arXiv subjects

Lijing Shao

Publications and source records attributed to Lijing Shao.

At least 109 records · Page 6Linked to original sources

arXiv2023

A tight universal relation between the shape eccentricity and the moment of inertia for rotating neutron stars

Universal relations that are insensitive to the equation of state (EoS) are useful in reducing the parameter space when measuring global quantities of neutron stars (NSs). In this paper, we reveal a new universal relation that connects the eccentricity to the radius and moment of inertia of rotating NSs. We demonstrate that the universality of this relation holds for both conventional NSs and bare quark stars (QSs) in the slow rotation approximation, albeit with different relations. The maximum relative deviation is approximately $1\%$ for conventional NSs and $0.1\%$ for QSs. Additionally, we show that the universality still exists for fast-rotating NSs if we use the dimensionless spin to characterize their rotation. The new universal relation will be a valuable tool to reduce the number of parameters used to describe the shape and multipoles of rotating NSs, and it may also be used to infer the eccentricity or moment of inertia of NSs in future X-ray observations.

astro-ph.HE↗

arXiv2023

Scientific Objectives of the Hot Universe Baryon Surveyor (HUBS) Mission

The Hot Universe Baryon Surveyor (HUBS) is a proposed space-based X-ray telescope for detecting X-ray emissions from the hot gas content in our universe. With its unprecedented spatially-resolved high-resolution spectroscopy and large field of view, the HUBS mission will be uniquely qualified to measure the physical and chemical properties of the hot gas in the interstellar medium, the circumgalactic medium, the intergalactic medium, and the intracluster medium. These measurements will be valuable for two key scientific goals of HUBS, namely to unravel the AGN and stellar feedback physics that governs the formation and evolution of galaxies, and to probe the baryon budget and multi-phase states from galactic to cosmological scales. In addition to these two goals, the HUBS mission will also help us solve some problems in the fields of galaxy clusters, AGNs, diffuse X-ray backgrounds, supernova remnants, and compact objects. This paper discusses the perspective of advancing these fields using the HUBS telescope.

astro-ph.GA↗

arXiv2023

High-altitude Magnetospheric Emissions from Two Pulsars

We discover three new weak pulse components in two known pulsars, one in PSR J0304+1932 and two in PSR J1518+4904. These components are emitted about half way between the main emission beam and the interpulse beam (beam from the opposite pole). They are separated from their main pulse peak by $99^{\circ}\pm{3}^{\circ}$ for J0304+1932, $123^{\circ}.6\pm{0^{\circ}.7}$ (leading) and $93^{^{\circ}}\pm 0^{\circ}.4$ (trailing) for J1518+4904, respectively. Their peak-intensity ratios to main pulses are: $\sim$ 0.06% for J0304+1932, $\sim$ 0.17% and $\sim$ 0.83% for J1518+4904. We also analyzed flux fluctuation and profile variation of the emissions for two pulsars. The results show correlations between the weak pulses and their main pulses, indicating that these emissions come from the same pole. We estimated the emission altitude of these weak pulses and derived a height of about half of the pulsar's light-cylinder radius. These pulse components are a unique sample of high-altitude emissions from pulsars, and challenge the current pulsar emission models.

astro-ph.HE↗

arXiv2023

Extended thermodynamics of the bumblebee black holes

As a vector-tensor theory including nonminimal coupling between the Ricci tensor and a vector field, the bumblebee gravity is a potential theory to test Lorentz symmetry violation. Recently, a new class of numerical spherical black holes in the bumblebee theory was constructed. In this paper, we investigate the associated local thermodynamic properties. By introducing a pair of conjugated thermodynamic quantities $X$ and $Y$, which can be interpreted as an extension of electric potential and charge of the Reissner Nordström black holes, we numerically construct a new first law of thermodynamics for bumblebee black holes. We then study the constant-$Y$ processes in the entropy-charge parameter space. For the constant-$Y$ processes, we also calculate the heat capacity to study the local thermodynamic stability of the bumblebee black holes. For a negative nonminimal coupling coefficient $ξ$, we find both divergent and smooth phase transitions. For a positive but small $ξ$, only a divergent phase transition is found. It turns out that there is a critical value $0.4κ<ξ_c < 0.5κ$ such that when $ξ_c < ξ<2κ$, even the divergent phase transition disappears and the bumblebee black holes thus become locally thermodynamically unstable regardless of the bumblebee charge. As for $ξ>2κ$, the smooth phase transition arises again but there no longer exists any discontinuous phase transition for the bumblebee black holes.

