Gravitational Wave Events Research Articles

Pulsar revival in neutron star mergers: multimessenger prospects for the discovery of pre-merger coherent radio emission

ABSTRACT We investigate pre-merger coherent radio emission from neutron star mergers arising due to the magnetospheric interaction between compact objects. We consider two plausible radiation mechanisms, and show that if one neutron star has a surface magnetic field Bs ≥ 1012G, coherent millisecond radio bursts with characteristic temporal morphology and inclination angle dependence are observable to Gpc distances with next-generation radio facilities. We explore multi-messenger and multi-wavelength methods of identification of a neutron star merger origin of radio bursts, such as in fast radio burst surveys, triggered observations of gamma-ray bursts and gravitational wave events, and optical/radio follow-up of fast radio bursts in search of kilonova and radio afterglow emission. We present our findings for current and future observing facilities, and make recommendations for verifying or constraining the model.

Deep learning model based on a bidirectional gated recurrent unit for the detection of gravitational wave signals

The application of deep learning to the detection of gravitational wave (GW) signals has attracted much attention in recent years, and nearly all of the existing deep learning-based GW detection algorithms are based on the convolutional neural network (CNN), which can be used to identify the GW merger moment and produce responding triggers. However, current CNN-based models are not best optimized tools in modeling the causal logic of sequential data in the time dimension to incorporate the information of the inspiral, merger and ringdown phases of the GW waveform. To address this issue we propose a deep learning model designed based on the bidirectional gated recurrent unit (BiGRU), a variant of the recurrent neural network (RNN) that is sensitive to the sequential logic of GW data. We train the neural network and implement the GW detection task by using the data obtained during the first, second and third observation runs (O1/O2/O3) of the LIGO-VIGRO scientific collaboration. We find that our BiGRU model achieves a false alarm probability (FAP) of $4.88\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ (corresponding false alarm rate is about 1 per 18.2 hours) and maintains a detection ratio of about 89.6%. We also construct a model ensemble containing two independently trained BiGRU models and find it does not trigger any false alarm events on our testing dataset. Meanwhile, our BiGRU model not only recovers all the reported GW events of O1-O2 runs, including the unique (as of today) confident (with observed electromagnetic counterparts) GW event originating from a binary neutron star (BNS) merger (GW170817), but also achieves a detection ratio of about 80% of the O3 run. By calculating and comparing the receiver operator characteristic (ROC) curves we demonstrate the superiority of the BiGRU model over traditional CNN-based models, and show that the BiGRU model, being optimized to deal with GW waveform that sequentially contains the inspiral, merger and ringdown phases, is a potentially powerful tool to study the GW data in the future.

Observational constraints on maximum mass limit and physical properties of anisotropic strange star models by gravitational decoupling in Einstein–Gauss–Bonnet gravity

ABSTRACT In this work, we are guided by the gravitational wave events GW 170817 and GW 190814 together with observations of neutron stars PSR J1614-2230, PSR J1903+6620, and LMC X-4 to model compact objects within the framework of Einstein–Gauss–Bonnet (EGB) gravity. In addition, we employ the extended gravitational decoupling (EGD) method to explore the impact of anisotropy by varying the decoupling parameter. We model strange quark stars in which the interior stellar fluid obeys the MIT Bag equation of state which represents a degenerated Fermi gas comprising of up, down, and strange quarks. In order to close the system of field equations describing the seed solution, we employ the Buchdahl ansatz for one of the metric functions. The θ sector is solved under the bifurcation: $\epsilon =\theta ^0_0$ and $P_r=\theta ^1_1$ leading to two new families of solutions. In order to test the physical viability of the models, we vary the EGB parameter (α) or the decoupling constant (β) to achieve the observed masses and radii of compact objects. Our models are able to account for low-mass stars for a range of β values while α is fixed. The present models mimic the secondary component of the GW 190814 with a mass range of 2.5–2.67 M⊙ and radii typically of the order of 11.76$^{+0.14}_{-0.19}$ km for large values of the EGB parameter and the decoupling constant. The energy exchange between fluids inside the stellar object is sensitive to model parameters which lead to stable configurations.

