arXiv subjects

X. Y. Lai

Publications and source records attributed to X. Y. Lai.

17 recordsLinked to original sources

arXiv2023

Pulsar Glitch in a Strangeon Star Model. III. The recovery

Strangeon star model has passed various observational tests, such as the massive pulsars and the tidal deformability during binary mergers. Pulsar glitch, as a useful probe for studying the interior structure of pulsars, has also been studied in strangeon star model in our previous papers, including the recovery coefficient, the waiting time of glitches and glitch activity. In this paper, the recovery process of a glitch is described in the strangeon star model, based on the starquake picture established in Paper I. After the starquake, the inner motion of the stellar matter would reduce the tangential pressure in the cracked places at the equatorial plane. The recovery (increase) of the tangential pressure would be achieved by a viscous flow towards the cracked places at equatorial plane, which leads to the exponential recovery of the spin frequency. A uniform viscous flow can reproduce the single exponential decay observed in some glitches, and the viscous time-scale $τ$ and the depth $h$ of the cracking place below the surface can be fitted by the recovery data. It is found that $h$ increases with glitch size $Δν/ν$, which is expected in the glitch scenario of strangeon stars. The magnitude of the recovery predicted in this recovery model is also consistent with that derived from observations. The single exponential decay reproduced by a uniform viscous flow can be generalized to two or more exponentials by the multi-component of viscous flows.

astro-ph.HE↗

arXiv2021

Merging strangeon stars II: the ejecta and light curves

The state of supranuclear matter in compact stars remains puzzling, and it is argued that pulsars could be strangeon stars. The consequences of merging double strangeon stars are worth exploring, especially in the new era of multi-messenger astronomy. To develop the "strangeon kilonova" scenario proposed in Paper I, we make a qualitative description about the evolution of ejecta and light curves for merging double strangeon stars. In the hot environment of the merger, the strangeon nuggets ejected by tidal disruption and hydrodynamical squeezing would suffer from evaporation, in which process particles, such as strangeons, neutrons and protons, are emitted. Taking into account both the evaporation of strangeon nuggets and the decay of strangeons, most of the strangeon nuggets would turn into neutrons and protons, within dozens of milliseconds after being ejected. The evaporation rates of different particles depend on temperature, and we find that the ejecta could end up with two components, with high and low opacity respectively. The high opacity component would be in the directions around the equatorial plane, and the low opacity component would be in a broad range of angular directions. The bolometric light curves show that even if the total ejected mass would be as low as $\sim 10^{-4} M_\odot$, the spin-down power of the long-lived remnant would account for the whole emission of kilonova AT2017gfo associated with GW 170817. The detailed picture of merging double strangeon stars is expected to be tested by future numerical simulations.

astro-ph.HE↗

arXiv2018

Merging Strangeon Stars

The state of supranuclear matter in compact star remains puzzling, and it is argued that pulsars could be strangeon stars. What 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 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 surface by 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 2017gfo 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 (GRB), is calculated. It is found that, after a promote burst, an X-ray plateau could follow in a timescale of $10^{2-3}$ s. Certainly, the results could be tested also by further observational synergies between gravitational wave detectors (e.g., aLIGO) and X-ray telescopes (e.g., Chinese HXMT and eXTP), and especially if the detected gravitational wave form is checked by peculiar equation of state provided by the numerical relativistical simulation.

astro-ph.HE↗

arXiv2018

Pulsar Glitches in a Strangeon Star Model

Pulsar-like compact stars provide us a unique laboratory to explore properties of dense matter at supra-nuclear densities. One of the models for pulsar-like stars is that they are totally composed of "strangeons", and in this paper we studied the pulsar glitches in a strangeon star model. Strangeon stars would be solidified during cooling, and the solid stars would be natural to have glitches as the result of starquakes. Based on the starquake model established before, we proposed that when the starquake occurs, the inner motion of the star which changes the moment of inertia and has impact on the glitch sizes, is divided into plastic flow and elastic motion. The plastic flow which is induced in the fractured part of the outer layer, would move tangentially to redistribute the matter of the star and would be hard to recover. The elastic motion, on the other hand, changes its shape and would recover significantly. Under this scenario, we could understand the behaviors of glitches without significant energy releasing, including the Crab and the Vela pulsars, in an uniform model. We derive the recovery coefficient as a function of glitch size, as well as the time interval between two successive glitches as the function of the released stress. Our results show consistency with observational data under reasonable ranges of parameters. The implications on the oblateness of the Crab and the Vela pulsars are discussed.

