arXiv subjects

Zi-Gao Dai

Publications and source records attributed to Zi-Gao Dai.

At least 55 records · Page 3Linked to original sources

arXiv2024

On the dynamical evolution of the asteroid belt in a massive star-neutron star binary

Some fast radio bursts (FRBs) exhibit repetitive behaviors and their origins remain enigmatic. It has been argued that repeating FRBs could be produced by the interaction between a neutron star and an asteroid belt. Here we consider the systems in which an asteroid belt dwells around a massive star, while a neutron star, as a companion of the massive star, interacts with the belt through gravitational force. Various orbital configurations are assumed for the system. Direct N-body simulations are performed to investigate the dynamical evolution of the asteroids belt. It is found that a larger orbital eccentricity of the neutron star will destroy the belt more quickly, with a large number of asteroids being scattered out of the system. A low inclination not only suppresses the collisions but also inhibits the ejection rate at early stages. However, highly inclined systems may undergo strong oscillations, resulting in the Kozai--Lidov instabilities. Among the various configurations, a clear periodicity is observed in the collision events for the case with an orbital eccentricity of 0.7 and mutual inclination of $0^{\circ}$. It is found that such a periodicity can be sustained for at least 8 neutron star orbital periods, supporting this mechanism as a possible explanation for periodically repeating FRBs. Our studies also suggest that the active stage of these kinds of FRB sources should be limited, since the asteroid belt would finally be destroyed by the neutron star after multiple passages.

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arXiv2024

Multiwavelength Radiation from the Interaction between Magnetar Bursts and Companion Star in a Binary System

Magnetars are young, highly magnetized neutron stars that are associated with magnetar short bursts (MSBs), magnetar giant flares (MGFs), and at least some fast radio bursts (FRBs). In this work, we consider a magnetar and a main sequence star in a binary system and analyze the properties of the electromagnetic signals generated by the interaction between the magnetar bursts and the companion star. During the preburst period, persistent radiation could be generated by the interaction between the $e^+e^-$-pair wind from the magnetar and the companion or its stellar wind. We find that for a newborn magnetar, the persistent preburst radiation from the strong magnetar wind can be dominant, and it is mainly at the optical and ultraviolet (UV) bands. For relatively old magnetars, the re-emission from a burst interacting with the companion is larger than the persistent preburst radiation and the luminosity of the companion itself. The transient re-emission produced by the heating process has a duration of $0.1 - 10^5 {\rm~s}$ at the optical, UV, and X-ray bands. Additionally, we find that if these phenomena occur in nearby galaxies within a few hundred kiloparsecs, they could be detected by current or future optical telescopes.

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arXiv2024

The white-light superflares from cool stars in GWAC triggers

M-type stars are the ones that flare most frequently, but how big their maximum flare energy can reach is still unknown. We present 163 flares from 162 individual M2 through L1-type stars that triggered the GWAC, with flare energies ranging from $10^{32.2}$ to $10^{36.4}$ erg . The flare amplitudes range from $\triangle G = 0.84$ to $\sim 10$ mag. Flare energy increases with stellar surface temperature ($T_{\rm eff}$) but both $\triangle G$ and equivalent duration $\log_{10}(ED)$ seem to be independent of $T_{\rm eff}$. Combining periods detected from light curves of TESS and K2, spectra from LAMOST, SDSS and the 2.16 m Telescope, and the Gaia DR3 data, we found that these GWAC flare stars are young. For the stars that have spectra, we found that these stars are in or very near to the saturation region, and $\log_{10}(L_{\rm Hα}/L_{\rm bol})$ is lower for M7-L1 stars than for M2-M6 stars. We also studied the relation between GWAC flare bolometric energy $E_{\rm bol}$ and stellar hemispherical area $S$, and found that $\log_{10}E_{\rm bol}$ (in erg) increases with increasing $S$ (in cm$^2$), and the maximum flare energy $\log_{10}E_{\rm bol, max} \geqslant \log_{10}S + 14.25$. For M7-L1 stars, there seem to be other factors limiting their maximum flare energies in addition to stellar hemispherical area.