A Fisher matrix for gravitational-wave population inference

ABSTRACT We derive a Fisher matrix for the parameters characterizing a population of gravitational-wave events. This provides a guide to the precision with which population parameters can be estimated with multiple observations, which becomes increasingly accurate as the number of events and the signal-to-noise ratio of the sampled events increase. The formalism takes into account individual event measurement uncertainties and selection effects, and can be applied to arbitrary population models. We illustrate the framework with two examples: an analytical calculation of the Fisher matrix for the mean and variance of a Gaussian model describing a population affected by selection effects, and an estimation of the precision with which the slope of a power-law distribution of supermassive black hole masses can be measured using extreme-mass-ratio inspiral observations. We compare the Fisher predictions to results from Monte Carlo analyses, finding very good agreement.

Strategy and Results of MASTER Network Follow-Up Observations of LIGO and Virgo Gravitational Wave Events within the Observational Sets O1, O2, and O3

Strategy and Results of MASTER Network Follow-Up Observations of LIGO and Virgo Gravitational Wave Events within the Observational Sets O1, O2, and O3

Repeated Mergers of Black Hole Binaries: Implications for GW190521

The gravitational wave event GW190521 involves the merger of two black holes of ∼85 M ⊙ and ∼66 M ⊙ forming an intermediate-mass black hole (IMBH) of mass ∼142 M ⊙. Both progenitors are challenging to explain within standard stellar evolution as they are within the upper black hole mass gap. We propose a dynamical formation pathway for this IMBH based on multiple mergers in the core of a globular cluster. We identify such scenarios from analysis of a set of 58 N-body simulations using NBODY6-gpu. In one of our simulations, we observe a stellar black hole undergoing a chain of seven binary mergers within 6 Gyr, attaining a final mass of 97.8 M ⊙. We discuss the dynamical interactions that lead to the final IMBH product, as well as the evolution of the black hole population in that simulation. We explore statistically the effects of gravitational recoil on the viability of such hierarchical mergers. From the analysis of all 58 simulations we observe additional smaller chains, tentatively inferring that an IMBH formation through hierarchical mergers is expected in the lifetime of a median-mass globular cluster with probability 0.01 ≲ p ≲ 0.1 without gravitational merger recoil. Using this order-of-magnitude estimate we show that our results are broadly consistent with the rate implied by GW190521, assuming that gravitational recoil ejection of progenitors has a low probability. We discuss implications for future gravitational wave detections, emphasizing the importance of studying such formation pathways for black holes within the upper mass gap as a means to constrain such modeling.

Contracting and Expanding Binary Black Holes in 3D Low-mass AGN Disks: The Importance of Separation

LIGO/Virgo has detected several binary black hole (BBH) merger events that may have originated in the accretion disks of active galactic nuclei (AGNs). These events require individual black hole masses that fall within the pair instability supernova mass gap, and therefore these black holes may have been grown from hierarchical mergers. AGN disks are a prime environment for hierarchical mergers and thus a potential location for the progenitors of BBH gravitational wave events. Understanding how a BBH embedded in an AGN disk interacts with the surrounding environment is thus crucial for determining whether this interaction can lead to its merger. However, there are few high-fidelity simulations of this process, and almost all are two-dimensional. We present the results from 3D, high-resolution, local shearing-box simulations of an embedded BBH interacting with an AGN disk. In these first simulations of their kind, we focus on determining the mass accretion rate and the orbital evolution rate at different BBH separations. We find that circular, equal-mass BBHs with separations greater than 10% of their Hill radius contract while accreting at a super-Eddington rate. At smaller separations, however, our 3D simulations find that BBHs expand their orbits. This result suggests that it may be difficult for an AGN disk to push a BBH to merger, but we discuss several mechanisms, including MHD turbulence and radiative and mechanical feedback, which could alleviate this difficulty.