astro-ph.HE↗

arXiv2017

Analytic quantum-interference conditions in Coulomb corrected photoelectron holography

We provide approximate analytic expressions for above-threshold ionization (ATI) transition probabilities and photoelectron angular distributions (PADs). These analytic expressions are more general than those existing in the literature and include the residual binding potential in the electron continuum propagation. They successfully reproduce the ATI side lobes and specific holographic structures such as the near-threshold fan-shaped pattern and the spider-like structure that extends up to relatively high photoelectron energies. We compare such expressions with the Coulomb quantum orbit strong-field approximation (CQSFA) and the full solution of the time-dependent Schrödinger equation for different driving-field frequencies and intensities, and provide an in-depth analysis of the physical mechanisms behind specific holographic structures. Our results shed additional light on what aspects of the CQSFA must be prioritized in order to obtain the key holographic features, and highlight the importance of forward scattered trajectories. Furthermore, we find that the holographic patterns change considerably for different field parameters, even if the Keldysh parameter is kept roughly the same.

physics.atom-ph↗

arXiv2016

Spontaneous Magnetization of Solid Quark-cluster Stars

Pulsar-like compact stars usually have strong magnetic fields, with the strength from $\sim 10^8$ to $\sim 10^{12}$ Gauss on surface. How such strong magnetic fields can be generated and maintained is still an unsolved problem, which is, in principle, related to the interior structure of compact stars, i.e., the equation of state of cold matter at supra-nuclear density. In this paper we are trying to solve the problem in the regime of solid quark-cluster stars. Inside quark-cluster stars, the extremely low ratio of number density of electrons to that of baryons $n_e/n_b$ and the screening effect from quark-clusters could reduce the long-range Coulomb interaction between electrons to short-range interaction. In this case, the Stoner's model could apply, and we find that the condition for ferromagnetism is consistent with that for validity of Stoner's model. Under the screened Coulomb repulsion, the electrons inside the stars could spontaneously magnetized and become ferromagnetic, and hence would contribute non-zero net magnetic momentum to the whole star. We conclude that, for most cases in solid quark-cluster stars, the amount of net magnetic momentum, which is proportional to the amount of unbalanced spins $ξ=(n_+-n_-)/n_e$ and depends on the number density of electrons $n_e=n_++n_-$, could be significant with non-zero $ξ$. The net magnetic moments of electron system in solid quark-cluster stars could be large enough to induce the observed magnetic fields for pulsars with $B\sim 10^{11}$ to $\sim 10^{13}$ Gauss.

astro-ph.SR↗

arXiv2015

Small glitches: the role of strange nuggets?

Pulsar glitches, i.e. the sudden spin-ups of pulsars, have been detected for most pulsars that we known. The mechanism giving rise to this kind of phenomenon is uncertain, although a large data set has been built. In the framework of star-quake model, based on~\cite{Baym1971}, the glitch-sizes (the relative increases of spin-frequencies during glitches) $ΔΩ/Ω$ depend on the released energies during glitches, with less released energies corresponding to smaller glitch sizes. On the other hand, as one of dark matter candidates, our Galaxy might be filled with the so called strange nuggets (SNs) which are the relics from the early Universe. In this case the collisions between pulsars and SNs is inevitable, and these collisions would lead to glitches when enough elastic energy has been accumulated during the spin-down process. The SNs-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 size glitches, whose values of $ΔΩ/Ω$ are smaller than that in standard star-quake 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 ATNF Pulsar Catalogue and Jodrell Bank glitch table, we find that in our Galaxy the incidences of small size glitches exhibit tendencies consistent with the collision rates of pulsars and strange nuggets. Further test of this scenario is expected by detecting more small glitches (e.g., by the Square Kilometre Array).

