arXiv subjects

Zhi-Chao Zhao

Publications and source records attributed to Zhi-Chao Zhao.

At least 37 records · Page 2Linked to original sources

arXiv2024

Unveiling the Graviton Mass Bounds through Analysis of 2023 Pulsar Timing Array Data Releases

Strong evidence for the Helling-Downs correlation curves have been reported by multiple pulsar timing array (PTA) collaborations in middle 2023. In this work, we investigate the graviton mass bounds by analyzing the observational data of the overlap ruduction functions from the NANOGrav 15-year data release and CPTA first data release. The results from our data analysis display the state-of-the-art upper limits on the graviton mass at 90\% confidence level, namely, $m_{g}\lesssim8.6\times10^{-24}\mathrm{eV}$ from NANOGrav and $m_{g}\lesssim3.8\times10^{-23}\mathrm{eV}$ from CPTA. We also study the cosmic-variance limit on the graviton mass bounds, i.e., $σ_{m_{g}}^{\mathrm{CV}}=4.8\times10^{-24}\mathrm{eV}\times f/(10~\mathrm{year})^{-1}$, with $f$ being a typical frequency band of PTA observations. This is equivalent to the cosmic-variance limit on the speed of gravitational waves, i.e., $σ_{v_{g}}^{\mathrm{CV}}=0.07c$, with $c$ being the speed of light. Moreover, we discuss potential implications of these results for scenarios of ultralight tensor dark matter.

astro-ph.HE↗

arXiv2024

Constraints On Scalar-Induced Gravitational Waves Up To Third Order From Joint Analysis of BBN, CMB, And PTA Data

Recently, strong evidence for a gravitational wave background has been reported by collaborations of pulsar timing arrays (PTA). In the framework of scalar-induced gravitational waves (SIGWs), we concurrently investigate the second and third order gravitational waves by jointly analyzing PTA data, alongside big-bang nucleosynthesis (BBN), and cosmic microwave background (CMB) datasets. We determine the primordial curvature spectral amplitude as $0.021<A_ζ<0.085$ and the spectral peak frequency as $10^{-7.3}\ \mathrm{Hz}<f_\ast<10^{-6.3}\ \mathrm{Hz}$ at a 95\% confidence interval, pointing towards a mass range for primordial black holes of $10^{-4.5}M_\odot<m_{\mathrm{PBH}}<10^{-2.5}M_\odot$. Our findings suggest that third order gravitational waves contribute more significantly to the integrated energy density than the second order ones when $A_ζ\gtrsim0.06$. Furthermore, we expect future PTA projects to validate these findings and provide robust means to investigate the genesis and evolution of the universe, especially inflation.

astro-ph.CO↗

arXiv2024

Detection of the Permanent Strain Offset Component of Gravitational-Wave Memory in Black Hole Mergers

We propose a novel approach to detecting the elusive gravitational-wave memory predicted by general relativity to accompany black hole mergers: direct measurement of the permanent space-time strain offset. Compared to previous techniques modeling and disentangling both the "chirp" and memory signals, this approach has several advantages: it targets the feature of the signal carrying nearly all its Shannon information, has great simplicity, circumvents the need for precise modeling of the time evolution of all components of the gravitational wave signal, and uses only data largely free of the more complicated chirp signal. The frequency spectrum of the predicted memory signal is roughly similar to that of the chirp signal. However its inclusion of lower frequencies, where noise and data calibration are problematic, makes detection difficult but not impossible. We applied this novel analysis, implemented with a template-like algorithm, to a selection of 67 observations of 41 black hole mergers in the LIGO/Virgo Gravitational Wave Transient Catalog. Statistical significance was assessed by analyzing many time-shifted intervals. The result: a few possible detections ($2σ-4σ$) and many upper limits. The probability that a random ensemble of 67 strain time series, with the same noise but no memory signals, will yield a particular figure-of-merit computed for the actual data is approximately 0.1. Several validation checks proved useless, partly due to large measurement and theoretical uncertainties, so these results should be viewed with reservation. Appendices contain MatLab code for various operations, including an algorithm for the complex Fourier transform of arbitrarily spaced data.