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arXiv2024

Forecast of cosmological constraints with superluminous supernovae from the Chinese Space Station Telescope

Superluminous supernovae (SLSNe) are a class of intense celestial events that can be standardized for measuring cosmological parameters, bridging the gap between type Ia supernovae and the cosmic microwave background. In this work, we discuss the cosmological applications of SLSNe from the Chinese Space Station Telescope (CSST). Our estimation suggests that SLSNe rate is biased tracing the cosmic star formation rate, exhibiting a factor of $(1+z)^{1.2}$. We futher predict that CSST is poised to observe $\sim 360$ SLSNe in the 10 square degrees ultra-deep field survey within a span of 2.5 years. A stringent constraint on cosmological parameters can be derived from their peak-color relationship. CSST is anticipated to uncover a substantial number of SLSNe, contributing to a deeper understanding of their central engines and shedding light on the nature of dark energy at high redshifts.

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arXiv2024

Super-Eddington Magnetized Neutron Star Accretion Flows: a Self-similar Analysis

The properties of super-Eddington accretion disks exhibit substantial distinctions from the sub- Eddington ones. In this paper, we investigate the accretion process of a magnetized neutron star (NS) surrounded by a super-Eddington disk. By constructing self-similar solutions for the disk structure, we study in detail an interaction between the NS magnetosphere and the inner region of the disk, revealing that this interaction takes place within a thin boundary layer. The magnetosphere truncation radius is found to be approximately proportional to the Alfvén radius, with a coefficient ranging between 0.34-0.71, influenced by the advection and twisting of a magnetic field, NS rotation, and radiation emitted from an NS accretion column. Under super-Eddington accretion, the NS can readily spin up to become a rapid rotator. The proposed model can be employed to explore the accretion and evolution of NSs in diverse astrophysical contexts, such as ultraluminous X-ray binaries or active galactic nucleus disks.

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arXiv2024

A Novel Model for the MeV Emission Line in GRB 221009A

Gamma-ray bursts (GRBs) have long been considered potential sources of ultra-high-energy cosmic rays (UHECRs; with energy $\gtrsim 10^{18} {\rm~eV}$). In this work, we propose a novel model generating MeV emission lines in GRB, which can constrain the properties of heavy nuclei that potentially exist in GRB jets. Specifically, we find that relativistic hydrogen-like high-atomic-number ions originating from the $β$ decay of unstable nuclei and/or the recombination entrained in the GRB jet can generate narrow MeV emission lines through the de-excitation of excited-electrons. This model can successfully explain the MeV emission line observed in the most luminous GRB ever recorded, GRB~221009A, with suitable parameters including a Lorentz factor $γ\sim 820-1700$ and a total mass of heavy nuclei $M_{\rm tot} \sim 10^{23} - 10^{26}$~g. Especially, the emission line broadening can be reasonably attributed to both the expansion of the jet shell and the thermal motion of nuclei, naturally resulting in a narrow width ($σ_{\rm line} / E_{\rm line} \lesssim 0.2$) consistent with the observation. Furthermore, we predict that different GRBs can exhibit lines in different bands with various evolving behaviors, which might be confirmed with further observations. Finally, our model provides indirect evidence that GRBs may be one of the sources of UHECRs.

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arXiv2024

A Unified Geometric Model of Repeating and Non-Repeating Fast Radio Bursts

Fast radio bursts (FRBs) are millisecond-duration extragalactic radio transients. They apparently fall into repeaters and non-repeaters. However, such a classification has lacked a motivation on the physical picture. Here we propose a unified geometric model to distinguish between the repeaters and non-repeaters, in which the quasi-tangential (QT) propagation effect within the magnetospheric polar cap of a neutron star is considered. In this model, the non-repeaters arise from the sources whose emitting region has a smaller impact angle with respect to the magnetic axis, while the repeaters come from the sources whose emitting region has a larger impact angle. The observational discriminant polarization properties between the repeaters and non-repeaters are an important clue to verifying this unified geometric model since the polarization is sensitive to the QT propagation effect. Moreover, our model effectively explains all of the other discriminant properties, including bandwidth, duration, peak luminosity, energy, brightness temperature, time-frequency downward drifting, and repetition rate, providing compelling evidence for the magnetospheric origin of FRBs.