astro-ph.HE↗

arXiv2013

A corresponding-state approach to quark-cluster matter

The state of super-dense matter is essential for us to understand the nature of pulsars, but the non- perturbative quantum chromodynamics (QCD) makes it very difficult for direct calculations of the state of cold matter at realistic baryon number densities inside compact stars. Nevertheless, from an observational point of view, it is conjectured that pulsars could be made up of quark clusters since the strong coupling between quarks might render quarks grouped in clusters. We are trying an effort to find an equation of state of condensed quark-cluster matter in a phenomenological way. Supposing that the quark-clusters could be analogized to inert gases, we apply here the corresponding-state approach to derive the equation of state of quark-cluster matter, as was similarly demonstrated for nuclear and neutron-star matter in 1970s. According to the calculations presented, the quark-cluster stars, which are composed of quark-cluster matter, could then have high maximum mass that is consistent with observations and, in turn, further observations of pulsar mass would also put constraints to the properties of quark-cluster matter. Moreover, the melting heat during solid-liquid phase conversion and the related astrophysical consequences are also briefly discussed.

astro-ph.SR↗

arXiv2013

H-cluster stars

The study of dense matter at ultra-high density has a very long history, which is meaningful for us to understand not only cosmic events in extreme circumstances but also fundamental laws of physics. It is well known that the state of cold matter at supra-nuclear density depends on the non-perturbative nature of quantum chromo-dynamics (QCD) and is essential for modeling pulsars. A so-called H-cluster matter is proposed in this paper as the nature of dense matter in reality. In compact stars at only a few nuclear densities but low temperature, quarks could be interacting strongly with each other there. That might render quarks grouped in clusters, although the hypothetical quark-clusters in cold dense matter has not been confirmed due to the lack of both theoretical and experimental evidence. Motivated by recent lattice QCD simulations of the H-dibaryons (with structure uuddss), we are therefore considering here a possible kind of quark-clusters, H-clusters, that could emerge inside compact stars during their initial cooling, as the dominant components inside (the degree of freedom could then be H-clusters there).Taking into account the in-medium stiffening effect, we find that at baryon densities of compact stars $H$-cluster matter could be more stable than nuclear matter. We also find that for the H-cluster matter with lattice structure, the equation of state could be so stiff that it would seem to be "superluminal" in most dense region. However, the real sound speed for H-cluster matter is in fact hard to calculate, so at this stage we do not put constraints on our model from the usual requirment of causality.

astro-ph.SR↗

arXiv2011

Ionization Suppression of Diatomic Molecules in Intense Midinfrared Laser Field

Diatomic molecules (e.g., O$_2$) in intense laser field exhibit a peculiar suppressed ionization behavior compared to their companion atoms. Several physical models have been proposed to account for this suppression while no consensus has been achieved. In this letter, we aim to clarify the underlying mechanisms behind this molecular ionization suppression. Experimental data recorded at midinfrared laser wavelength and its comparison with that at near-infrared wavelength revealed a peculiar wavelength and intensity dependence of the suppressed ionization of O$_2$ with respect to its companion atom of Xe, while N$_2$ behaves like a structureless atom. It is found that the S-matrix theory calculation can reproduce well the experimental observations and unambiguously identifies the significant role of two-center interference effect in the ionization suppression of O$_2$.

physics.atom-ph↗

arXiv2010

A note on the discovery of a 2M_\odot pulsar

It is conventionally thought that the state equation of dense matter softens and thus cannot result in high maximum mass if pulsars are quark stars, and that a recently discovered $2M_\odot$ pulsar (PSR J1614-2230) may make quark stars to be unlikely. However, this standard point of view would be revisited and updated if quark clustering could occur in cold quark matter because of the strong coupling between quarks at realistic baryon densities of compact stars, and it is addressed that the state equation of clustering quark matter stiffs to support compact stars with maximum mass $M_{\rm max}>2M_\odot$. In this brief note, it is demonstrated that large parameter spaces are allowed for $M_{\rm max}>2M_\odot$ in a Lennard-Jones model of clustered quark matter, and the newly measured highest mass of PSR J1614-2230 would be meaningful to constrain the number of quarks inside single quark-cluster, to be $N_q<\sim 10^3$.

astro-ph.SR↗

arXiv2009

From quantum to classical description of intense laser-atom physics with Bohmian trajectories

In this paper, Bohmian mechanics is introduced to the intense laser-atom physics. The motion of atomic electron in intense laser field is obtained from the Bohm-Newton equation. We find the quantum potential that dominates the quantum effect of a physical system becomes negligible as the electron is driven far away from the parent ion by the intense laser field, i.e. the behavior of the electron smoothly trends to be classical soon after the electron was ionized. Our numerical calculations present a direct positive evidence for the semiclassical trajectory methods in the intense laser-atom physics where the motion of the ionized electron is treated by the classical mechanics, while quantum mechanics is needed before the ionization.

