arXiv subjects

Xing-Jiang Zhu

Publications and source records attributed to Xing-Jiang Zhu.

At least 37 records · Page 2Linked to original sources

arXiv2022

Observational evidence for a spin-up line in the P-Pdot diagram of millisecond pulsars

It is believed that millisecond pulsars attain their fast spins by accreting matter and angular momentum from companion stars. Theoretical modelling of the accretion process suggests a spin-up line in the period-period derivative ($P$-$\dot{P}$) diagram of millisecond pulsars, which plays an important role in population studies of radio millisecond pulsars and accreting neutron stars in X-ray binaries. Here we present observational evidence for such a spin-up line using a sample of 143 radio pulsars with $P$ < 30 ms. We also find that PSRs~J1823$-$3021A and J1824$-$2452A, located near the classic spin-up line, are consistent with the broad population of millisecond pulsars. Finally, we show that our approach of Bayesian inference can probe accretion physics, allowing constraints to be placed on the accretion rate and the disk-magnetosphere interaction.

astro-ph.HE↗

arXiv2022

Consistency of the Parkes Pulsar Timing Array Signal with a Nanohertz Gravitational-wave Background

Pulsar timing array experiments have recently reported strong evidence for a common-spectrum stochastic process with a strain spectral index consistent with that expected of a nanohertz-frequency gravitational-wave background, but with negligible yet non-zero evidence for spatial correlations required for a definitive detection. However, it was pointed out by the Parkes Pulsar Timing Array (PPTA) collaboration that the same models used in recent analyses resulted in strong evidence for a common-spectrum process in simulations where none is present. In this work, we introduce a methodology to distinguish pulsar power spectra with the same amplitude from noise power spectra of similar but distinct amplitudes. The former is the signature of a spatially uncorrelated pulsar term of a nanohertz gravitational-wave background, whereas the latter could represent ensemble pulsar noise properties. We test the methodology on simulated data sets. We find that the reported common process in PPTA pulsars is indeed consistent with the spectral feature of a pulsar term. We recommend this methodology as one of the validity tests that the real astrophysical and cosmological backgrounds should pass, as well as for inferences about the spatially uncorrelated component of the background.

gr-qc↗

arXiv2021

Optimized localization for gravitational-waves from merging binaries

The Advanced LIGO and Virgo gravitational wave observatories have opened a new window with which to study the inspiral and mergers of binary compact objects. These observations are most powerful when coordinated with multi-messenger observations. This was underlined by the first observation of a binary neutron star merger GW170817, coincident with a short Gamma-ray burst, GRB170817A, and the identification of the host galaxy NGC~4993 from the optical counterpart AT~2017gfo. Finding the fast-fading optical counterpart critically depends on the rapid production of a sky-map based on LIGO/Virgo data. Currently, a rapid initial sky map is produced followed by a more accurate, high-latency, $\gtrsim{12}{\rm{hr}}$ sky map. We study optimization choices of the Bayesian prior and signal model which can be used alongside other approaches such as reduced order quadrature. We find these yield up to a $60\%$ reduction in the time required to produce the high-latency localisation for binary neutron star mergers.

gr-qc↗

arXiv2021

On the large apparent black hole spin-orbit misalignment angle in GW200115

GW200115 is one of the first two confidently detected gravitational-wave events of neutron star-black hole mergers. An interesting property of this merger is that the black hole, if spinning rapidly, has its spin axis negatively aligned (with a misalignment angle $> 90^{\circ}$) with the binary orbital angular momentum vector. Although such a large spin-orbit misalignment angle naturally points toward a dynamical origin, the measured neutron star-black hole merger rate exceeds theoretical predictions of the dynamical formation channel. In the canonical isolated binary formation scenario, the immediate progenitor of GW200115 is likely to be a binary consisting of a black hole and a helium star, with the latter forming a neutron star during a supernova explosion. Since the black hole is generally expected to spin along the pre-supernova binary orbital angular momentum axis, a large neutron star natal kick is required to produce the observed misalignment angle. Using simple kinematic arguments, we find that a misalignment angle $> 90^{\circ}$ in GW200115-like systems implies a kick velocity $\sim 600\, \text{km/s}$ and a kick direction within $\approx 30 ^{\circ}$ of the pre-supernova orbital plane. We discuss different interpretations of the large apparent black hole spin-orbit misalignment angle, including a non-spinning black hole.