gr-qc↗

arXiv2023

Primordial Non-Gaussianity $f_{\mathrm{NL}}$ and Anisotropies in Scalar-Induced Gravitational Waves

Primordial non-Gaussianity encodes vital information of the physics of the early universe, particularly during the inflationary epoch. To explore the local-type primordial non-Gaussianity $f_{\mathrm{NL}}$, we study the anisotropies in gravitational wave background induced by the linear cosmological scalar perturbations during radiation domination in the early universe. We provide the first complete analysis to the angular power spectrum of such scalar-induced gravitational waves. The spectrum is expressed in terms of the initial inhomogeneities, the Sachs-Wolfe effect, and their crossing. It is anticipated to have frequency dependence and multipole dependence, i.e., $C_\ell(ν)\propto [\ell(\ell+1)]^{-1}$ with $ν$ being a frequency and $\ell$ referring to the $\ell$-th spherical harmonic multipole. In particular, the initial inhomogeneites in this background depend on gravitational-wave frequency. These properties are potentially useful for the component separation, foreground removal, and breaking degeneracies in model parameters, making the non-Gaussian parameter $f_{\mathrm{NL}}$ measurable. Further, theoretical expectations may be tested by space-borne gravitational-wave detectors in future.

astro-ph.CO↗

arXiv2023

Implications of Pulsar Timing Array Data for Scalar-Induced Gravitational Waves and Primordial Black Holes: Primordial Non-Gaussianity $f_{\mathrm{NL}}$ Considered

Multiple pulsar-timing-array collaborations have reported strong evidence for the existence of a gravitational-wave background. We study physical implications of this signal for cosmology, assuming that it is attributed to scalar-induced gravitational waves. By incorporating primordial non-Gaussianity $f_{\mathrm{NL}}$, we specifically examine the nature of primordial curvature perturbations and primordial black holes. We find that the signal allows for a primordial non-Gaussianity $f_{\mathrm{NL}}$ in the range of $-4.1\lesssim f_{\mathrm{NL}} \lesssim 4.1$ (68\% confidence intervals) and a mass range for primordial black holes $m_{\mathrm{pbh}}$ spanning from $\sim10^{-5}M_{\odot}$ to $\sim10^{-2}M_{\odot}$. Furthermore, we find that the signal favors a negative non-Gaussianity, which can suppress the abundance of primordial black holes. We also demonstrate that the anisotropies of scalar-induced gravitational waves serve as a powerful tool to probe the non-Gaussianity $f_{\mathrm{NL}}$. We conduct a comprehensive analysis of the angular power spectrum within the nano-Hertz band. Looking ahead, we anticipate that future projects, such as the Square Kilometre Array, will have the potential to measure these anisotropies and provide further insights into the primordial universe.

astro-ph.CO↗

arXiv2023

Bayesian Implications for the Primordial Black Holes from NANOGrav's Pulsar-Timing Data Using the Scalar-Induced Gravitational Waves

Assuming that the common-spectrum process in the NANOGrav 12.5-year dataset has an origin of scalar-induced gravitational waves, we study the enhancement of primordial curvature perturbations and the mass function of primordial black holes, by performing the Bayesian parameter inference for the first time. We obtain lower limits on the spectral amplitude, i.e., $\mathcal{A}\gtrsim10^{-2}$ at 95\% confidence level, when assuming the power spectrum of primordial curvature perturbations to follow a log-normal distribution function with width $σ$. In the case of $σ\rightarrow0$, we find that the primordial black holes with $2\times10^{-4}-10^{-2}$ solar mass are allowed to compose at least a fraction $10^{-6}$ of dark matter. Such a mass range is shifted to more massive regimes for larger values of $σ$, e.g., to a regime of $4\times10^{-3}-0.2$ solar mass in the case of $σ=1$. We expect the planned gravitational-wave experiments to have their best sensitivity to $\mathcal{A}$ in the range of $10^{-4}$ to $10^{-7}$, depending on the experimental setups. With this level of sensitivity, we can search for primordial black holes throughout the entire parameter space, especially in the mass range of $10^{-16}$ to $10^{-11}$ solar masses, where they could account for all dark matter. In addition, the importance of multi-band detector networks is emphasized to accomplish our theoretical expectation.

