arXiv subjects

Weiwei Zhu

Publications and source records attributed to Weiwei Zhu.

At least 37 records · Page 2Linked to original sources

arXiv2025

The double neutron star PSR J1946+2052 I. Masses and tests of general relativity

We conducted high-precision timing of PSR J1946+2052 to determine the masses of the two neutron stars in the system, test general relativity (GR) and assessed the system's potential for future measurement of the moment of inertia of the pulsar. We analysed seven years of timing data from the Arecibo 305-m radio telescope, the Green Bank Telescope (GBT), and the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The data processing accounted for dispersion measure variations and relativistic spin precession-induced profile evolution. We employed both DDFWHE and DDGR binary models to measure the spin parameters, kinematic parameters and orbital parameters. The timing campaign has resulted in the precise measurement of five post-Keplerian parameters, which yield very precise masses for the system and three tests of general relativity. One of these is the second most precise test of the radiative properties of gravity to date: the intrinsic orbital decay, $\dot{P}_{\rm b,int}=-1.8288(16)\times10^{-12}\rm\,s\,s^{-1}$, represents $1.00005(91)$ of the GR prediction, indicating that the theory has passed this stringent test. The other two tests, of the Shapiro delay parameters, have precisions of 6\% and 5\% respectively; this is caused by the moderate orbital inclination of the system, $\sim 74^{\circ}$; the measurements of the Shapiro delay parameters also agree with the GR predictions. Additionally, we analysed the higher-order contributions of $\dotω$, including the Lense-Thirring contribution. Both the second post-Newtonian and the Lense-Thirring contributions are larger than the current uncertainty of $\dotω$ ($δ\dotω=4\times10^{-4}\,\rm deg\,yr^{-1}$), leading to the higher-order correction for the total mass.

astro-ph.HE↗

arXiv2025

Revealing the Temporally Stable Bimodal Energy Distribution of FRB 20121102A with a Tripled Burst Set from AI Detections

Active repeating Fast Radio Bursts (FRBs), with their large number of bursts, burst energy distribution, and their potential energy evolution, offer critical insights into the FRBs emission mechanisms. Traditional pipelines search for bursts through conducting dedispersion trials and looking for signals above certain fluence thresholds, both of which could result in missing weak and narrow-band bursts. In order to improve the completeness of the burst set, we develop an End-to-end DedispersE-agnostic Nonparametric AI model (EDEN), which directly detect bursts from dynamic spectrum and is the first detection pipeline that operates without attempting dedispersion. We apply EDEN to archival FAST L-band observations during the extreme active phase of the repeating source FRB 20121102A, resulting in the largest burst set for any FRB to date, which contains 5,927 individual bursts, tripling the original burst set. The much enhanced completeness enables a refined analysis of the temporal behavior of energy distribution, revealing that the bimodal energy distribution remains stable over time. It is rather an intrinsic feature of the emission mechanisms than a consequence of co-evolving with burst rate.

astro-ph.HE↗

arXiv2025

Dense Matter in Neutron Stars with eXTP

In this White Paper, we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission to constrain the equation of state of dense matter in neutron stars, exploring regimes not directly accessible to terrestrial experiments. By observing a diverse population of neutron stars - including isolated objects, X-ray bursters, and accreting systems - eXTP's unique combination of timing, spectroscopy, and polarimetry enables high-precision measurements of compactness, spin, surface temperature, polarimetric signals, and timing irregularity. These multifaceted observations, combined with advances in theoretical modeling, pave the way toward a comprehensive description of the properties and phases of dense matter from the crust to the core of neutron stars. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is planned to be launched in early 2030.

astro-ph.HE↗

arXiv2025

The magnetar model's energy crisis for a prolific repeating fast radio burst source