gr-qc↗

arXiv2023

Upgraded waveform model of eccentric binary black hole based on effective-one-body-numerical-relativity for spin-aligned binary black holes

Effective one body numerical relativity waveform models for spin aligned binary black holes (SEOBNR) are based on the effective one body theoretical framework and numerical relativity simulation results. SEOBNR models have evolved through version 1 to version 4. We recently extended SEOBNRv1 model to SEOBNRE (Effective One Body Numerical Relativity waveform models for Spin aligned binary black holes along Eccentric orbit) model which is also valid for spin aligned binary black hole coalescence along eccentric orbit. In this paper we update our previous SEOBNRE model to make it consistent to SEOBNRv4 which is the most widely used SEOBNR waveform model. This upgraded SEOBNRE model improves accuracy compared to previous SEOBNRE model, especially for highly spinning black holes. For spin aligned binary black holes with mass ratio $1\leq q\lesssim10$, dimensionless spin $-0.9\lesssimχ\lesssim0.995$ and orbital eccentricity $0\leq e_0\lesssim0.6$ at reference frequency $Mf_0=0.002$ ($M$ is the total mass of the binary black hole, $f_0\approx 40\frac{10{\rm M}_\odot}{M}$Hz), the upgraded SEOBNRE model can always fit numerical relativity waveform better than 98.2\%. For most cases the fitting factor can even be better than 99\%.

gr-qc↗

arXiv2023

BlackHoleCam -- Testing general relativity with pulsars orbiting Sagittarius A*

BlackHoleCam is a project funded by a European Research Council Synergy Grant to build a complete astrophysical description of nearby supermassive black holes by using a combination of radio imaging, pulsar observations, stellar astrometry and general relativistic magneto-hydrodynamic models. BlackHoleCam scientists are active partners of the Event Horizon Telescope Consortium. In this talk I will discuss the use of pulsars orbiting Sagittarius A* for tests of General Relativity, the current difficulties in detecting such sources, recent results from the Galactic Centre magnetar PSR J1745-2900 and how BlackHoleCam aims to search for undiscovered pulsars in the Galactic Centre.

astro-ph.HE↗

arXiv2023

The precursor of GRB211211A: a tide-induced giant quake?

The equilibrium configuration of a solid strange star in the final inspiral phase with another compact object is generally discussed, and the starquake-related issue is revisited, for a special purpose to understand the precursor emission of binary compact star merger events (e.g., that of GRB211211A). As the binary system inspirals inward due to gravitational wave radiation, the ellipticity of the solid strangeon star increases due to the growing tidal field of its compact companion. Elastic energy is hence accumulated during the inspiral stage which might trigger a starquake before the merger when exceeds a critical value. The energy released during such starquakes is calculated and compared to the precursor observation of GRB211211A. The result shows that the energy might be insufficient for binary strangeon-star case unless the entire solid strangeon star shatters, and hence favors a black hole-strangeon star scenario for GRB211211A. The timescale of the precursor as well as the frequency of the observed quasi-periodic-oscillation have also been discussed in the starquake model.

