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Lijing Shao

Publications and source records attributed to Lijing Shao.

At least 91 records · Page 5Linked to original sources
arXiv2024

Comparison between time-domain and frequency-domain Bayesian inferences to inspiral-merger-ringdown gravitational-wave signals

Time-domain (TD) Bayesian inference is important in ringdown analysis for gravitational wave (GW) astronomy. The validity of this method has been well studied by Isi and Farr [1]. Using GW190521 as an example, we study the TD method in detail by comparing it with the frequency-domain (FD) method as a complement to previous study. We argue that the autocovariance function (ACF) should be calculated from the inverse fast Fourier transform of the power spectral density (PSD), which is usually estimated by the Welch method. In addition, the total duration of the GW data that are used to estimate the PSD and the slice duration of the truncated ACF should be long enough. Only when these conditions are fully satisfied can the TD method be considered sufficiently equivalent to the FD method.

gr-qc↗
arXiv2024

Anatomy of parameter-estimation biases in overlapping gravitational-wave signals

In future gravitational-wave (GW) detections, a large number of overlapping GW signals will appear in the data stream of detectors. When extracting information from one signal, the presence of other signals can cause large parameter estimation biases. Using the Fisher matrix (FM), we develop a bias analysis procedure to investigate how each parameter of other signals affects the inference biases. Taking two-signal overlapping as an example, we show detailedly and quantitatively that the biases essentially originate from the overlapping of the frequency evolution. Furthermore, we find that the behaviors of the correlation coefficients between the parameters of the two signals are similar to the biases. Both of them can be used as characterization of the influence between signals. We also corroborate the bias results of the FM method with full Bayesian analysis. Our results can provide guidance for the development of new PE algorithms on overlapping signals, and the analysis methodology has the potential to generalize.

astro-ph.IM↗
arXiv2024

Constraints on charged black holes from merger-ringdown signals in GWTC-3 and prospects for the Einstein Telescope

Whether astrophysical black holes (BHs) can have charge is a question to be addressed by observations. In the era of gravitational wave (GW) astronomy, one can constrain the charge of a merged BH remnant using the merger-ringdown signal of the GW data. Extending earlier studies, we analyze five GW events in GWTC-3, assuming Kerr-Newman BHs. Our results show no strong evidence for a charged BH, and give a limit on the charge-to-mass-ratio $Q<0.37$ at $90\%$ credible level (CL). Due to the charge-spin degeneracy in the waveform and the limited signal-to-noise ratios (SNRs), it is challenging for LIGO/Virgo/KAGRA observations to provide better constraints. We further simulate data for the Einstein Telescope (ET), where SNRs can be as large as $\sim270$ in the ringdown signal. These simulated events allow us to consider the 220, 221, and 330 ringdown modes altogether, which can help break the charge-spin degeneracy. The analysis of a simulated GW150914-like signal shows that ET can improve the constraints on the charge-to-mass-ratio to $Q \lesssim 0.2$ at $90\%$ CL with one ringdown signal.

gr-qc↗
arXiv2023

Tests of Classical Gravity with Radio Pulsars

Tests of gravity are important to the development of our understanding of gravitation and spacetime. Binary pulsars provide a superb playground for testing gravity theories. In this chapter we pedagogically review the basics behind pulsar observations and pulsar timing. We illustrate various recent strong-field tests of the general relativity (GR) from the Hulse-Taylor pulsar PSR B1913+16, the double pulsar PSR J0737$-$3039, and the triple pulsar PSR J0337+1715. We also overview the inner structure of neutron stars (NSs) that may influence some gravity tests, and have used the scalar-tensor gravity and massive gravity theories as examples to demonstrate the usefulness of pulsar timing in constraining specific modified gravity theories. Outlooks to new radio telescopes for pulsar timing and synergies with other strong-field gravity tests are also presented.