Fast radio bursts (FRBs) are widely considered to originate from magnetars that power the explosion through releasing magnetic energy. Active repeating FRBs have been seen to produce hundreds of bursts per hour and can stay active for months, thus may provide stringent constraints on the energy budget of FRBs' central engine. Within a time span of 214 days, we detected 11,553 bursts from the hyper-active FRB 20240114A that reached a peak burst rate of 729 hr$^{-1}$. This is the largest burst sample from any single FRB source, exceeding the cumulative total of all published bursts from all known FRBs to date. Assuming typical values of radio efficiency and beaming factor, the estimated total isotropic burst energy of this source exceeds 86% of the dipolar magnetic energy of a typical magnetar. The total released energy from this source exceeds that of other known repeaters by about one and a half orders of magnitude, yielding the most stringent lower limit of $4.7\times10^{32}$ G cm$^3$ for the magnetar's magnetic moment. The source remained active at the end of this observation campaign. Our findings thus require either the FRB's central magnetar engine's possessing exceptionally high emission efficiency or a more powerful compact object than a typical magnetar.

astro-ph.HE↗

arXiv2025

Multi-year Polarimetric Monitoring of Four CHIME-Discovered Repeating Fast Radio Bursts with FAST

In this study, we report multi-year polarization measurements of four repeating FRBs initially discovered by CHIME: FRBs~20190117A, 20190208A, 20190303A, and 20190417A. We observed the four repeating FRBs with FAST, detecting a total of 66 bursts. Two bursts from FRB~20190417A exhibit a circular polarization signal-to-noise ratio greater than 7, with the highest circular polarization fraction recorded at 35.7%. While the bursts from FRBs 20190208A and 20190303A are highly linearly polarized, those from FRBs~20190117A and 20190417A show depolarization due to multi-path propagation, with σ_{\mathrm{RM}} = 2.78 \pm 0.05 rad m$^{-2}$ and 5.19 \pm 0.09 rad m$^{-2}$, respectively. The linear polarization distributions among five repeating FRB--FRBs~20190208A, 20190303A, 20201124A, 20220912A, and 20240114A--are nearly identical but show distinct differences from those of non-repeating FRBs. FRBs~20190117A, 20190303A, and 20190417A exhibit substantial rotation measure (RM) variations between bursts, joining other repeating FRBs in this behavior. Combining these findings with published results, 64% of repeating FRBs show RM variations greater than 50 rad m$^{-2}$, and 21\% exhibit RM reversals. A significant proportion of repeating FRBs reside in a dynamic magneto-ionic environment. The structure function of RM variations shows a power-law index of $γ\sim (0-0.8)$, corresponding to a shallow power spectrum $α= -(γ+ 2) \sim -(2.0-2.8)$ of turbulence, if the RM variations are attributed to turbulence. This suggests that the variations are dominated by small-scale RM density fluctuations. We perform K-S tests comparing the RMs of repeating and non-repeating FRBs, which reveal a marginal dichotomy in the distribution of their RMs.We caution that the observed dichotomy may be due to the small sample size and selection biases.

astro-ph.HE↗

arXiv2025

Polarization Position Angle Swing and the Rotating Vector Model of Repeating Fast Radio Bursts

Fast radio bursts (FRBs), typically highly polarized, usually have a nearly constant polarization position angle (PA) during each burst. Some bursts show significant PA variations, and one of them was claimed to have a PA variation pattern consistent with the prediction of the rotating vector model (RVM) commonly adopted to fit the PA variations in radio pulsars. We systematically study the PA evolution pattern of 1727 bursts from three active repeating FRB sources monitored by the Five-hundred-meter Aperture Spherical Telescope (FAST). We identify 46 bursts whose PA variations are fully consistent with the RVM. However, the inferred geometrical parameters and rotation periods derived from these RVM fitting are inconsistent from each other. This suggests that the magnetosphere of the FRB central engine is constantly distorted by the FRB emitter, and the magnetic configuration is dynamically evolving.

astro-ph.HE↗

arXiv2025

Periodic variation of magnetoionic environment of a fast radio burst source

Fast radio bursts (FRBs) are luminous, dispersed millisecond-duration radio bursts whose origin is poorly known. Recent observations suggest that some FRBs may reside in binary systems, even though conclusive evidence remains elusive. Here we report the detection of a 26.24$\pm$0.02 day periodicity in Faraday rotation measure (RM) of an actively repeating source named FRB 20201124A. The detection was made from 3,106 bursts collected with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) over ~365 days. The RM periodicity is coherently phase-connected across ~14 cycles over a 1-year duration. Our detection of RM periodicity corresponds to a natural logarithmic Bayesian factor of 1,168. The detection significances vary between 5.9-34 σ under different assumptions. Such a periodicity provides evidence for the binary nature of FRB 20201124A, where the periodic RM variations arise from the orbital motion of the FRB source within the magnetoionic environment of the system. Together with previous observations, our result suggests that being in binary systems may be a common feature for actively repeating FRB sources.