astro-ph.CO↗

arXiv2023

Multi-TeV photons from GRB 221009A: uncertainty of optical depth considered

It is reported that the Large High Altitude Air Shower Observatory (LHAASO) observed thousands of very-high-energy photons up to $\sim$18 TeV from GRB 221009A. We study the survival rate of these photons via considering the fact that they are absorbed by the extragalactic background light. By performing a set of $10^6$ Monte-Carlo simulations, we explore the parameter space allowed by current observations and estimate the probability of predicting that LHAASO detects at least one photon of 18 TeV from GRB 221009A. We find that the standard physics is compatible with the observations of 18 TeV photons within $3.5σ$ confidence interval. Our research method can be straightforwardly generalized to study more data sets of LHAASO and other experiments in the future.

astro-ph.HE↗

arXiv2022

Stochastic gravitational wave background due to gravitational wave memory

Gravitational wave memory is an important prediction of general relativity, which has not been detected yet. Amounts of memory events can form a stochastic gravitational wave memory background. Here we find that memory background can be described as a Brownian motion in the condition that the observation time is longer than the averaged time interval between two successive memory events. We investigate, for the first time, the memory background of binary black hole coalescences. We only consider the spectrum of the memory background for a relatively low frequency range. So we can use the step function to approximate the waveform for each memory event. Then we find that the spectrum is a power law with index -2. And the amplitude of the power law spectrum depends on and only on the merger rate of the binary black holes. Consequently, the memory background not only provides a brand new means to detect gravitational wave memory but also opens a new window to explore the event rate of binary black hole mergers and the gravity theory. Space-based detectors are ideal to detect the gravitational wave memory background which corresponds to supermassive binary black holes. Since gravitational wave memory is only sensitive to the merger stage of binary black hole coalescence, the memory background will be an ideal probe of the famous final parsec problem.

gr-qc↗

arXiv2022

Gravitational wave memory produced by cosmic background radiation

It is well known that energy fluxes will produce gravitational wave memory. The gravitational wave memory produced by background including cosmic microwave background (CMB), cosmic neutrino background (C$ν$B), and gravitational wave background is investigated in this work. We construct a theory relating the gravitational wave memory strength to the energy flux of a stochastic background. We find that the resulted gravitational wave memory behaves as a constantly varying metric tensor. Such a varying metric tensor will introduce a quadrupole structure to the universe expansion. The gravitational wave memory due to the CMB is too small to be detected. But the gravitational wave memory due to the C$ν$B and the gravitational wave background is marginally detectable. Interestingly, such detection can be used to estimate the neutrino masses and the properties of the gravitational wave background.

gr-qc↗

arXiv2022

Search for the birefringence of gravitational waves with the third observing run of Advanced LIGO-Virgo

Gravitational waves would attain birefringence during their propagation from distant sources to the Earth, when the CPT symmetry is broken. If it was sizeable enough, such birefringence could be measured by the Advanced LIGO, Virgo and KAGRA detector network. In this work, we place constraints on the birefringence of gravitational waves with the third observing run of this network, i.e. two catalogues GWTC-2 and GWTC-3. For the dispersion relation $ω^{2}=k^{2}\pm2ζk^{3}$, our analysis shows the up-to-date strictest limit on the CPT-violating parameter, i.e. \textcolor{black}{{$ζ=4.07^{+5.91}_{-5.79}\times10^{-17}\mathrm{m}$}}, at $68\%$ confidence level. This limit is stricter by $\sim$5 times than the existing one {($\sim 2\times10^{-16}$m)} and stands for the first $\sim$10GeV-scale test of the CPT symmetry in gravitational waves. The results of Bayes factor strongly disfavor the birefringence scenario of gravitational waves.

gr-qc↗

arXiv2022

GW200105 and GW200115 are compatible with a scenario of primordial black hole binary coalescences