On the detection of the electromagnetic counterparts from lensed gravitational wave events by binary neutron star mergers

ABSTRACT Future ground-based gravitational wave (GW) detectors, i.e. Einstein telescope (ET) and Cosmic Explorer (CE), are expected to detect a significant number of lensed binary neutron star (BNS) mergers, which may provide a unique tool to probe cosmology. In this paper, we investigate the detectability of the optical/infrared electromagnetic (EM) counterparts (kilonovae/afterglows) from these lensed BNS mergers by future GW detectors and EM telescopes using simple kilonova, afterglow, and lens models. ET and CE are expected to detect ${\sim}5.32^{+26.1}_{-5.10}$ and $67.3^{+332}_{-64.7}$ lensed BNS mergers per year. We find that the EM counterparts associated with all these mergers will be detectable by an all sky-survey in the H band with the limiting magnitude mlim ≳ 27, while the detectable fraction is ${\lesssim}0.4{{\ \rm per\ cent}}$ in the g/z band if with mlim ≲ 24. Generally, it is more efficient to search the lensed EM counterparts by adopting the infrared bands than the optical/UV bands with the same mlim. Future telescopes like Vera C. Rubin Observatory, China Space Station Telescope, and Euclid can hardly detect the EM counterparts of even one lensed BNS merger. Roman Space Telescope (RST) and James Webb Space Telescope (JWST) have the capability to detect about a few or more such events per year. Moreover, the time delays and separations between the lensed image pairs are typically in the ranges from minutes to months and from 0.1 to 1 arcsec, suggesting that both the GW and EM images of most lensed BNS mergers can be well resolved by not only CE/ET in the time domain but also RST/JWST spatially.

Dark-siren cosmology with Decihertz gravitational-wave detectors

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

GW190425, GW190521 and GW190814: Three candidate mergers of primordial black holes from the QCD epoch

The two recent gravitational-wave events GW190425 and GW190814 from the third observing run of LIGO/Virgo have both a companion which is unexpected if originated from a neutron star or a stellar black hole, with masses [ 1 . 6 − 2 . 5 ] M ⊙ and [ 2 . 5 − 2 . 7 ] M ⊙ and merging rates 46 0 − 360 + 1050 and 7 − 6 + 16 events/yr/Gpc 3 respectively, at 90% confidence level. Moreover, the recent event GW190521 has black hole components with masses 67 and 91 M ⊙ , and therefore lies in the so-called pair-instability mass gap, where there should not be direct formation of stellar black holes. The possibility that all of these compact objects are Primordial Black Holes (PBHs) is investigated. The known thermal history of the Universe predicts that PBH formation is boosted at the time of the QCD transition, inducing a peak in their distribution at this particular mass scale, and a bump around 30 − 50 M ⊙ . We find that the merging rates inferred from GW190425, GW190521 and GW190814 are consistent with PBH binaries formed by capture in dense halos in the matter era or in the early universe. At the same time, the rate of black hole mergers around 30 M ⊙ and of sub-solar PBH mergers do not exceed the LIGO/Virgo limits. Such PBHs could explain a significant fraction, or even the totality of the Dark Matter, but they must be sufficiently strongly clustered in order to be consistent with current astrophysical limits.