astro-ph.HE↗

arXiv2025

Topological $π/2$ modes in photonic waveguide arrays

Periodic driving is a powerful tool to generate exotic topological phases without static counterparts, such as the anomalous chiral edge modes from bulk bands with zero Chern number and topological $π$ modes exhibiting period-doubled dynamics. Recently, a new class of Floquet topological mode, namely the $π/2$ mode, which carries four-period periodicity and has potential applications in quantum computing, was proposed based on a square-root method and realized in an acoustic system. Here we propose a laser-written waveguide array lattice to realize topological $π/2$ modes in photonics. Our photonic model simulates a square-root periodically driven Su-Schrieffer-Heeger model and has a rich phase diagram allowing for the co-existence of conventional zero, $π$ modes, and the new $π/2$ modes. Through numerical simulations of the wave equation, we uncover the unique four-period evolution feature of the $π/2$ modes. Our model, which only contains four waveguides per unit cell and two driving steps, is easy to implement with current fabrication techniques and may find applications in quantum optics.

physics.optics↗

arXiv2025

Propagation-induced Frequency-dependent Polarization Properties of Fast Radio Burst

Frequency-dependent polarization properties provide crucial insights into the radiation mechanisms and magnetic environments of fast radio bursts (FRBs). We explore an analytical solution of radiative transfer of the polarization properties of FRBs as a strong incoming wave propagates in a homogeneous magnetized plasma. The cases of a thermal plasma is studied in detail. The rotational axis of the polarization spectrum undergoes precession with frequency on the Poincaré sphere when the medium has both strong Faraday rotation and conversion. Such precession on the Poincaré sphere could occur in hot or cold plasma with a strong magnetic field component perpendicular to the line of sight. Significant absorption can exist in a dense plasma medium, which may give rise to a highly circularly polarized outgoing wave. We apply the analytical solution with the mixing Faraday case to fit the observations of frequency-dependent Stokes parameters for FRB 20180301A and FRB 20201124A. The analytical solution offers a more physical description of FRBs' magnetic environment properties than the empirical ``generalized Faraday rotation'' method commonly adopted in the literature. The frequency-dependent Stokes parameters may be associated with reversing rotation measures or the presence of a persistent radio source around an FRB.

astro-ph.HE↗

arXiv2025

Discovery of 15 new pulsars at high Galactic Latitudes with FAST

We present the discovery and timing results of 15 pulsars discovered in a high Galactic latitude survey conducted with the Five-hundred-meter Aperture Spherical Telescope (FAST). The survey targeted a region as close as possible to the Galactic Center, encompassing an area near the Galactic Bulge. The newly discovered pulsars consist of eleven normal pulsars and four millisecond pulsars (MSPs). Among the MSPs, three are identified in binary systems with orbital periods of ~3.1, 4.6 and 12.5 days, respectively. We have successfully obtained coherent timing solutions for three of the normal pulsars (PSRs J1745-0059, J1746-0156 and J1800-0059). Furthermore, within our data set we found that four pulsars (three new and one known) show mode-changing and/or subpulse drifting phenomena. Comparing our discoveries with simulations of the Galactic disk and Bulge MSP populations indicates that these new pulsars are most likely located in the disk. Nonetheless, our discoveries demonstrate that deep surveys at high Galactic latitudes have significant potential to enhance our understanding of the MSP population in the direction of the Bulge.

astro-ph.HE↗

arXiv2025

PSR J1922+37: a 1.9-second pulsar discovered in the direction of the old open cluster NGC 6791