Two gravitational wave events, i.e. GW200105 and GW200115, were observed by the Advanced LIGO and Virgo detectors recently. In this work, we show that they can be explained by a scenario of primordial black hole binaries that are formed in the early Universe. The merger rate predicted by such a scenario could be consistent with the one estimated from LIGO and Virgo, even if primordial black holes constitute a fraction of cold dark matter. The required abundance of primordial black holes is compatible with the existing upper limits from microlensing, caustic crossing and cosmic microwave background observations.

astro-ph.CO↗

arXiv2021

Gravitational wave memory of the binary black hole events in GWTC-2

Gravitational wave (GW) memory is an important prediction of general relativity. Existing works on the GW memory detection focus on the waveform analysis. It is hard for waveform analysis method to detect the GW memory due to its quasi-direct current behavior and weakness. We implement a completely different scheme in this work to estimate the GW memory. In this scheme, we firstly apply the Bondi-Metzner-Sachs method to calculate the GW memory of binary black hole based on numerical relativity simulation. Then we construct a surrogate model to relate binary black hole's parameters and the GW memory. Afterwards we apply this surrogate model together with Bayesian techniques to estimate the GW memory of the 48 binary black hole events recorded in GWTC-2. The GW memory corresponding to the all 48 events has been estimated. The most interesting results are for GW190814. The corresponding GW memory is about $-1\times10^{-23}$ and $1\times10^{-23}$ for Hanford detector and Livingston detector respectively. At the same time we find with 3$σ$ C.L. that the memory strain of GW190814 is negative on Hanford detector while positive on Livingston detector.

gr-qc↗

arXiv2021

LTD064402+245919: A Subgiant with a 1-3 M$_{\odot}$ Undetected Companion Identified from LAMOST-TD Data

Single-line spectroscopic binaries recently contribute to the stellar-mass black hole discovery, independently of the X-ray transient method. We report the identification of a single-line binary system LTD064402+245919, with an orbital period of 14.50 days. The observed component is a subgiant with a mass of 2.77$\pm$0.68M$_{\odot}$, radius 15.5$\pm$2.5R$_{\odot}$, effective temperature $T_{\rm eff}$ 4500$\pm$200K, and surface gravity log\emph{g} 2.5$\pm$0.25dex. The discovery makes use of the LAMOST time-domain (LAMOST-TD) and ZTF survey. Our general-purpose software pipeline applies the Lomb-Scargle periodogram to determine the orbital period and uses machine-learning to classify the variable type from the folded light curves. We apply a combined model to estimate the orbital parameters from both the light and radial velocity curves, taking constraints on the primary star mass, mass function, and detection limit of secondary luminosity into consideration. We obtain a radial velocity semi-amplitude of 44.6$\pm$1.5 km s$^{-1}$, mass ratio of 0.73$\pm$0.07, and an undetected component mass of 2.02$\pm$0.49M$_{\odot}$ when the type of the undetected component is not set. We conclude that the inclination is not well constrained, and that the secondary mass is larger than 1M$_{\odot}$ when the undetected component is modelled as a compact object. According to our investigations using an MCMC simulation, increasing the spectra SNR by a factor of 3 would enable the secondary light to be distinguished (if present). The algorithm and software in this work are able to serve as general-purpose tools for the identification of compact objects quiescent in X-rays.

astro-ph.SR↗

arXiv2021

An Empirical Bayesian Approach to Limb-darkening in Modeling WASP-121b Transit Light Curves

We present a novel, iterative method using an empirical Bayesian approach for modeling the limb darkened WASP-121b transit from the TESS light curve. Our method is motivated by the need to improve $R_{p}/R_{\ast}$ estimates for exoplanet atmosphere modeling, and is particularly effective with the limb darkening (LD) quadratic law requiring no prior central value from stellar atmospheric models. With the non-linear LD law, the method has all the advantages of not needing atmospheric models but does not converge. The iterative method gives a different $R_{p}/R_{\ast}$ for WASP-121b at a significance level of 1$σ$ when compared with existing non-iterative methods. To assess the origins and implications of this difference, we generate and analyze light curves with known values of the limb darkening coefficients (LDCs). We find that non-iterative modeling with LDC priors from stellar atmospheric models results in an inconsistent $R_{p}/R_{\ast}$ at 1.5$σ$ level when the known LDC values are as those previously found when modeling real data by the iterative method. In contrast, the LDC values from the iterative modeling yields the correct value of $R_{p}/R_{\ast}$ to within 0.25$σ$. For more general cases with different known inputs, Monte Carlo simulations show that the iterative method obtains unbiased LDCs and correct $R_{p}/R_{\ast}$ to within a significance level of 0.3$σ$. Biased LDC priors can cause biased LDC posteriors and lead to bias in the $R_{p}/R_{\ast}$ of up to 0.82$\%$, 2.5$σ$ for the quadratic law and 0.32$\%$, 1.0$σ$ for the non-linear law. Our improvement in $R_{p}/R_{\ast}$ estimation is important when analyzing exoplanet atmospheres.