astro-ph.HE↗

arXiv2021

Heavy double neutron stars: birth, mid-life and death

Radio pulsar observations probe the lives of Galactic double neutron-star (DNS) systems while gravitational waves enable us to study extragalactic DNS in their final moments. By combining measurements from radio and gravitational-wave astronomy, we seek to gain a more complete understanding of DNS from formation to merger. We analyse the recent gravitational-wave binary neutron star mergers GW170817 and GW190425 in the context of other DNS known from radio astronomy. By employing a model for the birth and evolution of DNS, we measure the mass distribution of DNS at birth, at mid-life (in the radio), and at death (in gravitational waves). We consider the hypothesis that the high-mass gravitational-wave event GW190425 is part of a subpopulation formed through unstable case BB mass transfer, which quickly merge in $\sim 10-100~\mathrm{Myr}$. We find mild evidence to support this hypothesis and that GW190425 is not a clear outlier from the radio population as previously claimed. If there are fast-merging binaries, we estimate that they constitute $8-79\%$ of DNS at birth (90% credibility). We estimate the typical delay time between the birth and death of fast-merging binaries to be $\approx 5-401~\mathrm{Myr}$ (90% credibility). We discuss the implications for radio and gravitational-wave astronomy.

astro-ph.HE↗

arXiv2020

Standard-siren cosmology using gravitational waves from binary black holes

Gravitational-wave astronomy provides a unique new way to study the expansion history of the Universe. In this work, we investigate the impact future gravitational-wave observatories will have on cosmology. Third-generation observatories like the Einstein Telescope and Cosmic Explorer will be sensitive to essentially all of the binary black hole coalescence events in the Universe. Recent work by \cite{farr2019future} points out that features in the stellar-mass black hole population break the mass-redshift degeneracy, facilitating precise determination of the Hubble parameter without electromagnetic counterparts or host galaxy catalogues. Using a hierarchical Bayesian inference model, we show that with one year of observations by the Einstein Telescope, the Hubble constant will be measured to $\lesssim 1\%$. We also show that this method can be used to perform Bayesian model selection between cosmological models. As an illustrative example, we find that a decisive statement can be made comparing the $Λ$CDM and RHCT cosmological models using two weeks of data from the Einstein Telescope.

astro-ph.CO↗

arXiv2020

Identifying and mitigating noise sources in precision pulsar timing data sets

Pulsar timing array projects measure the pulse arrival times of millisecond pulsars for the primary purpose of detecting nanohertz-frequency gravitational waves. The measurements include contributions from a number of astrophysical and instrumental processes, which can either be deterministic or stochastic. It is necessary to develop robust statistical and physical models for these noise processes because incorrect models diminish sensitivity and may cause a spurious gravitational wave detection. Here we characterise noise processes for the 26 pulsars in the second data release of the Parkes Pulsar Timing Array using Bayesian inference. In addition to well-studied noise sources found previously in pulsar timing array data sets such as achromatic timing noise and dispersion measure variations, we identify new noise sources including time-correlated chromatic noise that we attribute to variations in pulse scattering. We also identify "exponential dip" events in four pulsars, which we attribute to magnetospheric effects as evidenced by pulse profile shape changes observed for three of the pulsars. This includes an event in PSR J1713$+$0747, which had previously been attributed to interstellar propagation. We present noise models to be used in searches for gravitational waves. We outline a robust methodology to evaluate the performance of noise models and identify unknown signals in the data. The detection of variations in pulse profiles highlights the need to develop efficient profile domain timing methods.

astro-ph.HE↗

arXiv2020

Characterizing Astrophysical Binary Neutron Stars with Gravitational Waves

Merging binary neutron stars are thought to be formed predominantly via isolated binary evolution. In this standard formation scenario, the first-born neutron star goes through a recycling process and might be rapidly spinning during the final inspiral, whereas the second-born star is expected to have effectively zero spin at merger. Based on this feature, we propose a new framework for the astrophysical characterization of binary neutron stars observed from their gravitational wave emission. We further propose a prior for the dimensionless spins of recycled neutron stars, given by a gamma distribution with a shape parameter of 2 and a scale parameter of 0.012, extrapolated from radio pulsar observations of Galactic binary neutron stars. Interpreting GW170817 and GW190425 in the context of the standard formation scenario and adopting the gamma-distribution prior, we find positive support (with a Bayes factor of 6, over the nonspinning hypothesis) for a spinning recycled neutron star in GW190425, whereas the spin of the recycled neutron star in GW170817 is small and consistent with our prior. We measure the masses of the recycled (slow) neutron stars in GW170817 and GW190425 to be $1.34_{-0.09}^{+0.12}$ $(1.38_{-0.11}^{+0.11})$ $M_{\odot}$ and $1.64_{-0.11}^{+0.13}$ $(1.66_{-0.12}^{+0.12})$ $M_{\odot}$, with $68\%$ credibility, respectively. We discuss implications for the astrophysical origins of these two events and outline future prospects of studying binary neutron stars using our framework.