A boosted gravitational wave background for primordial black holes with broad mass distributions and thermal features

Primordial black holes (PBHs) with a wide mass distribution imprinted by the thermal history of the Universe, which naturally produces a high peak at the solar mass scale, could explain the gravitational-wave events seen by LIGO/Virgo and up to the totality of the dark matter. We show that compared to monochromatic or log-normal mass functions, the gravitational wave backgrounds (GWBs) from early PBH binaries and from late binaries in clusters are strongly enhanced at low frequency and could even explain the NANOGrav observations. This enhancement comes from binaries with very low mass ratios, involving solar-mass and intermediate-mass PBHs at low frequency, solar-mass and subsolar-mass at high frequency. LISA could distinguish the various models, while in the frequency band of ground-based detectors, we find that the GWB from early binaries is just below the current LIGO/Virgo limits and above the astrophysical background, if they also explain black hole mergers. The GWB from binaries in clusters is less boosted but has a different spectral index than for neutron stars, astrophysical black holes or early PBH binaries. It is detectable with Einstein Telescope or even with the LIGO/Virgo design sensitivity.

Modeling Gamma-Ray Burst Afterglow Observations with an Off-axis Jet Emission

Abstract Gamma-ray bursts (GRBs) are fascinating extragalactic objects. They represent a fantastic opportunity to investigate unique properties not exhibited in other sources. Multiwavelength afterglow observations from some short- and long-duration GRBs reveal an atypical long-lasting emission that evolves differently from the canonical afterglow light curves favoring the off-axis emission. We present an analytical synchrotron afterglow scenario and the hydrodynamical evolution of an off-axis top-hat jet decelerated in a stratified surrounding environment. The analytical synchrotron afterglow model is shown during the coasting, deceleration (off- and on-axis emission), and post–jet break decay phases, and the hydrodynamical evolution is computed by numerical simulations showing the time evolution of the Doppler factor, the half-opening angle, the bulk Lorentz factor, and the deceleration radius. We show that numerical simulations are in good agreement with those derived with our analytical approach. We apply the current synchrotron model and successfully describe the delayed nonthermal emission observed in a sample of long and short GRBs with evidence of off-axis emission. Furthermore, we provide constraints on the possible afterglow emission by requiring the multiwavelength upper limits derived for the closest Swift-detected GRBs and promising gravitational-wave events.

Using Gravitational Waves to Distinguish between Neutron Stars and Black Holes in Compact Binary Mergers

In 2017 August, the first detection of a binary neutron star merger, GW170817, made it possible to study neutron stars in compact binary systems using gravitational waves. Despite being the loudest gravitational-wave event detected to date (in terms of signal-to-noise ratio), it was not possible to unequivocally determine that GW170817 was caused by the merger of two neutron stars instead of two black holes from the gravitational-wave data alone. That distinction was primarily due to the accompanying electromagnetic counterpart. This raises the question: under what circumstances can gravitational-wave data alone, in the absence of an electromagnetic signal, be used to distinguish between different types of mergers? Here, we study whether a neutron star–black hole binary merger can be distinguished from a binary black hole merger using gravitational-wave data alone. We build on earlier results using chiral effective field theory to explore whether the data from LIGO and Virgo, LIGO A+, LIGO Voyager, the Einstein Telescope, or Cosmic Explorer could lead to such a distinction. The results suggest that the present LIGO–Virgo detector network will most likely be unable to distinguish between these systems even with the planned near-term upgrades. However, given an event with favorable parameters, third-generation instruments such as Cosmic Explorer will be capable of making this distinction. This result further strengthens the science case for third-generation detectors.

Subtracting glitches from gravitational-wave detector data during the third LIGO-Virgo observing run

Data from ground-based gravitational-wave detectors contains numerous short-duration instrumental artifacts, called ‘glitches’. The high rate of these artifacts in turn results in a significant fraction of gravitational-wave signals from compact binary coalescences overlapping glitches. In LIGO-Virgo’s third observing run, ≈20% of gravitational-wave source candidates required some form of mitigation due to glitches. This was the first observing run where glitch subtraction was included as a part of LIGO-Virgo-KAGRA data analysis methods for a large fraction of detected gravitational-wave events. This work describes the methods to identify glitches, the decision process for deciding if mitigation was necessary, and the two algorithms, BayesWave and gwsubtract, that were used to model and subtract glitches. Through case studies of two events, GW190424_180648 and GW200129_065458, we evaluate the effectiveness of the glitch subtraction, compare the statistical uncertainties in the relevant glitch models, and identify potential limitations in these glitch subtraction methods. We finally outline the lessons learned from this first-of-its-kind effort for future observing runs.

