arXiv subjects

Yuan-Zhu Wang

Publications and source records attributed to Yuan-Zhu Wang.

At least 37 records · Page 2Linked to original sources

arXiv2024

The Heavier the Faster: A Sub-population of Heavy, Rapidly Spinning and Quickly Evolving Binary Black Holes

The spins of binary black holes (BBHs) measured from gravitational waves carry notable information of the formation pathways. Here we propose a quantity "dimensionless net spin" ($χ_{\rm N}$), which is related to the sum of angular momentum of component black holes in the system, to provide a novel perspective to study the origin(s) of BBHs. By performing hierarchical Bayesian inference on $χ_{\rm N}$, we find strong evidence that the marginal distribution of this quantity can be better fitted by two Gaussian components rather than one: there is a narrow peak at $χ_{\rm N} \sim 0.15$ and another extended peak at $χ_{\rm N} \sim 0.47$. We also find that the rapidly spinning systems likely dominate the high-mass end of the population and they evolve with redshift much quicker. These findings bring new challenges to the field binary scenario, and suggest that dynamical process should plays a key role in forming high total mass BBHs.

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arXiv2024

Black holes as the source of dark energy: a stringent test with high-redshift JWST AGNs

Studies have proposed that there is evidence for cosmological coupling of black holes (BHs) with an index of $k\approx 3$; hence, BHs serve as the astrophysical source of dark energy. However, the data sample is limited for the redshifts of $\leq 2.5$. In recent years, the James Webb Space Telescope (JWST) has detected many high-redshift active galactic nuclei (AGNs) and quasars. Among the JWST NIRSpec-/NIRCam-resolved AGNs, three are determined to be in early-type host galaxies with a redshift of $z\sim 4.5--7$. However, their $M_{\star}$ and $M_{\rm BH}$ are in tension with the predicted cosmological coupling of black holes with $k = 3$ at a confidence level of $\sim 2σ$, which challenges the hypothesis that BHs serve as the origin of dark energy. Future work on high-redshift AGNs using the JWST will further assess such a hypothesis by identifying more early-type host galaxies in the higher mass range.

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arXiv2023

Potential Subpopulations and Assembling Tendency of the Merging Black Holes

The origins of coalescing binary black holes (BBHs) detected by the advanced LIGO/Virgo are still under debate, and clues may be present in the joint mass-spin distribution of these merger events. Here we construct phenomenological models containing two sub-populations to investigate the BBH population detected in gravitational wave observations. We find that our models can explain the GWTC-3 data rather well, and several constraints to our model are required by the data: first, the maximum mass for the component with a stellar-origin, $m_{\rm max}$, is $39.1^{+2.4}_{-2.7}M_{\odot}$ at 90\% credibility; second, about $15\%$ of the mergers happen in dynamical environments, in which $7-16\%$ of events are hierarchical mergers, and these BHs have an average spin magnitude significantly larger than the first-generation mergers, with ${\rm d}μ_{\rm a} > 0.4 $ at $99\%$ credibility; third, the dynamical component BHs tend to pair with each other with larger total mass and higher mass ratio. An independent analysis focusing on spins is also carried out, and we find that the spin amplitude of component BHs can be divided into two groups according to a division mass $m_{\rm d} = 46.1^{+5.6}_{-5.1}M_{\odot}$. These constraints can be naturally explained by current formation channels, and our results suggest that some of the observed events were likely from AGN disks.

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arXiv2022

Searching for candidates of coalescing binary black holes formed through chemically homogeneous evolution in GWTC-3

The LIGO, Virgo, and KAGRA (LVK) collaboration has announced 90 coalescing binary black holes (BBHs) with $p_{\rm astro} > 50\%$ to date, however, the origin of their formation channels is still an open scientific question. Given various properties of BBHs (BH component masses and individual spins) inferred using the default priors by the LVK, independent groups have been trying to explain the formation of the BBHs with different formation channels. Of all formation scenarios, the chemically homogeneous evolution (CHE) channel has stood out with distinguishing features, namely, nearly-equal component masses and preferentially high individual spins aligned with the orbital angular momentum. We perform Bayesian inference on the BBH events officially reported in GWTC-3 with astrophysically-predicted priors representing different formation channels of the isolated binary evolution (CEE: common-envelope evolution channel; CHE; SMT: stable mass transfer). Given assumed models, we report strong evidence for GW190517\_055101 being most likely to have formed through the CHE channel. Assuming the BBH events in the subsample are all formed through one of the isolated binary evolution channels, we obtain the lower limits on the local merger rate density of these channels at $11.45 ~\mathrm{Gpc^{-3}~yr^{-1}}$ (CEE), $0.18 ~\mathrm{Gpc^{-3}~yr^{-1}}$ (CHE), and $0.63 ~\mathrm{Gpc^{-3}~yr^{-1}}$ (SMT) at $90\%$ credible level.