astro-ph.HE↗

arXiv2020

Toward the unambiguous identification of supermassive binary black holes through Bayesian inference

Supermassive binary black holes at sub-parsec orbital separations have yet to be discovered, with the possible exception of blazar OJ~287. In parallel to the global hunt for nanohertz gravitational waves from supermassive binaries using pulsar timing arrays, there has been a growing sample of candidates reported from electromagnetic surveys, particularly searches for periodic variations in optical light curves of quasars. However, the periodicity search is prone to false positives from quasar red noise and quasi-periodic oscillations from the accretion disc of a single supermassive black hole---especially when the data span fewer than a few signal cycles. We present a Bayesian method for the detection of quasar (quasi-)periodicity in the presence of red noise. We apply this method to the binary candidate PG1302$-$102, and show that a) there is very strong support (Bayes factor $>10^6$) for quasi-periodicity, and b) the data slightly favour a quasi-periodic oscillation over a sinusoidal signal, which we interpret as modest evidence against the binary black hole hypothesis. We also find that the prevalent damped random walk red-noise model is disfavored with more than 99.9\% credibility. Finally, we outline future work that may enable the unambiguous identification of supermassive binary black holes.

astro-ph.HE↗

arXiv2020

Is there a spectral turnover in the spin noise of millisecond pulsars?

Pulsar timing arrays provide a unique means to detect nanohertz gravitational waves through long-term measurements of pulse arrival times from an ensemble of millisecond pulsars. After years of observations, some timing array pulsars have been shown to be dominated by low-frequency red noise, including spin noise that might be associated with pulsar rotational irregularities. The power spectral density of pulsar timing red noise is usually modeled with a power law or a power law with a turnover frequency below which the noise power spectrum plateaus. If there is a turnover in the spin noise of millisecond pulsars, residing within the observation band of current and/or future pulsar timing measurements, it may be easier than projected to resolve the gravitational-wave background from supermassive binary black holes. Additionally, the spectral turnover can provide valuable insights on neutron star physics. In the recent study by Melatos and Link, the authors provided a derivation of the model for power spectral density of spin noise from superfluid turbulence in the core of a neutron star, from first principles. The model features a spectral turnover, which depends on the dynamical response time of the superfluid and the steady-state angular velocity lag between the crust and the core of the star. In this work, we search for a spectral turnover in spin noise using the first data release of the International Pulsar Timing Array. Through Bayesian model selection, we find no evidence of a spectral turnover. Our analysis also shows that data from pulsars J1939+2134, J1024$-$0719 and J1713+0747 prefers the power-law model to the superfluid turbulence model.

astro-ph.HE↗

arXiv2020

On the origin of GW190425

The LIGO/Virgo collaborations recently announced the detection of a likely binary neutron star merger, GW190425. The total mass of GW190425 is significantly larger than the masses of Galactic double neutron stars known through radio astronomy. This suggests that GW190425 formed differently from Galactic double neutron stars. We hypothesize that GW190425 formed via unstable "case BB" mass transfer. According to this hypothesis, the progenitor of GW190425 was a binary consisting of a neutron star and a ${\sim} 4-5 {M_\odot}$ helium star, which underwent a common-envelope process. Following the supernovae of the helium star core, a tight, eccentric, double neutron star was formed, which merged in ${\lesssim}$ 10 Myr. The helium star progenitor may explain the unusually large mass of GW190425, while the short time to merger may explain why we do not see similar systems in radio. In order to test this hypothesis, we measure the eccentricity of GW190425 using publicly available LIGO/Virgo data. We constrain the eccentricity at 10 Hz to be $e \leq 0.007$ with $90\%$ confidence. This result provides no evidence for or against the unstable mass transfer scenario because the binary is likely to have circularized to $e\lesssim10^{-4}$ by the time it entered the LIGO/Virgo band. Future detectors operating in lower frequency bands will enable us to discern the formation channel of mergers similar to GW190425 using eccentricity measurements.