physics.atom-ph↗

arXiv2009

State of matter for quark stars

It depends on the state of matter at supra-nuclear density to model pulsar's structure, which is unfortunately not certain due to the difficulties in physics. In cold quark matter at realistic baryon densities of compact stars (with an average value of $\sim 2-3ρ_0$), the interaction between quarks is so strong that they would condensate in position space to form quark-clusters. We argue that quarks in quark stars are grouped in clusters, then we apply two phenomenological models for quark stars, the polytropic model and Lennard-Jones model. Both of the two models have stiffer EoS, and larger maximum mass for quark stars (larger than 2 $M_\odot$). The gravitational energy releases during the AIQ process could explain the observed energy of three supergiant flares from soft gamma-ray repeaters ($\sim 10^{47}$ ergs).

astro-ph.SR↗

arXiv2009

Bohmian mechanics to high-order harmonic generation

This paper introduces Bohmian mechanics (BM) into the intense laser-atom physics to study high-order harmonic generation. In BM, the trajectories of atomic electrons in intense laser field can be obtained with the Bohm-Newton equation. The power spectrum with the trajectory of an atomic electron is calculated, which is found to be irregular. Next, the power spectrum associated with an atom ensemble from BM is considered, where the power spectrum becomes regular and consistent with that from quantum mechanics. Finally, the reason of the generation of the irregular spectrum is discussed.

physics.atom-ph↗

arXiv2009

Lennard-Jones quark matter and massive quark stars

Quark clustering could occur in cold quark matter because of the strong coupling between quarks at realistic baryon densities of compact stars. Although one may still not be able to calculate this conjectured matter from first principles, the inter-cluster interaction might be analogized to the interaction between inert molecules. Cold quark matter would then crystallize in a solid state if the inter-cluster potential is deep enough to trap the clusters in the wells. We apply the Lennard-Jones potential to describe the inter-cluster potential, and derive the equations of state, which are stiffer than that derived in conventional models (e.g., MIT bag model). If quark stars are composed of Lennard-Jones matter, they could have high maximum masses ($>2M_{\odot}$) as well as very low masses ($<10^{-3}M_{\odot}$). These features could be tested by observations.

astro-ph.HE↗

arXiv2009

Above-threshold ionization photoelectron spectrum from quantum trajectory

Many nonlinear quantum phenomena of intense laser-atom physics can be intuitively explained with the concept of trajectory. In this paper, Bohmian mechanics (BM) is introduced to study a multiphoton process of atoms interacting with the intense laser field: above-threshold ionization (ATI). Quantum trajectory of an atomic electron in intense laser field is obtained from the Bohm-Newton equation first and then the energy of the photoelectron is gained from its trajectory. With energies of an ensemble of photoelectrons, we obtain the ATI spectrum which is consistent with the previous theoretical and experimental results. Comparing BM with the classical trajectory Monte-Carlo method, we conclude that quantum potential may play a key role to reproduce the spectrum of ATI. Our work may present a new approach to understanding quantum phenomena in intense laser-atom physics with the image of trajectory.

physics.atom-ph↗

arXiv2008

A Polytropic Model of Quark Stars

A polytropic quark star model is suggested in order to establish a general framework in which theoretical quark star models could be tested by observations. The key difference between polytropic quark stars and the polytropic model studied previously for normal (i.e., non-quarkian) stars is related to two issues: (i) a constant term representing the contribution of vacuum energy may be added in the energy density and the pressure for a quark star, but not for a normal star; (ii) the quark star models with non-vanishing density at the stellar surface are not avoidable due to the strong interaction between quarks. The first one implies that the vacuum inside a quark star is different from that outside, while the second one is relevant to the effect of color confinement. The polytropic equations of state are stiffer than that derived in conventional realistic models (e.g., the bag model) for quark matter, and pulsar-like stars calculated with a polytropic equation of state could then have high maximum masses (> 2 M_sun). Quark stars can also be very low massive, and be still gravitationally stable even if the polytropic index, n, is greater than 3. All these would result in different mass-radius relations, which could be tested by observations. In addition, substantial strain energy would develop in a solid quark star during its accretion/spindown phase, and could be high enough to take a star-quake. The energy released during star-quakes could be as high as ~ 10^{47} ergs if the tangential pressure is ~ 10^{-6} higher than the radial one.

astro-ph↗