astro-ph.HE↗

arXiv2023

Ultra low-mass and small-radius white dwarfs made of heavy elements

Seven ultra low-mass and small-radius white dwarfs (LSPM J0815+1633, LP 240-30, BD+20 5125B, LP 462-12, WD J1257+5428, 2MASS J13453297+4200437, and SDSS J085557.46+053524.5) have been recently identified with masses ranging from $\sim$0.02 $M_\odot$ to $\sim$0.08 $M_\odot$ and radii from $\sim$ 4270 km to 10670 km. The mass-radius measurements of these white dwarfs pose challenges to traditional white dwarf models assuming they are mostly made of nuclei lighter than $^{56}$Fe. In this work we consider the possibility that those white dwarfs are made of heavier elements. Due to the small charge-to-mass ratios in heavy elements, the electron number density in white dwarf matter is effectively reduced, which reduces the pressure with additional contributions of lattice energy and electron polarization corrections. This consequently leads to white dwarfs with much smaller masses and radii, which coincide with the seven ultra low-mass and small-radius white dwarfs. The corresponding equation of state and matter contents of dense stellar matter with and without reaching the cold-catalyzed ground state are presented, which are obtained using the latest Atomic Mass Evaluation (AME 2020). Further observations are necessary to unveil the actual matter contents in those white dwarfs via, e.g., spectroscopy, asteroseismology, and discoveries of other ultra low-mass and small-radius white dwarfs.

astro-ph.SR↗

arXiv2023

Bumblebee black holes in light of Event Horizon Telescope observations

We report the existence of novel static spherical black-hole solutions in a vector-tensor gravitational theory called the bumblebee gravity model which extends the Einstein-Maxwell theory by allowing the vector to nonminimally couple to the Ricci curvature tensor. A test of the solutions in the strong-field regime is performed for the first time using the recent observations of the supermassive black-hole shadows in the galaxy M87 and the Milky Way from the Event Horizon Telescope Collaboration. The parameter space is found largely unexcluded and more experiments are needed to fully bound the theory.

gr-qc↗

arXiv2023

Probing vector hair of black holes with extreme mass ratio inspirals

The bumblebee gravity model, with a vector field nonminimally coupled to gravity, is a natural extension of the Einstein-Maxwell theory. In this theory, a black hole can carry a vector hair, making the metric deviate from the Schwarzschild metric. To investigate the detectability of the vector hair, we consider an Extreme Mass Ratio Inspiral (EMRI) system, where a stellar-mass black hole inspiraling into a supermassive black hole. We find that, with a one-year observation of an EMRI by a space-based gravitational-wave detector, we can probe the vector charge as small as $Q\sim 10^{-3}$ in the bumblebee gravity model, which is about three orders of magnitude tighter comparing to current EHT observations.

gr-qc↗

arXiv2022

Static spherical vacuum solutions in the bumblebee gravity model

The bumblebee gravity model is a vector-tensor theory of gravitation where the vector field nonminimally couples to the Ricci tensor. By investigating the vacuum field equations with spherical symmetry, we find two families of black-hole (BH) solutions in this model: one has a vanishing radial component of the vector field and the other has a vanishing radial component of the Ricci tensor. When the coupling between the vector field and the Ricci tensor is set to zero, the first family becomes the Reissner-Nordström solution while the second family degenerates to the Schwarzschild solution with the vector field being zero. General numerical solutions in both families are obtained for nonzero coupling between the vector field and the Ricci tensor. Besides BH solutions, we also reveal the existence of solutions that have a nonvanishing $tt$-component of the metric on the supposed event horizon where the $rr$-component of the metric diverges while the curvature scalars are finite. These solutions are not supported by existing observations but present certain properties that are of academic interests. We conclude the study by putting the BH solutions into tests against the Solar-system observations and the images of supermassive BHs.

gr-qc↗

arXiv2022

Precession of magnetars: dynamical evolutions and modulations on polarized electromagnetic waves

Magnetars are conjectured to be highly magnetized neutron stars (NSs). Strong internal magnetic field and elasticity in the crust may deform the stars and lead to free precession. We study the precession dynamics of triaxially-deformed NSs incorporating the near-field and the far-field electromagnetic torques. We obtain timing residuals for different NS geometries and torques. We also investigate the polarized X-ray and radio signals from precessing magnetars. The modulations on the Stokes parameters are obtained for thermal X-rays emitted from the surface of magnetars. For radio signals, we apply the simple rotating vector model (RVM) to give the modulations on the position angle (PA) of the polarization. Our results are comprehensive, ready to be used to search for magnetar precession with timing data and polarizations of X-ray and radio emissions. Future observations of precessing magnetars will give us valuable information on the geometry and the strength of the strong magnetic fields, the emission geometry, as well as the equation of state (EoS) of NSs.