astro-ph.EP↗

arXiv2020

Tests of CPT invariance in gravitational waves with LIGO-Virgo catalog GWTC-1

A discovery of gravitational waves from binary black holes raises a possibility that measurements of them can provide strict tests of CPT invariance in gravitational waves. When CPT violation exists, if any, gravitational waves with different circular polarizations could gain a slight difference in propagating speeds. Hence, the birefringence of gravitational waves is induced and there should be a rotation of plus and cross modes. For CPT-violating dispersion relation ${ω^{2}=k^{2}}$ ${\pm 2ζk^{3}}$, where a sign ${\pm}$ denotes different circular polarizations, we find no substantial deviations from CPT invariance in gravitational waves by analyzing a compilation of ten signals of binary black holes in the LIGO-Virgo catalog GWTC-1. We obtain a strict constraint on the CPT-violating parameter, i.e., $ζ=0.14^{+0.22}_{-0.31}\times10^{-15}\text{m}$, which is around two orders of magnitude better than the existing one. Therefore, this study stands for the up-to-date strictest tests of CPT invariance in gravitational waves.

gr-qc↗

arXiv2020

Impacts of the simulated standard siren on estimating cosmological parameters

We propose a method based on the process of extracting gravitational wave (GW) parameters from GW signals to simulate the binary neutron-star (BNS) merging events. We simulate 1000 GW standard sirens based on the observation of the Einstein Telescope (ET). Almost all the simulated GW data are in the redshift range of $[0,3]$. The role of the GW standard siren in the inference of the cosmological parameters is investigated. We find that the GW data can help improve the accuracy of cosmological parameters. Moreover, the degeneracy of cosmological parameters is broken by the GW data. The GW standard siren is helpful for the constraint of the cosmological parameters.

astro-ph.CO↗

arXiv2019

The prospects of gravitational waves on constraining the anisotropy of the Universe

The observation of GW150914 indicated a new independent measurement of the luminosity distance of a gravitational wave event. In this paper, we constrain the anisotropy of the Universe by using gravitational wave events. We simulate hundreds of events of binary neutron star merging that may be observed by Einstein Telescope. Full simulation for producing process of gravitational wave data is employed. We find that 200 of binary neutron star merging in redshift $(0,1)$ observed by Einstein Telescope may constrain the anisotropy with an accuracy comparable to the result from Union2.1 supernovae. This result shows that gravitational waves can be a powerful tool in investigating the cosmological anisotropy.

gr-qc↗

arXiv2018

Searching for a possible dipole anisotropy on acceleration scale with 147 rotationally supported galaxies

We report a possible dipole anisotropy on acceleration scale $g_†$ with 147 rotationally supported galaxies in local Universe. It is found that a monopole and dipole correction for the radial acceleration relation can better describe the SPARC data set. The monopole term is negligible but the dipole magnitude is significant. It is also found that the dipole correction is mostly induced by the anisotropy on the acceleration scale. The magnitude of $\hat{g}_†$-dipole reaches up to $0.25\pm0.04$, and its direction is aligned to $(l,b) = (171.30^{\circ}\pm7.18^{\circ}, -15.41^{\circ}\pm4.87^{\circ})$, which is very close to the maximum anisotropy direction from the hemisphere comparison method. Furthermore, robust check shows that the dipole anisotropy couldn't be reproduced by isotropic mock data set. However, it is still premature to claim that the Universe is anisotropic due to the small data samples and uncertainty in the current observations.

astro-ph.CO↗