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arXiv2024

Scattering Cross Sections of Magnetized Particles within Intense Electromagnetic Waves: Application to Fast Radio Bursts

Recently, Beloborodov suggested that there exists a resonance phenomenon between an extremely intense electromagnetic wave and internal magnetized particles. The particles exchange energy with the wave at frequent resonance events and then reach the radiation reaction limit immediately. This process greatly enhances the scattering cross section of the particles. Note that these results only involve an extraordinary (X) mode wave. In this paper, we focus on an intense ordinary (O) mode wave propagating through magnetized particles and compare it with the case of the X-mode wave. Our result shows that the scattering cross section of the particles in the O-mode wave is significantly smaller than that in the X-mode wave. This has important implications for the transparency of a fast radio burst (FRB) inside the magnetosphere of a magnetar. We argue that there is a strong scattering region in the stellar magnetosphere, within which an O-mode wave is more transparent than an X-mode wave for an FRB.

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arXiv2024

Reverse Shock Emission in an Off-axis Top-hat Jet Model for Gamma-Ray Bursts

The afterglow of a gamma-ray burst (GRB) has been widely argued to arise from the interaction of a relativistic outflow with its ambient medium. During such an interaction, a pair of shocks are generated: a forward shock that propagates into the medium, and a reverse shock that propagates into the outflow. Extensive studies have been conducted on the emission from the forward shock viewed off-axis. Furthermore, the observation of a reverse shock in an on-axis short GRB suggests that the reverse shock can produce an electromagnetic counterpart to a gravitational wave-detected merger. In this paper, we investigate the contribution of the reverse shock to the afterglow from a top-hat jet viewed off-axis, and apply our model to some short GRBs previously modeled by an off-axis emission. We employ the Markov Chain Monte Carlo (MCMC) method to get the model parameters (i.e., the jet's half-opening angle $θ_j$, the viewing angle $θ_\text{obs}$, the initial Lorentz factor $Γ_0$, and the isotropic energy $E_\mathrm{iso}$). Our model successfully reproduces off-axis afterglow emission without a structured jet. In addition, our calculations suggest that the reverse shock may produce a prominent feature in an early afterglow, which can be potentially observed in an orphan optical afterglow.

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arXiv2024

Magnetar as the Central Engine of AT2018cow: Optical, Soft X-Ray, and Hard X-Ray Emission

AT2018cow is the most extensively observed and widely studied fast blue optical transient to date; its unique observational properties challenge all existing standard models. In this paper, we model the luminosity evolution of the optical, soft X-ray, and hard X-ray emission, as well as the X-ray spectrum of AT2018cow with a magnetar-centered engine model. We consider a two-zone model with a striped magnetar wind in the interior and an expanding ejecta outside. The soft and hard X-ray emission of AT2018cow can be explained by the leakage of high-energy photons produced by internal gradual magnetic dissipation in the striped magnetar wind, while the luminous thermal UV/optical emission results from the thermalization of the ejecta by the captured photons. The two-component energy spectrum yielded by our model with a quasi-thermal component from the optically thick region of the wind superimposed on an optically thin synchrotron component well reproduces the X-ray spectral shape of AT2018cow. The Markov Chain Monte Carlo fitting results suggest that in order to explain the very short rise time to peak of the thermal optical emission, a low ejecta mass $M_{\rm ej}\approx0.1~M_\odot$ and high ejecta velocity $v_{\rm SN}\approx0.17c$ are required. A millisecond magnetar with $P_0\approx3.7~\rm ms$ and $B_p\approx2.4\times10^{14}~\rm G$ is needed to serve as the central engine of AT2018cow.

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arXiv2024

The Very Early Soft X-ray Plateau of GRB 230307A: Signature of an Evolving Radiative Efficiency in Magnetar Wind Dissipation?

Very recently, a particularly long gamma-ray burst (GRB) 230307A was reported and proposed to originate from a compact binary merger based on its host galaxy property, kilonova, and heavy elements. More intriguingly, a very early plateau followed by a rapid decline in soft X-ray band was detected in its light curve by the Lobster Eye Imager for Astronomy, indicating strong evidence of the existence of a magnetar as the merger product. This work explores that the Magnetar Wind Internal Gradual MAgnetic Dissipation (MIGMAD) model, in which the radiative efficiency evolves over time, successfully fits it to the observed data. Our results reinforce the notion that the X-ray plateau serves as a powerful indicator of a magnetar and imply that an evolving efficiency is likely to be a common feature in X-ray plateaus of GRB afterglows. In addition, we also discuss the explanations for the prompt emission, GRB afterglows, as well as kilonova, and predict possible kilonova afterglows in a magnetar central engine.