astro-ph.HE↗

arXiv2022

Polarizations of Gravitational Waves in the Bumblebee Gravity Model

Lorentz violation modifies the dispersion relation of gravitational waves (GWs), and induces birefringence and anisotropy in propagation. Our study shows that Lorentz violation can also activate multiple polarizations of GWs. We use the gauge invariants to investigate the polarizations of GWs in the bumblebee gravity model, and obtain the following results. (i) For a vector background $b^μ$ with only a nonzero temporal component $b^t$, there are five independent propagating degrees of freedom (DOFs), which is simlar to the Einstein-aether theory. (ii) The presence of a spatial component in the background defines a preferred spatial direction which breaks rotational symmetry. We denote $\hat{\bf b}$ as the direction of the spatial part of the background and $b_s$ as its length. If GWs propagate along $\hat{\bf b}$, the polarization content is similar to the purely timelike case. (iii) If the propagation direction of GWs is separated by an angle $β$ to $\hat{\bf b}$, and $β=\arccos(b^t/b_s)$, there are only two tensor polarizations. (iv) If $β\neq \arccos(b^t/b_s)$, there are only two independent DOFs, and the vector and scalar modes degenerate with the tensor modes. The tensor perturbations can activate a mixture of all six polarizations simultaneously. Finally, we point out the difference in GWs between the bumblebee gravity model and the minimal Standard-Model Extension framework in the linearized regime. Current observations have placed stringent constraints on the anisotropy induced by the background, while our theoretical study still reveals some novel phenomena and provides more understanding about the interaction between the Lorentz-violating vector field and gravity.

gr-qc↗

arXiv2022

Kilonova and Optical Afterglow from Binary Neutron Star Mergers. II. Optimal Search Strategy for Serendipitous Observations and Target-of-opportunity Observations of Gravitational-wave Triggers

In the second work of this series, we explore the optimal search strategy for serendipitous and gravitational-wave-triggered target-of-opportunity (ToO) observations of kilonovae and optical short-duration gamma-ray burst (sGRB) afterglows from binary neutron star (BNS) mergers, assuming that cosmological kilonovae are AT2017gfo-like (but with viewing-angle dependence) and that the properties of afterglows are consistent with those of cosmological sGRB afterglows. A one-day cadence serendipitous search strategy with an exposure time of $\sim30\,$s can always achieve an optimal search strategy of kilonovae and afterglows for various survey projects. We show that the optimal detection rates of the kilonovae (afterglows) are $\sim0.3/0.6/1/20\,$yr$^{-1}$ ($\sim50/60/100/800\,$yr$^{-1}$) for ZTF/Mephisto/WFST/LSST, respectively A better search strategy for SiTian than the current design is to increase the exposure time. In principle, a fully built SiTian can detect $\sim7({2000})\,$yr$^{-1}$ kilonovae (afterglows). Population properties of electromagnetic (EM) signals detected via the serendipitous observations are studied in detail. For ToO observations, we predict that one can detect $\sim11\,{\rm{yr}}^{-1}$ BNS gravitational wave (GW) events during the fourth observing run (O4) by considering an exact duty cycle of the third observing run. The median GW sky localization area is expected to be $\sim10\,{\rm{deg}}^2$ for detectable BNS GW events. In O4, we predict that ZTF/Mephisto/WFST/LSST can detect $\sim5/4/3/3$ kilonovae ($\sim1/1/1/1$ afterglows) per year, respectively. The GW detection rates, GW population properties, GW sky localizations, and optimistic ToO detection rates of detectable EM counterparts for BNS GW events at the Advanced Plus, LIGO Voyager and ET\&CE eras are detailedly simulated in this paper.

astro-ph.HE↗

arXiv2022

Dark-siren Cosmology with Decihertz Gravitational-wave Detectors

Gravitational waves (GWs) originated from mergers of stellar-mass binary black holes (SBBHs) are considered as dark sirens in cosmology since they usually do not have electromagnetic counterparts. In order to study cosmos with these events, we not only need the luminosity distances extracted from GW signals, but also require the redshift information of sources via, say, matching GW sky localization with galaxy catalogs. Based on such a methodology, we explore how well decihertz GW detectors, DO-Optimal and DECIGO, can constrain cosmological parameters. Using Monte-Carlo simulated dark sirens, we find that DO-Optimal can constrain the Hubble parameter to ${σ_{H_0}} / {H_0}\, \lesssim 0.23\%$ when estimating $H_0$ alone, while DECIGO performs better by a factor of 5 with ${σ_{H_0}} / {H_0}\lesssim 0.043\%$. Such a good precision of $H_0$ will shed light on the $H_0$ tension. For multiple-parameter estimation, DECIGO can still reach a level of relative uncertainty smaller than $7\%$. The reason why decihertz detectors perform well is explained by their large numbers of SBBH GW events with good distance and angular resolution.