astro-ph.HE↗
arXiv2023

Probing the vector charge of Sagittarius A* with pulsar timing

Timing a pulsar orbiting around Sagittarius A* (Sgr A*) can provide us with a unique opportunity of testing gravity theories. We investigate the detectability of a vector charge carried by the Sgr A* black hole (BH) in the bumblebee gravity model with simulated future pulsar timing observations. The spacetime of a bumblebee BH introduces characteristic changes to the orbital dynamics of the pulsar and the light propagation of radio signals. Assuming a timing precision of 1 ms, our simulation shows that a 5-yr observation of a pulsar with an orbital period $P_b\sim 0.5\,{\rm yr}$ and an orbital eccentricity $e\sim 0.8$ can probe a vector charge-to-mass ratio as small as $Q/M\sim 10^{-3}$, which is much more stringent than the current constraint from the Event Horizon Telescope (EHT) observations, and comparable to the prospective constraint from extreme mass-ratio inspirals with the Laser Interferometer Space Antenna (LISA).

astro-ph.HE↗
arXiv2023

Fundamental Physics Opportunities with the Next-Generation Event Horizon Telescope

The Event Horizon Telescope (EHT) Collaboration recently published the first images of the supermassive black holes in the cores of the Messier 87 and Milky Way galaxies. These observations have provided a new means to study supermassive black holes and probe physical processes occurring in the strong-field regime. We review the prospects of future observations and theoretical studies of supermassive black hole systems with the next-generation Event Horizon Telescope (ngEHT), which will greatly enhance the capabilities of the existing EHT array. These enhancements will open up several previously inaccessible avenues of investigation, thereby providing important new insights into the properties of supermassive black holes and their environments. This review describes the current state of knowledge for five key science cases, summarising the unique challenges and opportunities for fundamental physics investigations that the ngEHT will enable.

astro-ph.HE↗
arXiv2023

Prospects for probing small-scale dark matter models with pulsars around Sagittarius A*

Future observations with next-generation large-area radio telescopes are expected to discover radio pulsars (PSRs) closely orbiting around Sagittarius~A* (Sgr~A*), the supermassive black hole (SMBH) dwelling at our Galactic Center (GC). Such a system can provide a unique laboratory for testing General Relativity (GR), as well as the astrophysics around the GC. In this paper, we provide a numerical timing model for PSR-SMBH systems based on the post-Newtonian (PN) equation of motion, and use it to explore the prospects of measuring the black hole (BH) properties with pulsar timing. We further consider the perturbation caused by the dark matter (DM) distribution around Sgr~A*, and the possibility of constraining DM models with PSR-SMBH systems. Assuming a 5-year observation of a normal pulsar in an eccentric ($e=0.8$) orbit with an orbital period $P_b = 0.5\,$yr, we find that -- with weekly recorded times of arrival (TOAs) and a timing precision of 1 ms -- the power-law index of DM density distribution near the GC can be constrained to about 20%. Such a measurement is comparable to those measurements at the Galactic length scale but can reveal small-scale properties of the DM.

astro-ph.HE↗
arXiv2023

Effect of Noise Estimation in Time-Domain Ringdown Analysis: A Case Study with GW150914

Accurate noise estimation from gravitational wave (GW) data is critical for Bayesian inference. However, recent studies on ringdown signal, such as those by Isi et al. [1], Cotesta et al. [2], and Isi and Farr [3], have encountered disagreement in noise estimation, leading to inconsistent results. The key discrepancy between these studies lies in the usage of different noise estimation methods, augmented by the usage of different sampling rates. We achieved consistent results across various sampling rates by correctly managing noise estimation, shown in the case study of the GW150914 ringdown signal. By conducting a time-domain Bayesian inference analysis on GW data, starting from the peak of the signal, we discovered that the first overtone mode is weakly supported by the amplitude distribution, with a confidence level of $1.6σ$, and is slightly disfavored by the log-Bayes factor. Overall, in our time-domain analysis we conclude there is no strong evidence for overtones in GW150914.

gr-qc↗
arXiv2023

Quasi-periodic oscillations during magnetar giant flares in the strangeon star model

