaller glitch sizes. On the other hand, as one of the dark matter candidates,our Galaxy might be filled with so called strange nuggets(SNs) which are relics from the early Universe.In this case collisions between pulsars and SNs are inevitable, and these collisions would lead to glitches when enough elastic energy has been accumulated during the spin-down process. The SN-triggered glitches could release less energy, because the accumulated elastic energy would be less than that in the scenario of glitches without SNs. Therefore, if a pulsar is hit frequently by SNs, it would tend to have more small glitches, whose values of ??/? are smaller than those in the standard starquake model(with larger amounts of released energy). Based on the assumption that in our Galaxy the distribution of SNs is similar to that of dark matter, as well as on the glitch data in the ATNF Pulsar Catalogue and Jodrell Bank glitch table, we find that in our Galaxy the incidences of small glitches exhibit tendencies consistent with the collision rates between pulsars and SNs. Further testing of this scenario is expected by detecting more small glitches(e.g.,by the Square Kilometre Array).

Understanding the X-ray spectrum of anomalous X-ray pulsars and soft gamma-ray repeaters

Hard X-rays above 10 ke V are detected from several anomalous X-ray pulsars(AXPs)and soft gamma-ray repeaters(SGRs),and different models have been proposed to explain the physical origin within the frame of either a magnetar model or a fallback disk system.Using data from Suzaku and INTEGRAL,we study the soft and hard X-ray spectra of four AXPs/SGRs:1RXS J170849–400910,1E 1547.0–5408,SGR 1806–20 and SGR 0501+4516.It is found that the spectra could be well reproduced by the bulk-motion Comptonization(BMC)process as was first suggested by Tr¨umper et al.,showing that the accretion scenario could be compatible with Xray emission from AXPs/SGRs.Simulated results from the Hard X-ray Modulation Telescope using the BMC model show that the spectra would have discrepancies from the power-law,especially the cutoff at～200 ke V.Thus future observations will allow researchers to distinguish different models of the hard X-ray emission and will help us understand the nature of AXPs/SGRs.

X-ray and optical plateaus following the main bursts in GRBs and SNe Ⅱ-P: a hint about similar late injection behaviors?

We analyze the emission plateaus in the X-ray afterglow light curves of gamma-ray bursts (GRBs) and those in the optical light curves of type Ⅱ plateau supernovae (SNe Ⅱ-P) in order to study whether they have similar late energy injection behaviors. We show that correlations of bolometric energies (or luminosities) between the prompt explosions and the plateaus for the two phenomena are similar. The energy emitted by SNe Ⅱ-P are at the lower end of the range of possible energies for GRBs. The bolometric energies (or luminosities) in the prompt phase Eexpl (or Lexpl ) and in the plateau phase Eplateau (or Lplateau ) share relations of E expl ∝ Eplateau 0.73 ±0.14 and Lexpl ∝ Lplateau～0.70 . These results may indicate a similar late energy injection behavior that produces the observed plateaus in these two phenomena.

The timing behavior of magnetar Swift J1822.3—1606: timing noise or a decreasing period derivative?

The different timing results of the magnetar Swift J1822.3—1606 are analyzed and understood theoretically.It is noted that different timing solutions are caused not only by timing noise,but also because the period derivative is decreasing after the outburst.Both the decreasing period derivative and the large timing noise may originate from wind braking associated with the magnetar.Future timing of Swift J1822.3—1606 will help clarify whether or not its period derivative is decreasing with time.

Pulsating magneto-dipole radiation of a quaking neutron star powered by energy of Alfvn seismic vibrations

We compute the characteristic parameters of the magneto-dipole radiation of a neutron star undergoing torsional seismic vibrations under the action of Lorentz restoring force about an axis of a dipolar magnetic field experiencing decay.After a brief outline of the general theoretical background of the model of a vibration-powered neutron star,we present numerical estimates of basic vibration and radiation characteristics,such as frequency,lifetime and luminosity,and investigate their time dependence on magnetic field decay.The presented analysis suggests that a gradual decrease in frequencies of pulsating high-energy emission detected from a handful of currently monitored AXP/SGR-like X-ray sources can be explained as being produced by the vibration-powered magneto-dipole radiation of quaking magnetars.

The optical/ultraviolet excess of isolated neutron stars in the resonant cyclotron scattering model

X-ray dim isolated neutron stars are peculiar pulsar-like objects,characterized by their Planck-like spectrum.In studying their spectral energy distributions,optical/ultraviolet(UV) excess is a long standing problem.Recently Kaplan et al.measured the optical/UV excess for all seven sources,which is understandable in the resonant cyclotron scattering(RCS) model previously addressed.The RCS model calculations show that the RCS process can account for the observed optical/UV excess for most sources.The ?at spectrum of RX J2143.0+0654 may be due to contributions from the bremsstrahlung emission of the electron system in addition to the RCS process.

Origins of short gamma-ray bursts deduced from offsets in their host galaxies revisited

The spatial distribution of short Gamma-ray bursts (GRBs) in their host galaxies provides us with an opportunity to investigate their origins. Based on the currently observed distribution of short GRBs relative to their host galaxies, we obtain the fraction of the component that traces the mergers of binary compact objects and the one that traces star formation rate (such as massive stars) in earlyand late-type host galaxies. From the analysis of projected offset distribution and only based on population synthesis and massive star models, we find that the fraction of massive stars is 0.37+-00..4327 with an error at the 1σ level for a sample with 22 short GRBs in the literature. From these results, it is hard to accept that the origin of short GRBs with observed statistics is well described by current models using only the offset distribution. The uncertainties in observational localizations of short GRBs also strongly affect the resulting fraction.