The two-Gaussian jet structure and its formation process: Constraints from the afterglows of GRB 170817A

The short gamma-ray burst (SGRB) GRB 170817A was the first observed GRB associated with a gravitational-wave event, providing a valuable opportunity to further study the producing mechanism of SGRBs. Its multi-wavelength afterglow emissions probably reveal the angular structure of the jet to us. And, if the jet structure is described phenomenologically by a Gaussian function, large sample statistical studies on SGRBs show that we probably need to introduce two-Gaussian functions to unify the emissions from GRB 170817A with other normal SGRBs. In other words, GRB 170817A's prompt emission is likely from an additional component of a jet with a larger angle. Therefore, in order to test the compatibility between this two-Gaussian jet structure and the afterglow data of GRB 170817A, we calculate the multi-wavelength afterglow light curves from such an off-axis jet structure, and find that the introduction of the outer Gaussian component can well explain the early uptrend of the GRB 170817A afterglow data. On this basis, we also calculate the propagation and breakout processes of the jet in the merger ejecta, which shows that these processes can naturally lead to the two-Gaussian jet structure obtained by the observational constraint, thus showing the overall self-consistency of the model. Furthermore, the observational constraint on the jet propagation process also provides a new clue for understanding the properties of merger ejecta.

Formation, Possible Detection and Consequences of Highly Magnetized Compact Stars

Over the past several years, there has been enormous interest in massive neutron stars and white dwarfs due to either their direct or indirect evidence. The recent detection of gravitational wave event GW190814 has confirmed the existence of compact stars with masses as high as ∼2.5–2.67 M⊙ within the so-called mass gap, indicating the existence of highly massive neutron stars. One of the primary goals to invoke massive compact objects was to explain the recent detections of over a dozen Type Ia supernovae, whose peculiarity lies with their unusual light curve, in particular the high luminosity and low ejecta velocity. In a series of recent papers, our group has proposed that highly magnetised white dwarfs with super-Chandrasekhar masses can be promising candidates for the progenitors of these peculiar supernovae. The mass-radius relations of these magnetised stars are significantly different from those of their non-magnetised counterparts, which leads to a revised super-Chandrasekhar mass-limit. These compact stars have wider ranging implications, including those for soft gamma-ray repeaters, anomalous X-ray pulsars, white dwarf pulsars and gravitational radiation. Here we review the development of the subject over the last decade or so, describing the overall state of the art of the subject as it stands now. We mainly touch upon the possible formation channels of these intriguing stars as well as the effectiveness of direct detection methods. These magnetised stars can have many interesting consequences, including reconsideration of them as possible standard candles.

Deep learning and Bayesian inference of gravitational-wave populations: Hierarchical black-hole mergers

The catalog of gravitational-wave events is growing, and so are our hopes of constraining the underlying astrophysics of stellar-mass black-hole mergers by inferring the distributions of, e.g., masses and spins. While conventional analyses parametrize this population with simple phenomenological models, we propose an emulation-based approach that can compare astrophysical simulations against gravitational-wave data. We combine state-of-the-art deep-learning techniques with hierarchical Bayesian inference and exploit our approach to constrain the properties of repeated black-hole mergers from the gravitational-wave events in the most recent LIGO/Virgo catalog. Deep neural networks allow us to (i) construct a flexible single-channel population model that accurately emulates simple parametrized numerical simulations of hierarchical mergers, (ii) estimate selection effects, and (iii) recover the branching ratios of repeated-merger generations. Among our results, we find the following: The distribution of host-environment escape speeds favors values less than $100~\mathrm{km\,s^{-1}}$ but is relatively flat, with around $37\%$ of first-generation mergers retained in their host environments; first-generation black holes are born with a maximum mass that is compatible with current estimates from pair-instability supernovae; there is multimodal substructure in both the mass and spin distributions, which, in our model, can be explained by repeated mergers; and binaries with a higher-generation component make up at least $14\%$ of the underlying population. Though these results are inferred through emulation of a simplified model, the deep-learning pipeline we present is readily applicable to realistic astrophysical simulations