More than 300 pulsars have been discovered in Galactic globular clusters; however, none have been found in open clusters. Here we present results from 20-hour pulsar searching observations in seven open clusters with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Our first discovery is a 1.9-second pulsar (J1922+37) found in the direction of the old open cluster NGC 6791. The measured dispersion measure (DM) implies a distance of 4.79 kpc and 8.92 kpc based on the NE2001 and YMW16 electron density models, respectively. Given the large uncertainty of DM distance estimates, it is likely that PSR J1922+37 is indeed a member of NGC 6791, for which the distance is $4.19\pm0.02$ kpc based on Gaia Data Release 3. If confirmed, PSR J1922+37 will be the first pulsar found in Galactic open clusters. We outline future observations that can confirm this pulsar-open cluster association and discuss the general prospects of finding pulsars in open clusters.

astro-ph.HE↗

arXiv2024

Radio Pulsar B0950$+$08: Radiation in Magnetosphere and Sparks above Surface

We observed the nearby 100$\%$-duty-cycle radio pulsar B0950+08 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We obtained the polarization profile for its entire rotation, which enabled us to investigate its magnetospheric radiation geometry and the polar cap sparking pattern. After we excluded part of the profile in which the linear polarization factor is low ($\lesssim 30 \%$) and potentially contaminated by position angle jumps, the rest of the polarization position angle swing fits a classical rotating vector model (RVM) well. The bestfit RVM indicates that the inclination angle, $α$, and the impact angle, $β$, of this pulsar, are 100.5$^{\circ}$ and $-$33.2$^{\circ}$, respectively, suggesting that the radio emission comes from two poles.We find that, in such RVM geometry, either the annular vacuum gap or the core vacuum gap model would require that the radio emissions come from a high-altitude magnetosphere with heights from $\sim 0.25~R_{\rm LC}$ to $\sim 0.56~R_{\rm LC}$, with $R_{\rm LC}$ being the light cylinder radius. Both the main and inter-pulses' sparking points are located away from the magnetic pole, which could be relevant to the physical conditions on the pulsar surface.

astro-ph.HE↗

arXiv2024

Flatbands from Bound States in the Continuum for Orbital Angular Momentum Localization

A flatband material is a system characterized by energy bands with zero dispersion, allowing for the compact localization of wavefunctions in real space. This compact localization significantly enhances inter-particle correlations and light-matter interactions, leading to notable advancements such as fractional Chern insulators in condensed matter systems and flat-band lasers in photonics. Previous flatband platforms, including twisted bilayer graphene and artificial kagome/Lieb lattices, typically focused on nondegenerate flatbands, lacking access to the high degeneracy that can facilitate the localization of orbital angular momentum (OAM). Here, we propose a general framework to construct highly degenerate flatbands from bound states in the continuum (BICs)--a concept originating from quantum theory but significantly developed in photonics and acoustics in recent years. The degeneracy of flatbands is determined by the number of BICs within each unit cell in a lattice. We experimentally validate this approach in two-dimensional (2D) and three-dimensional (3D) acoustic crystals, demonstrating flatbands with 4-fold and 12-fold degeneracies, respectively. The high degeneracy provides sufficient internal degrees of freedom, enabling the selective excitation of localized OAM at any position in any direction. Our results pave the way for exploring BIC-constructed flatbands and their localization properties.

physics.optics↗

arXiv2024

Characterizing generalized Floquet topological states in hybrid space-time dimensions

In spatiotemporally modulated systems, topological states exist not only in energy gaps but also in momentum gaps. Such unconventional topological states impose challenges on topological physics. The underlying models also make the conventional Hamiltonian descriptions complicated. Here, we propose to describe such systems with space- and time-direction transfer matrices which substantially simplify the underlying theory and give direct information on the topological properties of the quasienergy and quasimomentum gaps. In particular, we find that the space- and time-direction reflection phases can serve as signatures for distinguishing various topological phases of the quasienergy and quasimomentum gaps. This approach directly reveals the topological properties of the band gap, avoiding the complexity in calculating bulk band topology in hybrid energy-moment space. By investigating two concrete models, we show that the method works well for both Hermitian and non-Hermitian systems. Furthermore, we uncover an unconventional topological state, called the anomalous Floquet quasimomentum gap, whose topological properties are invariant for different choices of the unit-cell center. This work advances the study of topological phenomena in hybrid space-time (energy-momentum) dimension that are attracting much interest due to the development of spatiotemporally modulated materials.