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arXiv2022

Divergence in mass ratio distributions between low-mass and high-mass coalescing binary black holes

Coalescing binary black hole (BBH) systems are likely formed via several channels, and it is challenging to understand their formation / evolutionary processes. Some features in the mass function of the primary components ($m_1$), such as the distinct Gaussian-like peak located at $\sim 34M_\odot$, have been previously found. In this work, we investigate the possible dependence of the mass ratio ($q=m_2/m_1$) distribution on the primary mass. We find a Bayesian odds ratio of 18.1 in favor of divergence in the mass ratio distributions between the low- and high-mass ranges over an invariable mass ratio distribution. The BBHs with $m_1\gtrsim29M_{\odot}$ have a stronger preference to be symmetric compared to those with $m_1\lesssim29M_{\odot}$ at a 97.6\% credible level. Additionally, we find mild evidence that the BBHs with $m_1$ located in the Gaussian-like peak have a mass ratio distribution different from that of other BBHs. Our findings may be in favor of some formation channels, such as the chemically homogeneous evolution and the dynamical assembly in globular clusters/nuclear star clusters, which are more likely to provide symmetric BBHs in the high-mass range.

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arXiv2022

Hypercritical Accretion for Black Hole High Spin in Cygnus X-1

Recent observations of AdLIGO and Virgo have shown that the spin measurements in binary black hole (BH) systems are typically small, which is consistent with the predictions by the classical isolated binary evolution channel. In this standard formation channel, the progenitor of the first-born BH is assumed to have efficient angular momentum transport. The BH spins in high-mass X-ray binaries (HMXBs), however, have been found consistently to be extremely high. In order to explain the high BH spins, the inefficient angular momentum transport inside the BH progenitor is required. This requirement, however, is incompatible with the current understanding of conventional efficient angular momentum transport mechanism. We find that this tension can be highly alleviated as long as the hypercritical accretion is allowed. We show that, for a case study of Cygnus X-1, the hypercritical accretion cannot only be a good solution for the inconsistent assumption upon the angular momentum transport within massive stars, but match its other properties reported recently.

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arXiv2021

On the Angular Momentum Transport Efficiency within the Star Constrained from Gravitational-wave Observations

The LIGO Scientific Collaboration and Virgo Collaboration (LVC) have recently reported in GWTC-2.1 eight additional candidate events with a probability of astrophysical origin greater than 0.5 in the LVC deeper search on O3a running. In GWTC-2.1, the majority of the effective inspiral spins ($χ_{\rm eff}$) show magnitudes consistent with zero, while two (GW190403$_{-}$051519 and GW190805$_{-}$211137) of the eight new events have $χ_{\rm eff}$ $> 0$ (at 90% credibility). We note that GW190403$_{-}$051519 was reported with $χ_{\rm eff}$ = $0.70^{+0.15}_{-0.27}$ and mass ratio $q$ = $0.25^{+0.54}_{-0.11}$, respectively. Assuming a uniform prior probability between 0 and 1 for each black hole's dimensionless spin magnitude, GW190403$_{-}$051519 was reported with the dimensionless spin of the more massive black hole, $χ_1$ = $0.92^{+0.07}_{-0.22}$. This is the fastest first-born black hole ever measured in all current gravitational-wave events. If GW190403$_{-}$051519 is formed through isolated binary evolution channel, this extremely high spin challenges, at least in that case, the existence of efficient angular momentum transport mechanism between the stellar core and the radiative envelope of massive stars, as for instance predicted by the Tayler-Spruit dynamo (Spruit 2002) or its revised version by Fuller et al. 2019.