astro-ph.HE↗

arXiv2019

The mass distribution of Galactic double neutron stars

The conventional wisdom, dating back to 2012, is that the mass distribution of Galactic double neutron stars is well-fit by a Gaussian distribution with a mean of $1.33 M_\odot$ and a width of $0.09 M_\odot$. With the recent discovery of new Galactic double neutron stars and GW170817, the first neutron star merger event to be observed with gravitational waves, it is timely to revisit this model. In order to constrain the mass distribution of double neutron stars, we perform Bayesian inference using a sample of 17 Galactic double neutron stars effectively doubling the sample used in previous studies. We expand the space of models so that the recycled neutron star need not be drawn from the same distribution as the non-recycled companion. Moreover, we consider different functional forms including uniform, single-Gaussian, and two-Gaussian distributions. While there is insufficient data to draw firm conclusions, we find positive support (a Bayes factor of 9) for the hypothesis that recycled and non-recycled neutron stars have distinct mass distributions. The most probable model---preferred with a Bayes factor of 29 over the conventional model---is one in which the recycled neutron star mass is distributed according to a two-Gaussian distribution and the non-recycled neutron star mass is distributed uniformly. We show that precise component mass measurements of $\approx 20$ double neutron stars are required in order to determine with high confidence (a Bayes factor of 150) if recycled and non-recycled neutron stars come from a common distribution. Approximately $60$ are needed in order to establish the detailed shape of the distributions.

astro-ph.HE↗

arXiv2018

The minimum and maximum gravitational-wave background from supermassive binary black holes

The gravitational-wave background from supermassive binary black holes (SMBBHs) has yet to be detected. This has led to speculations as to whether current pulsar timing array limits are in tension with theoretical predictions. In this paper, we use electromagnetic observations to constrain the SMBBH background from above and below. To derive the {\em maximum} amplitude of the background, we suppose that equal-mass SMBBH mergers fully account for the local black hole number density. This yields a maximum characteristic signal amplitude at a period of one year $A_{\rm{yr}}<2.4\times 10^{-15}$, which is comparable to the pulsar timing limits. To derive the {\em minimum} amplitude requires an electromagnetic observation of an SMBBH. While a number of candidates have been put forward, there are no universally-accepted electromagnetic detections in the nanohertz band. We show the candidate 3C 66B implies a lower limit, which is inconsistent with limits from pulsar timing, casting doubt on its binary nature. Alternatively, if the parameters of OJ 287 are known accurately, then $A_{\rm{yr}}> 6.1\times 10^{-17}$ at 95\% confidence level. If one of the current candidates can be established as a bona fide SMBBH, it will immediately imply an astrophysically interesting lower limit.

astro-ph.GA↗

arXiv2018

Inferring the population properties of binary neutron stars with gravitational-wave measurements of spin

The recent LIGO-Virgo detection of gravitational waves from a binary neutron star inspiral event GW170817 and the discovery of its accompanying electromagnetic signals mark a new era for multimessenger astronomy. In the coming years, advanced gravitational-wave detectors are likely to detect tens to hundreds of similar events. Neutron stars in binaries can possess significant spin, which is imprinted on the gravitational waveform via the effective spin parameter $χ_\text{eff}$. We explore the astrophysical inferences made possible by gravitational-wave measurements of $χ_\text{eff}$. First, using a fiducial model informed by radio observations, we estimate that $\approx15-30\%$ of binary neutron stars should have spins measurable at $\gtrsim 90\%$ confidence level by advanced detectors assuming the spin axis of the recycled neutron star aligns with the total orbital angular momentum of the binary. Second, using Bayesian inference, we show that it is possible to tell whether or not the spin axis of the recycled neutron star tends to be aligned with the binary orbit using $\gtrsim 30$ detections. Finally, interesting constraints can be placed on neutron star magnetic field decay after $\gtrsim 300$ detections, if the spin periods and magnetic field strengths of Galactic binary neutron stars are representative of the merging population.

astro-ph.HE↗

arXiv2015

Detecting nanohertz gravitational waves with pulsar timing arrays

Complementary to ground-based laser interferometers, pulsar timing array experiments are being carried out to search for nanohertz gravitational waves. Using the world's most powerful radio telescopes, three major international collaborations have collected $\sim$10-year high precision timing data for tens of millisecond pulsars. In this paper we give an overview on pulsar timing experiments, gravitational wave detection in the nanohertz regime, and recent results obtained by various timing array projects.

astro-ph.IM↗

arXiv2015

Detection and localization of single-source gravitational waves with pulsar timing arrays