Soft gamma-ray repeaters (SGRs) are widely understood as slowly rotating isolated neutron stars. Their generally large spin-down rates, high magnetic fields, and strong outburst energies render them different from ordinary pulsars. In a few giant flares (GFs) and short bursts of SGRs, high-confidence quasi-periodic oscillations (QPOs) were observed. Although remaining an open question, many theoretical studies suggest that the torsional oscillations caused by starquakes could explain QPOs. Motivated by this scenario, we systematically investigate torsional oscillation frequencies based on the strangeon-star (SS) model with various values of harmonic indices and overtones. To characterize the strong-repulsive interaction at short distances and the non-relativistic nature of strangeons, a phenomenological Lennard-Jones model is adopted. We show that, attributing to the large shear modulus of SSs, our results explain well the high-frequency QPOs ($\gtrsim 150\,\mathrm{Hz}$) during the GFs. The low-frequency QPOs ($\lesssim 150\,\mathrm{Hz}$) can also be interpreted when the ocean-crust interface modes are included. We also discuss possible effects of the magnetic field on the torsional mode frequencies. Considering realistic models with general-relativistic corrections and magnetic fields, we further calculate torsional oscillation frequencies for quark stars. We show that it would be difficult for quark stars to explain all QPOs in GFs. Our work advances the understanding of the nature of QPOs and magnetar asteroseismology.

astro-ph.HE↗
arXiv2023

Continuous gravitational waves from trapped magnetar ejecta and the connection to glitches and antiglitches

Gravitational waves from isolated sources have eluded detection so far. The upper limit of long-lasting continuous gravitational wave emission can now probe physically-motivated models with the most optimistic being strongly constrained. Naturally, one might want to relax the assumption of the gravitational wave being quasi-infinite in duration, leading to the idea of transient continuous gravitational waves. In this paper, we outline how to get transient continuous waves from magnetars (or strongly-magnetised neutron stars) that exhibit glitches and/or antiglitches and apply the model to magnetar SGR J1935+2154. The toy model hypothesizes that at a glitch or antiglitch, mass is ejected from the magnetar but becomes trapped on its outward journey through the magnetosphere. Depending on the height of the trapped ejecta and the magnetic inclination angle, we are able to reproduce both glitches and antiglitches from simple angular momentum arguments. The trapped ejecta causes the magnetar to precess leading to gravitational wave emission at once and twice the magnetar's spin frequency, for a duration equal to however long the ejecta is trapped for. We find that the gravitational waves are more detectable when the magnetar is: closer, rotating faster, or has larger glitches/antiglitches. The detectability also improves when the ejecta height and magnetic inclination angle have values near their critical values, though this requires more mass to be ejected to remain consistent with the observed glitch/antiglitch. We find it unlikely that gravitational waves will be detected from SGR J1935+2154 when using the trapped ejecta model.

astro-ph.HE↗
arXiv2023

Superradiant instabilities of massive bosons around exotic compact objects

Superradiantly unstable ultralight particles around a classical rotating black hole (BH) can form an exponentially growing bosonic cloud, which have been shown to provide an astrophysical probe to detect ultralight particles and constrain their mass. However, the classical BH picture has been questioned, and different theoretical alternatives have been proposed. Exotic compact objects (ECOs) are horizonless alternatives to BHs featuring a reflective surface (with a reflectivity $\mathcal{K}$) in place of the event horizon. In this work, we study superradiant instabilities around ECOs, particularly focusing on the influence of the boundary reflection. We calculate the growth rate of superradiant instabilities around ECOs, and show that the result can be related to the BH case by a correction factor $g_{\mathcal{K}}$, for which we find an explicit analytical expression and a clear physical interpretation. Additionally, we consider the time evolution of superradiant instabilities and find that the boundary reflection can either shorten or prolong the growth timescale. As a result, the boundary reflection alters the superradiance exclusion region on the Regge plane, potentially affecting constraints on the mass of ultralight particles. For a mildly reflective surface ($|\mathcal{K}|\lesssim 0.5$), the exclusion region is not substantially changed, while significant effects from the boundary reflection can occur for an extreme reflectivity ($|\mathcal{K}|\gtrsim0.9$).