cond-mat.mes-hall↗

arXiv2024

An extremely active repeating fast radio burst source in a likely non-magneto-ionic environment

Fast radio bursts (FRBs) are bright radio bursts originating at cosmological distances. Only three repeating FRBs FRB 20121102A, FRB 20190520B and FRB 20201124A among $\sim$ 60 known repeating FRBs have circular polarization. We observed the FRB 20220912A with the Robert C. Byrd Green Bank Telescope (GBT) at L-band on 24 October 2022 and detected 128 bursts in 1.4 hours, corresponding to a burst rate of about 90 hr$^{-1}$, which is the highest yet for FRBs observed by the GBT. The average rotation measure (RM) was $-$0.4$\pm$0.3$\,$rad$\,$m$^{-2}$ with negligible intraday RM change, indicating a likely non-magneto-ionic environment. 61% bursts have linear polarization fraction greater than 90%. Approximately 56% of the bright bursts have circular polarization. A downward drift in frequency and polarization angle swings were found in our sample. The characterization of FRB 20220912A indicates that the circular polarization is unlikely to be caused by the magneto-ionic environment for at least some of the repeating FRB population.

astro-ph.HE↗

arXiv2024

X-ray hardening preceding the onset of SGR 1935+2154's radio pulsar phase

Magnetars are neutron stars with extremely strong magnetic fields, frequently powering high-energy activity in X-rays. Pulsed radio emission following some X-ray outbursts have been detected (\citealt{Camilo2006,camilo2007a}), albeit its physical origin is unclear. It has long been speculated that the origin of magnetars' radio signals is different from those from canonical pulsars, although convincing evidence is still lacking. Five months after magnetar SGR 1935+2154's X-ray outburst and its associated Fast Radio Burst (FRB) 20200428, a radio pulsar phase was discovered. Here we report the discovery of X-ray spectral hardening associated with the emergence of periodic radio pulsations from SGR 1935+2154 and a detailed analysis of the properties of the radio pulses. The observations suggest that radio emission originates from the outer magnetosphere of the magnetar, and the surface heating due to the bombardment of inward-going particles from the radio emission region is responsible for the observed X-ray spectral hardening.

astro-ph.HE↗

arXiv2024

The FAST Core Array

The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) Core Array is a proposed extension of FAST, integrating 24 secondary 40-m antennas implanted within 5 km of the FAST site. This original array design will combine the unprecedented sensitivity of FAST with a high angular resolution (4.3" at a frequency of 1.4 GHz), thereby exceeding the capabilities at similar frequencies of next-generation arrays such as the Square Kilometre Array Phase 1 or the next-generation Very Large Array. This article presents the technical specifications of the FAST Core Array, evaluates its potential relatively to existing radio telescope arrays, and describes its expected scientific prospects. The proposed array will be equipped with technologically advanced backend devices, such as real-time signal processing systems. A phased array feed receiver will be mounted on FAST to improve the survey efficiency of the FAST Core Array, whose broad frequency coverage and large field of view (FOV) will be essential to study transient cosmic phenomena such as fast radio bursts and gravitational wave events, to conduct surveys and resolve structures in neutral hydrogen galaxies, to monitor or detect pulsars, and to investigate exoplanetary systems. Finally, the FAST Core Array can strengthen China's major role in the global radio astronomy community, owing to a wide range of potential scientific applications from cosmology to exoplanet science.

astro-ph.IM↗

arXiv2024

Floquet $π$ Exceptional Points

We report a new kind of exceptional points in periodically driven system, called Floquet $π$ exceptional points, whose eigenvectors rotate on Bloch sphere and accumulate $π$ geometric phase in one time period. The merging of two such kind exceptional points are constrained by their dynamical structure, meaning two order-1/2 exceptional points with same dynamical structure can merge to one order-1 one while those with opposite dynamical structure can not. We show they exist in Floquet bipartite lattices, and the order-1 Floquet $π$ exceptional points appear at the phase transition point between quasimomentum gap phases and quasienergy gap phases. The scattering properties around the order-1 Floquet $π$ exceptional points is quite novel, which is perfect transparency but detectable in reflection for one of two sides.

cond-mat.mes-hall↗