Pulsar timing arrays (PTAs) can be used to search for very low frequency ($10^{-9}$--$10^{-7}$ Hz) gravitational waves (GWs). In this paper we present a general method for the detection and localization of single-source GWs using PTAs. We demonstrate the effectiveness of this new method for three types of signals: monochromatic waves as expected from individual supermassive binary black holes in circular orbits, GWs from eccentric binaries and GW bursts. We also test its implementation in realistic data sets that include effects such as uneven sampling and heterogeneous data spans and measurement precision. It is shown that our method, which works in the frequency domain, performs as well as published time-domain methods. In particular, we find it equivalent to the $\mathcal{F}_{e}$-statistic for monochromatic waves. We also discuss the construction of null streams -- data streams that have null response to GWs, and the prospect of using null streams as a consistency check in the case of detected GW signals. Finally, we present sensitivities to individual supermassive binary black holes in eccentric orbits. We find that a monochromatic search that is designed for circular binaries can efficiently detect eccentric binaries with both high and low eccentricities, while a harmonic summing technique provides greater sensitivities only for binaries with moderate eccentricities.

astro-ph.IM↗

arXiv2013

On the gravitational wave background from compact binary coalescences in the band of ground-based interferometers

This paper reports a comprehensive study on the gravitational wave (GW) background from compact binary coalescences. We consider in our calculations newly available observation-based neutron star and black hole mass distributions and complete analytical waveforms that include post-Newtonian amplitude corrections. Our results show that: (i) post-Newtonian effects cause a small reduction in the GW background signal; (ii) below 100 Hz the background depends primarily on the local coalescence rate $r_0$ and the average chirp mass and is independent of the chirp mass distribution; (iii) the effects of cosmic star formation rates and delay times between the formation and merger of binaries are linear below 100 Hz and can be represented by a single parameter within a factor of ~ 2; (iv) a simple power law model of the energy density parameter $Ω_{GW}(f) ~ f^{2/3}$ up to 50-100 Hz is sufficient to be used as a search template for ground-based interferometers. In terms of the detection prospects of the background signal, we show that: (i) detection (a signal-to-noise ratio of 3) within one year of observation by the Advanced LIGO detectors (H1-L1) requires a coalescence rate of $r_0 = 3 (0.2) Mpc^{-3} Myr^{-1}$ for binary neutron stars (binary black holes); (ii) this limit on $r_0$ could be reduced 3-fold for two co-located detectors, whereas the currently proposed worldwide network of advanced instruments gives only ~ 30% improvement in detectability; (iii) the improved sensitivity of the planned Einstein Telescope allows not only confident detection of the background but also the high frequency components of the spectrum to be measured. Finally we show that sub-threshold binary neutron star merger events produce a strong foreground, which could be an issue for future terrestrial stochastic searches of primordial GWs.

gr-qc↗

arXiv2011

Stochastic Gravitational Wave Background from Coalescing Binary Black Holes

We estimate the stochastic gravitational wave (GW) background signal from the field population of coalescing binary stellar mass black holes (BHs) throughout the Universe. This study is motivated by recent observations of BH-Wolf-Rayet star systems and by new estimates in the metallicity abundances of star forming galaxies that imply BH-BH systems are more common than previously assumed. Using recent analytical results of the inspiral-merger-ringdown waveforms for coalescing binary BH systems, we estimate the resulting stochastic GW background signal. Assuming average quantities for the single source energy emissions, we explore the parameter space of chirp mass and local rate density required for detection by advanced and third generation interferometric GW detectors. For an average chirp mass of 8.7$M_{\odot}$, we find that detection through 3 years of cross-correlation by two advanced detectors will require a rate density, $r_0 \geq 0.5 \rm{Mpc}^{-3} \rm{Myr}^{-1}$. Combining data from multiple pairs of detectors can reduce this limit by up to 40%. Investigating the full parameter space we find that detection could be achieved at rates $r_0 \sim 0.1 \rm{Mpc}^{-3} \rm{Myr}^{-1}$ for populations of coalescing binary BH systems with average chirp masses of $\sim 15M_{\odot}$ which are predicted by recent studies of BH-Wolf-Rayet star systems. While this scenario is at the high end of theoretical estimates, cross-correlation of data by two Einstein Telescopes could detect this signal under the condition $r_0 \geq 10^{-3} \rm{Mpc}^{-3} \rm{Myr}^{-1}$. Such a signal could potentially mask a primordial GW background signal of dimensionless energy density, $Ω_{\rm{GW}}\sim 10^{-10}$, around the (1--500) Hz frequency range.

gr-qc↗