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arXiv2021

Black Hole Gravitational Potential Enhanced Fallback Accretion onto the Nascent Lighter Compact Object: Tentative Evidence in the O3 Run Data of LIGO/Virgo

In a binary system, the gravitational potential of the primary black hole may play an important role in enhancing the fallback accretion onto the lighter compact object newly formed in the second supernova explosion. As a result, the final masses of the binary compact objects would be correlated, as suggested recently by Safarzadeh \& Wysocki. In this work, we analyze the mass distribution of four gravitational-wave events, which are characterized by both a small mass ratio and a low mass ($\leq 5M_\odot$) of the light component, and find tentative evidence for a mass correlation among the objects. To evaluate the feasibility of testing such a hypothesis with upcoming observations, we carry out simulations with a mock population and perform Bayesian hierarchical inference for the mass distribution. We find that with dozens of low mass ratio events, whether there exists correlation in the component mass distributions or not can be robustly tested and the correlation, if it exists, can be well determined.

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arXiv2021

Population properties of neutron stars in the coalescing compact binaries

We perform a hierarchical Bayesian inference to investigate the population properties of the coalescing compact binaries involving at least one neutron star (NS). With the current observation data, we can not rule out either of the Double Gaussian, Single Gaussian and Uniform NS mass distribution models, although the mass distribution of the Galactic NSs is slightly preferred by the gravitational wave (GW) observations. The mass distribution of black holes (BHs) in the neutron star-black hole (NSBH) population is found to be similar to that for the Galactic X-ray binaries. Additionally, the ratio of the merger rate densities between NSBHs and BNSs is estimated to be about 3 : 7. The spin properties of the binaries, though constrained relatively poor, play nontrivial role in reconstructing the mass distribution of NSs and BHs. We find that a perfectly aligned spin distribution can be ruled out, while a purely isotropic distribution of spin orientation is still allowed.

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arXiv2021

Mass and effective spin distributions of binary black hole events: Evidence for a population of hierarchical black hole mergers in active galactic nuclei disks

The origins of the coalescing binary black holes (BBHs) detected by the advanced LIGO/Virgo are still in debate and clues may present in the mass and effective spin ($χ_{\rm eff}$) distributions of these merger events. Here we analyze the GWTC-1 and GWTC-2.1 BBH events, and find evidence for a primary mass dependent $χ_{\rm eff}$ distribution. Below (above) the mass of $\sim 50M_\odot$, a nearly zero (relatively high) effective spin is typical. In the joint analysis of the primary mass and $χ_{\rm eff}$ distributions, we identify a sub-population of BBHs characterized by both a bump-like primary mass distribution extending to $\sim 100M_\odot$ and a relatively high $χ_{\rm eff}\sim 0.3$, consistent with the expectation for the $\geq 2$g merger events in the disks of active galactic nuclei (AGNs). Our results suggest that the merger rate in AGN disks is around $1.7~{\rm Gpc}^{-3}{\rm yr}^{-1}$.

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arXiv2021

A flexible Gaussian process reconstruction method and the mass function of the coalescing binary black hole systems

We develop a new method based on Gaussian process to reconstruct the mass distribution of binary black holes (BBHs). Instead of prespecifying the formalisms of mass distribution, we introduce a more flexible and nonparametric model with which the distribution can be mainly determined by the observed data. We first test our method with simulated data, and find that it can well recover the injected distribution. Then we apply this method to analyze the data of BBHs' observations from LIGO-Virgo Gravitational-Wave Transient Catalog 2. By reconstructing the chirp mass distribution, we find that there is a peak or a platform locating at $20-30\,M_{\odot}$ rather than a single-power-law-like decrease from low mass to high mass. Moreover, one or two peaks in the chirp mass range of $\mathcal{M}<20\,M_{\odot}$ may be favored by the data. Assuming a mass-independent mass ratio distribution of $p(q)\propto q^{1.4}$, we further obtain a distribution of primary mass, and find that there is a feature locating in the range of $(30, 40)\,M_{\odot}$, which can be related to \textsc{Broken Power Law} and \textsc{Power Law + Peak} distributions described in The LIGO Scientific Collaboration et al. (2020). Besides, the merger rate of BBHs is estimated to $\mathcal{R} = 26.29^{+14.21}_{-8.96}~{\rm Gpc^{-3}~yr^{-1}}$ supposing there is no redshift evolution.

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arXiv2021

Black Hole Mass Function of Coalescing Binary Black Hole Systems: Is There a Pulsational Pair Instability Mass Cutoff?

