Gravitational Wave Event GW170817 Research Articles

Radioactive Gamma-Ray Lines from Long-lived Neutron Star Merger Remnants

The observation of a kilonova AT2017gfo associated with the gravitational wave event GW170817 provides the first strong evidence that neutron star mergers are dominant contributors to the production of heavy r-process elements. Radioactive gamma-ray lines emitted from neutron star merger remnants provide a unique probe for investigating the nuclide composition and tracking its evolution. In this work, we studied the gamma-ray line features arising from the radioactive decay of heavy nuclei in the merger remnants based on the r-process nuclear reaction network and the astrophysical inputs derived from numerical relativity simulations. The decay chain of 50126 Sn (T 1/2 = 230 kyr) → 51126 Sb (T 1/2 = 12.35 days) → 52126 Te (stable) produces several bright gamma-ray lines with energies of 415, 667, and 695 keV, making it the most promising decay chain during the remnant phase. The photon fluxes of these bright gamma-ray lines reach ∼10−5 γ cm−2 s−1 for Galactic merger remnants with ages less than 100 kyr, which can be detected by the high energy resolution MeV gamma-ray detectors like the MASS mission.

On non-detection of gamma-ray bursts in three compact binary merger events detected by LIGO

ABSTRACT The joint detection of the gravitational wave (GW) event GW170817 and the short-duration gamma-ray burst (SGRB) event GRB 170817A, marked the beginning of GW multimessenger astronomy and confirmed that binary neutron star mergers are progenitors of at least some SGRBs. An estimated joint detection rate of 0.3–1.7 per year between the LIGO-Hanford, LIGO-Livingston, and Virgo GW network at design sensitivity, and the Fermi Gamma-ray Burst Monitor was predicted. However, to date, the GW170817/GRB 170817A joint detection has been the only event of its kind so far. Taking into account that SGRBs are narrowly beamed and are emitted perpendicular to the orbital plane of the binary system, we propose that previous mergers involving neutron stars, were orientated such that observation of the emitted SGRB along this narrow jet was not possible. To support this hypothesis we have estimated the inclination of the binary systems for previously detected Binary Neutron Star (BNS) and Black Hole Neutron Star (BHNS) mergers through GW analysis. This analysis was performed using bilby, a python based Bayesian inference library, to estimate the inclination of the BNS events GW170817 and GW190425, and the BHNS events GW190917_114630 and GW200115_042309. The results obtained in this study indicate that these binaries may have had inclinations greater than 33° with respect to the line of sight from Earth, an upper limit on the viewing angle set from observations of GRB 170817A. This then suggests that the observation of the emitted SGRB from these past mergers might not have been possible.

The S-PLUS Transient Extension Program: imaging pipeline, transient identification, and survey optimization for multimessenger astronomy

ABSTRACT We present the S-PLUS Transient Extension Program (STEP): a supernova and fast transient survey conducted in the southern hemisphere using data from the Southern Photometric Local Universe Survey (S-PLUS) Main Survey and the T80-South telescope. Transient astrophysical phenomena have a range of interest that goes through different fields of astrophysics and cosmology. With the detection of an electromagnetic counterpart to the gravitational wave (GW) event GW170817 from a binary neutron stars merger, new techniques and resources to study fast astrophysical transients in the multimessenger context have increased. In this paper, we present the STEP overview, the SN follow-up data obtained, data reduction, analysis of new transients and deep learning algorithms to optimize transient candidate selection. Additionally, we present prospects and optimized strategy for the search of gravitational wave counterparts in the current LIGO/Virgo/Kagra observational run (O4) in the context of T80-South telescope.

Cocoon cooling emission in neutron star mergers

ABSTRACT In the gravitational wave event GW170817, there was a ∼10 h gap before electromagnetic (EM) observations, without detection of the cocoon. The cocoon is heated by a short gamma-ray burst (sGRB) jet propagating through the ejecta of a neutron star (NS) merger, and a part of the cocoon escapes the ejecta with an opening angle of 20°–30°. Here, we model the cocoon and calculate its EM emission. Our 2D hydrodynamic simulations suggest that the density and energy distributions, after entering homologous expansion, are well-fitted with power-law functions, in each of the relativistic and non-relativistic parts of the escaped cocoon. Modelling these features, we calculate the cooling emission analytically. We find that the cocoon outshines the r-process kilonova/macronova at early times (10–103 s), peaking at UV bands. The relativistic velocity of the cocoon’s photosphere is measurable with instruments such as Swift, ULTRASAT, and LSST. We also imply that energetic cocoons, including failed jets, might be detected as X-ray flashes. Our model clarifies the physics and parameter dependence, covering a wide variety of central engines and ejecta of NS mergers and sGRBs in the multimessenger era.