astro-ph.CO↗

arXiv2022

Prospects for Constraining the Yukawa Gravity with Pulsars around Sagittarius A*

The discovery of radio pulsars (PSRs) around the supermassive black hole (SMBH) in our Galactic Center (GC), Sagittarius A* (Sgr A*), will have significant implications for tests of gravity. In this paper, we predict restrictions on the parameters of the Yukawa gravity by timing a pulsar around Sgr A* with a variety of orbital parameters. Based on a realistic timing accuracy of the times of arrival (TOAs), $σ_{\rm TOA}=100\,μ{\rm s}$, and using a number of 960 TOAs in a 20-yr observation, our numerical simulations show that the PSR-SMBH system will improve current tests of the Yukawa gravity when the range of the Yukawa interaction varies between $10^{1}$-$10^{4}\,{\rm AU}$, and it can limit the graviton mass to be $m_g \lesssim 10^{-24}\,{\rm eV}/c^2$.

astro-ph.HE↗

arXiv2022

Tests of Gravitational-Wave Birefringence with the Open Gravitational-Wave Catalog

We report the results of testing gravitational-wave birefringence using the largest population of gravitational-wave events currently available. Gravitational-wave birefringence, which can arise from the effective field theory extension of general relativity, occurs when the parity symmetry is broken, causing the left- and right-handed polarizations to propagate following different equations of motion. We perform Bayesian inference on the 94 events reported by the 4th-Open Gravitational-wave Catalog (4-OGC) using a parity-violating waveform. We find no evidence for a violation of general relativity in the vast majority of events. However, the most massive event, GW190521, and the second most massive event, GW191109, show intriguing non-zero results for gravitational-wave birefringence. We find that the probability of association between GW190521 and the possible electromagnetic (EM) counterpart reported by Zwicky Transient Facility (ZTF) is increased when assuming birefringence. Excluding GW190521 and GW191109, the parity-violating energy scale is constrained to $M_\mathrm{PV} > 0.05$ GeV at $90\%$ credible interval, which is an improvement over previous results from twelve events by a factor of five. We discuss the implications of our results on modified gravity and possible alternative explanations such as waveform systematics. More detections of massive binary black hole mergers from the upcoming LIGO/Virgo/KAGRA run will shed light on the true origin of the apparent birefringence.

astro-ph.HE↗

arXiv2022

Oscillation Modes and Gravitational Waves from Strangeon Stars

The strong interaction at low energy scales determines the equation of state (EOS) of supranuclear matters in neutron stars (NSs). It is conjectured that the bulk dense matter may be composed of strangeons, which are quark clusters with nearly equal numbers of $u$, $d$, and $s$ quarks. To characterize the strong-repulsive interaction at short distance and the nonrelativistic nature of strangeons, a phenomenological Lennard-Jones model with two parameters is used to describe the EOS of strangeon stars (SSs). For the first time, we investigate the oscillation modes of non-rotating SSs and obtain their frequencies for various parameterizations of the EOS. We find that the properties of radial oscillations of SSs are different from those of NSs, especially for stars with relatively low central energy densities. Moreover, we calculate the $f$-mode frequency of nonradial oscillations of SSs within the relativistic Cowling approximation. The frequencies of the $f$-mode of SSs are found to be in the range from $6.7\,$kHz to $ 8.7\,\rm{kHz}$. Finally, we study the universal relations between the $f$-mode frequency and global properties of SSs, such as the compactness and the tidal deformability. The results we obtained are relevant to pulsar timing and gravitational waves, and will help to probe NSs' EOSs and infer nonperturbative behaviours in quantum chromodynamics.

gr-qc↗