gr-qc↗
arXiv2023

Moment of Inertia for Axisymmetric Neutron Stars in the Standard-Model Extension

We develop a consistent approach to calculate the moment of inertia (MOI) for axisymmetric neutron stars (NSs) in the Lorentz-violating Standard-Model Extension (SME) framework. To our knowledge, this is the first relativistic MOI calculation for axisymmetric NSs in a Lorentz-violating gravity theory other than deformed, rotating NSs in the General Relativity. Under Lorentz violation, there is a specific direction in the spacetime and NSs get stretched or compressed along that direction. When a NS is spinning stationarily along this direction, a conserved angular momentum and the concept of MOI are well defined. In the SME framework, we calculate the partial differential equation governing the rotation and solve it numerically with the finite element method to get the MOI for axisymmetric NSs caused by Lorentz violation. Besides, we study an approximate case where the correction to the MOI is regarded solely from the deformation of the NS and compare it with its counterpart in the Newtonian gravity. Our formalism and the numerical method can be extended to other theories of gravity for static axisymmetric NSs.

gr-qc↗
arXiv2023

Search for anisotropic, birefringent spacetime-symmetry breaking in gravitational wave propagation from GWTC-3

An effective field theory framework, the Standard-Model Extension, is used to investigate the existence of Lorentz and CPT-violating effects during gravitational wave propagation. We implement a modified equation for the dispersion of gravitational waves, that includes isotropic, anisotropic and birefringent dispersion. Using the LIGO-Virgo-KAGRA algorithm library suite, we perform a joint Bayesian inference of the source parameters and coefficients for spacetime symmetry breaking. From a sample of 45 high confidence events selected in the GWTC-3 catalog, we obtain a maximal bound of $3.19 \times 10^{-15}$~m at 90\% CI for the isotropic coefficient $k_{(V)00}^{(5)}$ when assuming the anisotropic coefficients to be zero. The combined measurement of all the dispersion parameters yields limits on the order of $10^{-13}$~m for the 16 $k_{(V)ij}^{(5)}$ coefficients. We study the robustness of our inference by comparing the constraints obtained with different waveform models, and find that a lack of physics in the simulated waveform may appear as spacetime symmetry breaking-induced dispersion for a subset of events.

gr-qc↗
arXiv2023

A search for pulsars around Sgr A* in the first Event Horizon Telescope dataset

The Event Horizon Telescope (EHT) observed in 2017 the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz ($λ$=1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT datasets. The high observing frequency means that pulsars - which typically exhibit steep emission spectra - are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic Center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most-sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the Fast-Folding-Algorithm and single pulse search targeting both pulsars and burst-like transient emission; using the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction ($\lesssim$2.2%) of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region.

astro-ph.HE↗
arXiv2023

Including higher harmonics in gravitational-wave parameter estimation and cosmological implications for LISA

Massive black holes (MBHs) are crucial in shaping their host galaxies. How the MBH co-evolves with its host galaxy is a pressing problem in astrophysics and cosmology. The valuable information carried by the binary MBH is encoded in the gravitational waves (GWs), which will be detectable by the space-borne GW detector LISA. In the GW data analysis, usually, only the dominant $(2,2)$ mode of the GW signal is considered in the parameter estimation for LISA. However, including the higher harmonics in parameter estimation can break the degeneracy between the parameters, especially for the inclination angle and luminosity distance. This may enable the identification of GW signals without electromagnetic counterparts, known as ''dark sirens''. Thus, incorporating higher harmonics will be beneficial to resolve the Hubble tension and constrain the cosmological model. In this paper, we investigate the role of higher harmonics in the parameter estimation for GWs emitted by binary MBHs. We demonstrate that including $(3,3)$ mode can lead to a $10^3$-times improvement in angular resolution and a $10^4$-times improvement in luminosity distance. Meanwhile, our results indicate that considering higher harmonics increases the probability of identifying over 70% host galaxies from $10^{-2}\,\rm{Gpc}^3$ cosmological volume threshold (corresponding $10^5$ host galaxies), while the probability less than 8% for only the $(2,2)$ mode. Thus, our results underscore the importance of including higher modes in the GW signal from binary MBHs, for LISA at least $(3,3)$ mode.