Eighteen Years of Kilonova Discoveries with Swift

Swift has now completed 18 years of mission, during which it discovered thousands of gamma-ray bursts as well as new classes of high-energy transient phenomena. Its first breakthrough result was the localization of short duration GRBs, which enabled for redshift measurements and kilonova searches. Swift, in synergy with the Hubble Space Telescope and a wide array of ground-based telescopes, provided the first tantalizing evidence of a kilonova in the aftermath of a short GRB. In 2017, Swift observations of the gravitational wave event GW170817 captured the early UV photons from the kilonova AT2017gfo, opening a new window into the physics of kilonovae. Since then, Swift has continued to expand the sample of known kilonovae, leading to the surprising discovery of a kilonova in a long duration GRB. This article will discuss recent advances in the study of kilonovae driven by the fundamental contribution of Swift.

Target-of-Opportunity Observation Detectability of Kilonovae with WFST

Kilonovae are approximately thermal transients, produced by the mergers of binary neutron stars (BNSs) and neutron star (NS)–black hole binaries. As the optical counterpart of the gravitational-wave event GW170817, AT2017gfo is the first kilonova detected with smoking-gun evidence. Its observation offers vital information for constraining the Hubble constant, the sources of cosmic r-process enrichment, and the equation of state of NSs. The 2.5 m Wide-Field Survey Telescope (WFST) operates in six bands (u, g, r, i, z, w), spanning from 320 to 925 nm. It will be completed in the first half of 2023, and with a field-of-view diameter of 3°, aims to detect kilonovae in the near future. In this article, considering the influence of the host galaxies and sky brightness, we generate simulated images to investigate WFST’s ability to detect AT2017gfo-like kilonovae. Due to their spectra, host galaxies can significantly impact kilonova detection at longer wavelengths. When kilonovae are at peak luminosity, we find that WFST performs better in the g and r bands and can detect 90% (50%) of kilonovae at a luminosity distance of 248 Mpc (338 Mpc) with 30 s exposures. Furthermore, to reflect the actual efficiency under target-of-opportunity observations, we calculate the total time of follow up under various localization areas and distances. We find that if the localization areas of most BNS events detected during the fourth observing (O4) run of LIGO and Virgo are hundreds of deg2, WFST is expected to find ∼30% of kilonovae in the first two nights following the detection of a GW event produced by a BNS during the O4 period.

Revisiting the Parameter Space of Binary Neutron Star Merger Event GW170817

Since the gravitational wave event GW170817 and gamma-ray burst GW170817A, there have been numerous studies constraining the burst properties through analysis of the afterglow light curves. Most agree that the burst was viewed off-axis with a ratio of the observer angle to the jet angle (θ obs/θ j ) between 4 and 6. We use a parameterized model and broadband synchrotron data up to ∼800 days post-merger to constrain parameters of the burst. To reproduce the hydrodynamics of a gamma-ray burst outflow, we use a two-parameter “boosted fireball” model. The structure of a boosted fireball is determined by the specific internal energy, η 0, and the bulk Lorentz factor, γ B ( ∼ 1/θ j ), with shapes varying smoothly from a quasi-spherical outflow for low values of γ B to a highly collimated jet for high values. We run simulations with γ B in the range 1–20 and η 0 in the range 2–15. To calculate light curves, we use a synchrotron radiation model characterized by F peak, ν m , and ν c , and calculate millions of spectra at different times and θ obs values using the boxfit radiation code. We can tabulate the spectral parameter values from our spectra and rapidly generate arbitrary light curves for comparison to data in MCMC analysis. We find that our model prefers a gamma-ray burst with jet energy E j ∼ 1050 erg and with an observer angle of radians and ratio to the jet opening angle of (θ obs/θ j ) = .

Spherical symmetry in the kilonova AT2017gfo/GW170817.

The mergers of neutron stars expel a heavy-element enriched fireball that can be observed as a kilonova1-4. The kilonova's geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. Current hydrodynamical merger models typically show aspherical ejecta5-7. Previously, Sr+ was identified in the spectrum8 of the only well-studied kilonova9-11 AT2017gfo12, associated with the gravitational wave event GW170817. Here we combine the strong Sr+ P Cygni absorption-emission spectral feature and the blackbody nature of kilonova spectrum to determine that the kilonova is highly spherical at early epochs. Line shape analysis combined with the known inclination angle of the source13 also show the same sphericity independently. We conclude that energy injection by radioactive decay is insufficient to make the ejecta spherical. A magnetar wind or jet from the black-hole disk could inject enough energy to induce a more spherical distribution in the overall ejecta; however, an additional process seems necessary to make the element distribution uniform.