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arXiv2023

The Propagation of Fast Radio Bursts in the Magnetosphere Shapes Their Waiting-time and Flux Distributions

The field of fast radio bursts (FRBs) has entered the age of fine characterization as observational results from different radio telescopes become more and more abundant. The large FRB sample is suitable for a statistical study. There is an interesting finding that the waiting-time distributions of very active repeating FRBs show a universal double-peaked feature, with left peaks lower than right ones. Assuming these two peaks are independent and initially comparable, we show that the observed asymmetric shape can be ascribed to the propagational effect in the magnetosphere. An FRB passing through the magnetized plasma will induce the circular motion of charged particles to form a current loop. This further leads to an induced magnetic field with opposite direction respect to the background field. As the effective field strength changes, the scattering absorption probability of the following FRB will be influenced. The absorption can be important under certain physical conditions and bursts with smaller time-lags are easier to be absorbed. Also there will be an imprint on the flux distribution as the scattering optical depth depends on burst luminosity as well.

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arXiv2023

Electromagnetic Counterparts Powered by Kicked Remnants of Black Hole Binary Mergers in AGN Disks

The disk of an active galactic nucleus (AGN) is widely regarded as a prominent formation channel of binary black hole (BBH) mergers that can be detected through gravitational waves (GWs). Besides, the presence of dense environmental gas offers the potential for an embedded BBH merger to produce electromagnetic (EM) counterparts. In this paper, we investigate EM emission powered by the kicked remnant of a BBH merger occurring within the AGN disk. The remnant BH will launch a jet via accreting magnetized medium as it traverses the disk. The resulting jet will decelerate and dissipate energy into a lateral cocoon during its propagation. We explore three radiation mechanisms of the jet cocoon system: jet breakout emission, disk cocoon cooling emission, and jet cocoon cooling emission, and find that the jet cocoon cooling emission is more likely to be detected in its own frequency bands. We predict a soft X-ray transient, lasting for O($10^3$) s, to serve as an EM counterpart, of which the time delay O(10) days after the GW trigger contributes to follow-up observations. Consequently, BBH mergers in the AGN disk represent a novel multimessenger source. In the future, enhanced precision in measuring and localizing GWs, coupled with diligent searches for such associated EM signal, will effectively validate or restrict the origin of BBH mergers in the AGN disk.

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arXiv2023

Coherent Cherenkov Radiation by Bunches in Fast Radio Bursts

Fast radio bursts (FRBs) are extragalactic radio transients with extremely high brightness temperature, which strongly suggests the presence of coherent emission mechanisms. In this study, we introduce a novel radiation mechanism for FRBs involving coherent Cherenkov radiation (ChR) emitted by bunched particles that may originate within the magnetosphere of a magnetar. We assume that some relativistic particles are emitted from the polar cap of a magnetar and move along magnetic field lines through a charge-separated magnetic plasma, emitting coherent ChR along their trajectory. The crucial condition for ChR to occur is that the refractive index of the plasma medium, denoted as $n_r$, must satisfy the condition $n_r^2 > 1$. We conduct comprehensive calculations to determine various characteristics of ChR, including its characteristic frequency, emission power, required parallel electric field, and coherence factor. Notably, our proposed bunched coherent ChR mechanism has the remarkable advantage of generating a narrower-band spectrum. Furthermore, a frequency downward drifting pattern, and $\sim100\%$ linearly polarized emission can be predicted within the framework of this emission mechanism.