astro-ph.HE↗
arXiv2023

GRB 211211A-like Events and How Gravitational Waves May Tell Their Origins

GRB 211211A is a rare burst with a genuinely long duration, yet its prominent kilonova association provides compelling evidence that this peculiar burst was the result of a compact binary merger. However, the exact nature of the merging objects, whether they were neutron star pairs, neutron star--black hole systems, or neutron star--white dwarf systems, remains unsettled. This {\it Letter} delves into the rarity of this event and the possibility of using current and next-generation gravitational wave detectors to distinguish between the various types of binary systems. Our research reveals an event rate density of $\gtrsim 5.67^{+13.04}_{-4.69} \times 10^{-3}\ \rm Gpc^{-3}\ yr^{-1}$ for GRB 211211A-like gamma-ray bursts (GRBs), which, assuming GRB 211211A is the only example of such a burst, is significantly smaller than that of typical long- and short-GRB populations. We further calculated that if the origin of GRB 211211A is a result of a neutron star--black hole merger, it would be detectable with a significant signal-to-noise ratio (S/N), given the LIGO-Virgo-KAGRA designed sensitivity. On the other hand, a neutron star--white dwarf binary would also produce a considerable S/N during the inspiral phase at decihertz and is detectable by next-generation spaceborne detectors DECIGO and the Big Bang Observer. However, to detect this type of system with millihertz spaceborne detectors like LISA, Taiji, and TianQin, the event must be very close, approximately 3 Mpc in distance or smaller.

astro-ph.HE↗
arXiv2023

$g$-mode of neutron stars in pseudo-Newtonian gravity

The equation of state (EOS) of nuclear dense matter plays a crucial role in many astrophysical phenomena associated with neutron stars (NSs). Fluid oscillations are one of the most fundamental properties therein. NSs support a family of gravity $g$-modes, which are related to buoyancy. We study the gravity $g$-modes caused by composition gradient and density discontinuity in the framework of pseudo-Newtonian gravity. The mode frequencies are calculated in detail and compared with Newtonian and general-relativistic (GR) solutions. We find that the $g$-mode frequencies in one of the pseudo-Newtonian treatments can approximate remarkably well the GR solutions, with relative errors in the order of $1\%$. Our findings suggest that, with much less computational cost, pseudo-Newtonian gravity can be utilized to accurately analyze oscillation of NSs constructed from an EOS with a first-order phase transition between nuclear and quark matter, as well as to provide an excellent approximation of GR effects in core-collapse supernova (CCSN) simulations.

gr-qc↗
arXiv2023

Improved bounds on the bosonic dark matter with pulsars in the Milky Way

Neutron stars (NSs) can be used to constrain dark matter (DM) since a NS can transform into a black hole (BH) if it captures sufficient DM particles and exceeds the Chandrasekhar limit. We extend earlier work and for the first time take into account the Galactic motion of individual NSs, which changes the amount of the captured DM by as large as one to two orders of magnitude. We systematically apply the analysis to 413 NSs in the Milky Way, and constrain the DM particle mass and its interaction with nucleon simultaneously. We find that the most stringent bound is placed by a few NSs and the bound becomes stronger after considering the Galactic motion. The survival of observed NSs already excludes a cross section $σ_{nX}\gtrsim 10^{-45} \, {\rm cm}^2$ for DM particles with mass from $100\, {\rm MeV}$ to $10^3 \, {\rm GeV}$. Especially for a mass around $10 \, {\rm GeV}$, the constraint on the cross section is as stringent as $σ_{nX}\lesssim 10^{-49} \, {\rm cm}^2$.

astro-ph.HE↗