Relativistic Hybrid Stars with Sequential First-order Phase Transitions in Light of Multimessenger Constraints

In this work, we consider the properties of compact stars in which quark matter has low- and high-density phases that are separated by a first-order phase transition. Thus, unlike the commonly considered case of a single phase transition from hadronic to quark matter, our models of hybrid stars contain sequential phase transitions from hadronic matter to low- and then to high-density quark matter phases. We extend our previous study of the parameter space of hybrid stars with a single phase transition to those with sequential phase transitions, taking into account the constraints on the mass and radius of neutron stars from the NICER experiment, the experimental inferences of the neutron skin thickness of the lead nucleus by the PREX-II experiment, and constraints on the tidal deformability from the gravitational-wave event GW170817. We determine the range of the masses for which both twin and triplet configurations, i.e., identical-mass stars with two and three different values of radii, arise.

Higher-Order mimetic gravity after GW170817

On 17 August 2017, the thriving discovery of gravitational wave event GW170817 and its optical counterpart GRB170817A, owing to coalescing of two neutron stars, divulged a very small amount of difference of around [Formula: see text] between traveling speed of light and the velocity of gravitational waves [Formula: see text]. This small deviation can be used as a strong constraint on modified gravity models. We concentrate on the Higher-Order expansion of Mimetic Gravity (HOMimG) model to specify the parametric space of three parameters of our model ([Formula: see text], [Formula: see text], and [Formula: see text]) utilizing the observational constraint from GW170817–GRB170817A on [Formula: see text], besides two theoretical constraints on [Formula: see text] and [Formula: see text] due to assurance of the stability of the model and subluminal promulgation of the scalar and tensor perturbations. Thereafter, we increase the accuracy of the parametric space with the aid of imposing further limitation of [Formula: see text] parameter (related to the age of the universe). In pursuance of determining the other parameter of the model [Formula: see text], the potential of the model is specified, and another observational bound related to the Equation of State parameter of dark energy is taken into account. In consequence, we attain a viable HOMimG model confined to numbers of observational and theoretical constraints. At the end, regarding the concluded numerical ranges for the model parameters, and cogitating two different potential (quadratic and quartic potentials) to specify [Formula: see text] parameter, we illustrate that the values of the model parameters are independent of the form of potential.

Cocoon breakout and escape from the ejecta of neutron star mergers

ABSTRACT The cocoon is an inevitable product of a jet propagating through ambient matter, and takes a fair fraction of the jet energy. In short gamma-ray bursts (sGRBs), the ambient matter is the ejecta from the merger of neutron stars, expanding with a high velocity ∼0.2c, in contrast to the static stellar envelope in collapsars. Using 2D relativistic hydrodynamic simulations with the ejecta density profile as ρ ∝ r−2, we find that the expansion makes a big difference; only 0.5–5 per cent of the cocoon mass escapes from (faster than) the ejecta, with an opening angle 20°–30°, while it is $\sim 100{{\ \rm per\ cent}}$ and spherical in collapsars. We also analytically obtain the shares of mass and energies for the escaped and trapped cocoons. Because the mass of the escaped cocoon is small and the trapped cocoon is concealed by the ejecta and the escaped cocoon, we suggest that it is unlikely that cooling emission from the sGRB-jet heated cocoon was observed as a counterpart to the gravitational wave event GW170817.

Effective-one-body waveforms for precessing coalescing compact binaries with post-Newtonian twist

Spin precession is a generic feature of compact binary coalescences, which leaves clear imprints in the gravitational waveforms. Building on previous work, we present an efficient time domain inspiral-merger-ringdown effective-one-body model (EOB) for precessing binary black holes, which incorporates subdominant modes beyond $\ell=2$, and the first EOB frequency domain approximant for precessing binary neutron stars. We validate our model against 99 ``short'' numerical relativity precessing waveforms, where we find median mismatches of $5\times 10^{-3}$, $7 \times 10^{-3}$ at inclinations of $0$, $\pi/3$, and 21 ``long'' waveforms with median mismatches of $4 \times 10^{-3}$ and $5 \times 10^{-3}$ at the same inclinations. Further comparisons against the state-of-the-art $\texttt{NRSur7dq4}$ waveform model yield median mismatches of $4\times 10^{-3}, 1.8 \times 10^{-2}$ at inclinations of $0, \pi/3$ for 5000 precessing configurations with the precession parameter $\chi_p$ up to 0.8 and mass ratios up to 4. To demonstrate the computational efficiency of our model we apply it to parameter estimation and re-analyze the gravitational-wave events GW150914, GW190412, and GW170817.