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arXiv2023

Polarization Evolution of Fast Radio Burst Sources in Binary Systems

Recently, some fast radio bursts (FRBs) have been reported to exhibit complex and diverse variations in Faraday rotation measurements (RM) and polarization, suggesting that dynamically evolving magnetization environments may surround them. In this paper, we investigate the Faraday conversion (FC) effect in a binary system involving an FRB source and analyze the polarization evolution of FRBs. For an strongly magnetized high-mass companion binary (HMCB), when an FRB with $\sim100\%$ linear polarization passes through the radial magnetic field of the companion star, the circular polarization (CP) component will be induced and oscillate symmetrically around the point with the CP degree equal to zero, the rate and amplitude of the oscillation decrease as the frequency increases. The very strong plasma column density in the HMCBs can cause CP to oscillate with frequency at a very drastic rate, which may lead to depolarization. Near the superior conjunction of the binary orbit, the DM varies significantly due to the dense plasma near the companion, and the significant FC also occurs in this region. As the pulsar moves away from the superior conjunction, the CP gradually tends towards zero and then returns to its value before incidence. We also investigate the effect of the rotation of the companion star. We find that a sufficiently significant RM reversal can be produced at large magnetic inclinations and the RM variation is very diverse. Finally, we apply this model to explain some polarization observations of PSR B1744-24A and FRB 20201124A.

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arXiv2023

Photospheric velocity evolution of SN 2020bvc: signature of $r$-process nucleosynthesis from a collapsar

Whether binary neutron star mergers are the only astrophysical site of rapid neutron-capture process ($r$-process) nucleosynthesis remains unknown. Collapsars associated with long gamma-ray bursts (GRBs) and hypernovae are promising candidates. Simulations have shown that outflows from collapsar accretion disks can produce enough $r$-process materials to explain the abundances in the universe. However, there is no observational evidence to confirm this result at present. SN 2020bvc is a broad-lined type Ic (Ic-BL) supernova (SN) possibly associated with a low-luminosity GRB. Based on semi-analytic SN emission models with and without $r$-process materials, we perform a fitting to the multi-band light curves and photospheric velocities of SN 2020bvc. We find that in a $r$-process-enriched model the mixing of $r$-process materials slows down the photospheric recession and therefore matches the velocity evolution better. The fitting results show that $r$-process materials with mass of $\approx0.36~M_\odot$ and opacity of $\approx4~\rm cm^2~g^{-1}$ is needed to mix with about half of the SN ejecta. Our fitting results are weakly dependent on the nebular emission. Future statistical analysis of a sample of type Ic-BL SNe helps us understand the contribution of collapsars to the $r$-process abundance.

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arXiv2023

Afterglow polarizations in a stratified medium with effect of the equal arrival time surface

The environment of gamma-ray burst (GRB) has an important influence on the evolution of jet dynamics and of its afterglow. Here we investigate the afterglow polarizations in a stratified medium with the equal arrival time surface (EATS) effect. Polarizations of multi-band afterglows are predicted. The effects of the parameters of the stratified medium on the afterglow polarizations are also investigated. We found the influences of the EATS effect on the afterglow polarizations become important for off-axis detections and PD bumps move to later times with the EATS effect. Even the magnetic field configurations, jet structure and observational angles are fixed, polarization properties of the jet emission could still evolve. Here, we assume a large-scale ordered magnetic field in the reverse-shock region and a two-dimensional random field in the forward-shock region. Then PD evolution is mainly determined by the evolution of $f_{32}$ parameter (the flux ratio between the reverse-shock region and forward-shock region) at early stage and by the evolution of the bulk Lorentz factor $γ$ at late stage. Through the influences on the $f_{32}$ or $γ$, the observational energy band, observational angles, and the parameters of the stratified medium will finally affect the afterglow polarizations.

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arXiv2023

Implication of GRB 221009A: Can TeV Emission Come from the GRB Prompt Phase?

Recently, the B.O.A.T. ("brightest of all time") gamma-ray burst, dubbed GRB 221009A, was detected by various instruments. Unprecedentedly, the GRB presented very-high-energy (VHE, energy above 0.1 TeV) gamma-ray emission with energy extending above 10 TeV, as reported by the Large High Altitude Air Shower Observatory (LHAASO). We here demonstrate that the VHE and especially >10 TeV emission may originate from the internal hadronic dissipation of the GRB, without the need of invoking any exotic processes as suggested by some previous studies. We also discuss the constraints on the properties of the GRB ejecta from multiwavelength and multi-messenger observations, which favors a magnetically dominated GRB ejecta. The suggested Poynting-flux-dominated GRB ejecta in this work supports the Blandford & Znajek (BZ) mechanism as the possible central engine model of GRB.

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