Searches for Ultra-High-Energy Photons at the Pierre Auger Observatory

The Pierre Auger Observatory, which is the largest air-shower experiment in the world, offers unprecedented exposure to neutral particles at the highest energies. Since the start of data collection more than 18 years ago, various searches for ultra-high-energy (UHE, E≳1017eV) photons have been performed, either for a diffuse flux of UHE photons, for point sources of UHE photons or for UHE photons associated with transient events such as gravitational wave events. In the present paper, we summarize these searches and review the current results obtained using the wealth of data collected by the Pierre Auger Observatory.

Tidal Deformability of Neutron Stars in Unimodular Gravity

Unimodular gravity is a modified theory with respect to general relativity by an extra condition that the determinant of the metric is fixed. Especially, if the energy-momentum tensor is not imposed to be conserved separately, a new geometric structure will appear with potential observational signatures. In this paper, we study the tidal deformability of a compact star in unimodular gravity under the assumption of a non-conserved energy-momentum tensor. Both the electric-type and magnetic-type quadrupole tidal Love numbers are calculated for neutron stars using the polytrope model. It is found that the electric-type tidal Love numbers are monotonically increasing, but the magnetic-type ones are decreasing, with the increase in the non-conservation parameter. Compared with the observational data from the detected gravitational-wave events, a small negative non-conservation parameter is favored.

From 3D hydrodynamic simulations of common-envelope interaction to gravitational-wave mergers

Modelling the evolution of progenitors of gravitational-wave merger events in binary stars faces two major uncertainties: the common-envelope phase and supernova kicks. These two processes are critical for the final orbital configuration of double compact-object systems with neutron stars and black holes. Predictive one-dimensional models of common-envelope interaction are lacking and multidimensional simulations are challenged by the vast range of relevant spatial and temporal scales. Here, we present three-dimensional, magnetohydrodynamic simulations of the common-envelope interaction of an initially 10 M⊙ red supergiant primary star with a black-hole and a neutron-star companion. Employing the moving-mesh code AREPO and replacing the core of the primary star and the companion with point masses, we show that the high-mass regime is accessible to full ab initio simulations. About half of the common envelope is dynamically ejected at the end of our simulations and the ejecta mass fraction keeps growing. Almost complete envelope ejection seems possible if all ionised gas left over at the end of our simulation eventually recombines and the released energy continues to help unbind the envelope. We find that the dynamical plunge-in of both companions terminates at orbital separations that are too wide for gravitational waves to merge the systems in a Hubble time. However, the orbital separations at the end of our simulations are still decreasing such that the true final value at the end of the common-envelope phase remains uncertain. We discuss the further evolution of the system based on analytical estimates. A subsequent mass-transfer episode from the remaining 3 M⊙ core of the supergiant to the compact companion does not shrink the orbit sufficiently either. A neutron-star–neutron-star and neutron-star–black-hole merger is still expected for a fraction of the systems if the supernova kick aligns favourably with the orbital motion. For double neutron star (neutron-star–black-hole) systems we estimate mergers in about 9% (1%) of cases while about 77% (94%) of binaries are disrupted; that is, supernova kicks actually enable gravitational-wave mergers in the binary systems studied here. Assuming orbits smaller by one-third after the common-envelope phase enhances the merger rates by about a factor of two. However, the large post-common-envelope orbital separations found in our simulations mean that a reduction in predicted gravitational-wave merger events appears possible.