Stripping Model for Short Gamma-Ray Bursts in Neutron Star Mergers

We overview the current status of the stripping model for short gamma-ray bursts. After the historical joint detection of the gravitational wave event GW170817 and the accompanying gamma-ray burst GRB170817A, the relation between short gamma-ray bursts and neutron star mergers has been reliably confirmed. Many properties of GRB170817A, which turned out to be peculiar in comparison with other short gamma-ray bursts, are naturally explained in the stripping model, suggested by one of us in 1984. We point out the role of late Dmitriy Nadyozhin (1937–2020) in predicting the GRB and kilonova properties in 1990. We also review the problems to be solved in the context of this model.

Effects of relativistic parameter sets on tidal deformabilities and f-mode oscillations of neutron stars

The implications of relativistic parameter sets established at saturation density on the tidal deformabilities and f-mode oscillations of neutron stars (NSs) are examined using constraints from the gravitational wave (GW) event GW170817 and NICER. According to our findings, the isovector saturation parameters have a greater impact on the radii and tidal deformabilities of NSs than the isoscalar saturation parameters. Our analysis also examines the impact of saturation properties on f-mode frequencies and finds that f-mode frequencies with 1.4 M ⊙ (solar mass) are roughly between 1.95 and 2.15 kHz. These findings could be confirmed by future advanced GW detectors. A good linear parameter-independent correlation between f-mode frequencies inferred from saturation parameters in the entire region is also observed, and we attempt to fit an updated version of this universal relationship. Furthermore, we used chiral effective theory (χ EFT) together with the multi-messenger astronomy constraints to further reinforce the rationality of the conclusions we have reached.

Model systematics in time domain tests of binary black hole evolution

We perform several consistency tests between different phases of binary black hole dynamics; the inspiral, the merger, and the ringdown on the gravitational wave events GW150914 and GW170814. These tests are performed explicitly in the time domain, without any spectral leakage between the different phases. We compute posterior distributions on the mass and spin of the initial black holes and the final black hole. We also compute the initial areas of the two individual black holes and the final area from the parameters describing the remnant black hole. This facilitates a test of Hawking's black hole area theorem. We use different waveform models to quantify systematic waveform uncertainties for the area increase law with the two events. We find that these errors may lead to overstating the confidence with which the area theorem is confirmed. For example, we find $>99\%$ agreement with the area theorem for GW150914 if a damped sinusoid consisting of a single-mode is used at merger to estimate the final area. This is because this model overestimates the final mass. Including an overtone of the dominant mode decreases the confidence to $\sim94\%$; using a full merger-ringdown model further decreases the confidence to $\sim 85-90\%$. We find that comparing the measured change in the area to the expected change in area yields a more robust test, as it also captures over estimates in the change of area. We find good agreement with GR when applying this test to GW150914 and GW170814.

Fallback Accretion Halted by R-process Heating in Neutron Star Mergers and Gamma-Ray Bursts

Abstract The gravitational wave event GW170817 with a macronova/kilonova shows that a merger of two neutron stars ejects matter with radioactivity including r-process nucleosynthesis. A part of the ejecta inevitably falls back to the central object, possibly powering long-lasting activities of a short gamma-ray burst (sGRB), such as extended and plateau emissions. We investigate fallback accretion with r-process heating by performing one-dimensional hydrodynamic simulations and developing a semi-analytical model. We show that the usual fallback rate dM/dt ∝ t −5/3 is halted by the heating because pressure gradients accelerate ejecta beyond an escape velocity. The suppression is steeper than Chevalier’s power-law model through Bondi accretion within a turn-around radius. The characteristic halting timescale is ∼104–108 s for the GW170817-like r-process heating, which is longer than the typical timescale of the long-lasting emission of sGRBs. The halting timescale is sensitive to the uncertainty of the r-process. Future observations of fallback halting could constrain the r-process heating on the scale of a year.