We analyze the LIGO/Virgo GWTC-2 catalog to study the primary mass distribution of the merging black holes. We perform hierarchical Bayesian analysis, and examine whether the mass distribution has a sharp cutoff for primary black hole masses below $65 M_\odot$, as predicted in pulsational pair instability supernova model. We construct two empirical mass functions. One is a piece-wise function with two power-law segments jointed by a sudden drop. The other consists of a main truncated power-law component, a Gaussian component, and a third very massive component. Both models can reasonably fit the data and a sharp drop of the mass distribution is found at $\sim 50M_\odot$, suggesting that the majority of the observed black holes can be explained by the stellar evolution scenarios in which the pulsational pair-instability process takes place. On the other hand, the very massive sub-population, which accounts for at most several percents of the total, may be formed through hierarchical mergers or other processes.

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arXiv2020

GW190426_152155: a merger of neutron star-black hole or low mass binary black holes?

GW190426_152155 was recently reported as one of the 39 candidate gravitational wave (GW) events in \citet{2020arXiv201014527A}, which has an unusual source-frame chirp mass $\sim 2.4M_{\odot}$ and may be the first GW signal from a neutron star-black hole (NSBH) merger. Assuming an astrophysical origin, we reanalyze GW190426_152155 using several waveforms with different characteristics, and consider two different priors for the mass ratio of the binary (Uniform and LogUniform). We find that the results are influenced by the priors of mass ratio, and this candidate could also be from the merger of two low mass black holes (BH). In the case for a binary black hole (BBH) merger, the effective spin is likely negative and the effective precession spin is non-negligible. As for the NSBH merger, supposing the mass of the light object follow the distribution of current neutron stars (NSs) with a reasonably measured/constrained mass, the spin of the low mass BH is so small that is hard to generate bright electromagnetic emission. Finally, we estimate a merger rate of GW190426\_152155-like systems to be $59^{+137}_{-51}~{\rm Gpc}^{-3}~{\rm yr}^{-1}$.

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arXiv2020

GW190521 and the GWTC-1 Events: Implication on the Black Hole Mass Function of Coalescing Binary Black Hole Systems

With the black hole mass function (BHMF; assuming an exponential cutoff at a mass of $\sim 40\,M_\odot$) of coalescing binary black hole systems constructed with the events detected in the O1 run of the advanced LIGO/Virgo network, Liang et al.(2017) predicted that the birth of the lightest intermediate mass black holes (LIMBHs; with a final mass of $\gtrsim 100\,M_\odot$) is very likely to be caught by the advanced LIGO/Virgo detectors in their O3 run. The O1 and O2 observation run data, however, strongly favor a cutoff of the BHMF much sharper than the exponential one. In this work we show that a power-law function followed by a sudden drop at $\sim 40\,M_\odot$ by a factor of $\sim $a few tens and then a new power-law component extending to $\geq 100M_\odot$ are consistent with the O1 and O2 observation run data. With this new BHMF, quite a few LIMBH events can be detected in the O3 observation run of advanced LIGO/Virgo. The first LIMBH born in GW190521, an event detected in the early stage of the O3 run of advanced LIGO/Virgo network, provides additional motivation for our hypothesis.

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arXiv2020

Discovery of a Universal Correlation For Long and Short GRBs and Its Application on the Study of Luminosity Function and Formation Rate

Gamma-ray bursts (GRBs) are known to be the most violent explosions in the universe, and a variety of correlations between observable GRB properties have been proposed in literature, but none of these correlations is valid for both long GRBs and short GRBs. In this paper we report the discovery of a universal correlation which is suitable for both long and short GRBs using three prompt emission properties of GRBs, i.e. the isotropic peak luminosity $L_{\rm iso}$, the peak energy of the time-integtated prompt emission spectrum $E_{\rm peak}$, and the "high signal" timescale $T_{\rm 0.45}$, $L_{\rm iso} \propto E_{\rm peak}^{1.94} T_{0.45}^{0.37}$. This universal correlation just involves properties of GRB prompt emission and does not require any information of afterglow phase, which can be used as a relatively unbiased redshift estimator. Here we use this correlation to estimate the pseudo-redshifts for short Gamma Ray Bursts and then use Lynden-Bell method to obtain a non-parametric estimate of their luminosity function and formation rate. The luminosity function is $ψ(L_0)\propto{L_0^{-0.63\pm{0.07}}}$ for dim SGRBs and $ψ(L_0)\propto{L_0^{-1.96\pm{0.28}}}$ for bright SGRBs, with the break point $6.95_{-0.76}^{+0.84}\times10^{50} erg/s$. The local formation rate of SGRBs is about 15 events $\rm Gpc^{-3}yr^{-1}$ . This universal correlation may have important implications for GRB physics, implying that the long and short GRBs should share similar radiation processes.