Neutron stars in f(mathtt {R,L_m}) gravity with realistic equations of state: joint-constrains with GW170817, massive pulsars, and the PSR J0030+0451 mass-radius from NICER data

In this work, we investigate neutron stars (NS) in f(mathtt {R,L_m}) theory of gravity for the case f(mathtt {R,L_m})= mathtt {R}+ mathtt {L_m}+ sigma mathtt {R}mathtt {L_m}, where mathtt {R} is the Ricci scalar and mathtt {L_m} the Lagrangian matter density. In the term sigma mathtt {R}mathtt {L_m}, sigma represents the coupling between the gravitational and particles fields. For the first time the hydrostatic equilibrium equations in the theory are solved considering realistic equations of state and NS masses and radii obtained are subject to joint constrains from massive pulsars, the gravitational wave event GW170817 and from the PSR J0030+0451 mass-radius from NASA’s Neutron Star Interior Composition Explorer (NICER) data. We show that in this theory of gravity, the mass-radius results can accommodate massive pulsars, while the general theory of relativity can hardly do it. The theory also can explain the observed NS within the radius region constrained by the GW170817 and PSR J0030+0451 observations for masses around 1.4~M_{odot }.

First Constraints on Nuclear Coupling of Axionlike Particles from the Binary Neutron Star Gravitational Wave Event GW170817.

Light axion fields, if they exist, can be sourced by neutron stars due to their coupling to nuclear matter, and play a role in binary neutron star mergers. We report on a search for such axions by analyzing the gravitational waves from the binary neutron star inspiral GW170817. We find no evidence of axions in the sampled parameter space. The null result allows us to impose constraints on axions with masses below 10^{-11} eV by excluding the ones with decay constants ranging from 1.6×10^{16} to 10^{18} GeV at a 3σ confidence level. Our analysis provides the first constraints on axions from neutron star inspirals, and rules out a large region in parameter space that has not been probed by the existing experiments.

Is GW170817 a multimessenger neutron star-primordial black hole merger?

We investigate the possibility of the gravitational-wave event GW170817 being a light, solar-mass black hole (BH) — neutron star (NS) merger. We explore two exotic scenarios involving primordial black holes (PBH) that could produce such an event, taking into account available observational information on NGC 4993. First, we entertain the possibility of dynamical NS-PBH binary formation where a solar-mass PBH and a NS form a binary through gravitational interaction. We find that while dynamical NS-PBH formation could account for the GW170817 event, the rate is highly dependent on unknown density contrast factors and could potentially be affected by galaxy mergers. We also find that PBH-PBH binaries would likely have a larger merger rate, assuming the density contrast boost factor of an order similar to the NS-PBH case. These exotic merger formations could provide new channels to account for the volumetric rate of compact-object mergers reported by LIGO/Virgo. Secondly, we consider the case where one of the NS's in a binary NS system is imploded by a microscopic PBH. We find that the predicted rate for NS implosion into a BH is very small, at least for the specific environment of NGC 4993. We discuss how our analysis can be applied to similar existing (GW190425 and GW190814) and future observations.

Gamma-Ray Emission Produced by r-process Elements from Neutron Star Mergers

Abstract The observation of a radioactively powered kilonova AT 2017gfo associated with the gravitational wave event GW170817 from a binary neutron star merger proves that these events are ideal sites for the production of heavy r-process elements. The gamma-ray photons produced by the radioactive decay of heavy elements are unique probes for the detailed nuclide compositions. Based on the detailed r-process nucleosynthesis calculations and considering radiative transport calculations for the gamma rays in different shells, we study the gamma-ray emission in a merger ejecta on a timescale of a few days. It is found that the total gamma-ray energy generation rate evolution is roughly depicted as E ̇ ∝ t − 1.3 . For the dynamical ejecta with a low electron fraction (Y e ≲ 0.20), the dominant contributors of gamma-ray energy are the nuclides around the second r-process peak (A ∼ 130) and the decay chain of 132Te (t 1/2 = 3.21 days) → 132I (t 1/2 = 0.10 days) → 132Xe produces gamma-ray lines at 228, 668, and 773 keV. For the case of a wind ejecta with Y e ≳ 0.30, the dominant contributors of gamma-ray energy are the nuclides around the first r-process peak (A ∼ 80) and the decay chain of 72Zn (t 1/2 = 1.93 days) → 72Ga (t 1/2 = 0.59 days) → 72Ge produces gamma-ray lines at 145, 834, 2202, and 2508 keV. The peak fluxes of these lines are 10−9 ∼ 10−7 ph cm−2 s−1, which are marginally detectable with the next-generation MeV gamma-ray detector ETCC if the source is at a distance of 40 Mpc.