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arXiv2020

PSR J0030+0451, GW170817 and the nuclear data: joint constraints on equation of state and bulk properties of neutron stars

Very recently the NICER collaboration has published the first-ever accurate measurement of mass and radius together for PSR J0030+0451, a nearby isolated quickly-rotating neutron star (NS). In this work we set the joint constraints on the equation of state (EoS) and some bulk properties of NSs with the data of PSR J0030+0451, GW170817 and some nuclear experiments. The piecewise polytropic expansion method and the spectral decomposition method have been adopted to parameterize the EoS. The resulting constraints are consistent with each other. Assuming the maximal gravitational mass of non-rotating NS $M_{\rm TOV}$ lies between $2.04 \rm M_{\odot}$ and $2.4 \rm M_{\odot}$, with the piecewise method the pressure at twice nuclear saturation density is measured to be $3.38^{+2.43}_{-1.50}\times 10^{34}~{\rm dyn~cm^{-2}}$ at the $90\%$ level. For a NS with canonical mass of $1.4 \rm M_\odot$, we have the moment of inertia $I_{1.4} = {1.43}^{+0.28}_{-0.13} \times 10^{38}~{\rm kg \cdot m^2}$, tidal deformability $Λ_{1.4} = 390_{-140}^{+320}$, radius $R_{1.4} = 12.2_{-0.9}^{+1.0}~{\rm km}$, and binding energy $BE_{1.4} = {0.16}^{+0.01}_{-0.02} \rm M_{\odot}$ at the $90\%$ level, which are improved in comparison to the constraints with the sole data of GW170817.

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arXiv2020

Estimating the maximum gravitational mass of nonrotating neutron stars from the GW170817/GRB 170817A/AT2017gfo observation

Assuming that the differential rotation of the massive neutron star (NS) formed in the double NS (DNS) mergers has been effectively terminated by the magnetic braking and a uniform rotation has been subsequently established (i.e., a supramassive NS is formed), we analytically derive in this work an approximated expression for the critical total gravitational mass ($M_{\rm tot,c}$) to form supramassive NS (SMNS) in the DNS mergers, benefited from some equation of state (EoS) insensitive relationships. The maximum gravitational mass of the nonrotating NSs ($M_{\rm TOV}$) as well as the dimensionless angular momentum of the remnant ($j$) play the dominant roles in modifying $M_{\rm tot,c}$, while the radius and mass differences of the premerger NSs do not. The GW170817/GRB 170817A/AT2017gfo observations have provided so far the best opportunity to quantitatively evaluate $M_{\rm TOV}$. Supposing the central engine for GRB 170817A is a black hole quickly formed in the collapse of an SMNS, we find $M_{\rm TOV}=2.13^{+0.09}_{-0.08}M_\odot$ (68.3% credibility interval, including also the uncertainties of the EoS insensitive relationships), which is consistent with the constraints set by current NS mass measurements.

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arXiv2020

Black Hole Mass Function of Coalescing Neutron Star-Black Hole Binary Systems: The Prospect of Reconstruction with the Gravitational Wave Observations

The discovery of gravitational waves from compact objects coalescence opens a brand-new window to observe the universe. With more events being detected in the future, statistical examinations would be essential to better understand the underlying astrophysical processes. In this work we investigate the prospect of measuring the mass function of black holes that are merging with the neutron stars. Applying Bayesian parameter estimation for hundreds of simulated neutron star$-$black hole (NSBH) mergers, we find that the parameters for most of the injected events can be well recovered. We also take a Bayesian hierarchical model to reconstruct the population properties of the masses of black holes, in the presence of a low mass gap, both the mass gap and power-law index ($α$) of black hole mass function can be well measured, thus we can reveal where the $α$ is different for binary black hole (BBH) and NSBH systems. In the absence of a low mass gap, the gravitational wave data as well as the electromagnetic data can be used to pin down the nature of the merger event and then measure the mass of these very light black holes. However, as a result of the misclassification of BBH into NSBH, the measurement of $α$ is more challenging and further dedicated efforts are needed.

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