Binary Neutron Star Merger GW170817 Research Articles

Stringent constraints on neutron-star radii from multimessenger observations and nuclear theory

The properties of neutron stars are determined by the nature of the matter that they contain. These properties can be constrained by measurements of the star's size. We obtain stringent constraints on neutron-star radii by combining multimessenger observations of the binary neutron-star merger GW170817 with nuclear theory that best accounts for density-dependent uncertainties in the equation of state. We construct equations of state constrained by chiral effective field theory and marginalize over these using the gravitational-wave observations. Combining this with the electromagnetic observations of the merger remnant that imply the presence of a short-lived hyper-massive neutron star, we find that the radius of a $1.4\,\rm{M}_\odot$ neutron star is $R_{1.4\,\mathrm{M}_\odot} = 11.0^{+0.9}_{-0.6}~{\rm km}$ (90% credible interval). Using this constraint, we show that neutron stars are unlikely to be disrupted in neutron-star black-hole mergers; subsequently, such events will not produce observable electromagnetic emission.

GW170817 constraints analyzed with Gogny forces and momentum-dependent interactions

A set of equations of state obtained from finite-range Gogny forces and momentum-dependent interactions is used to investigate the recent observation of gravitational waves from the binary neutron star merger GW170817 event. For this set of interactions, we have calculated the neutron star tidal deformabilities (related to the second Love number), the mass-radius diagram, and the moment of inertia (I). The I-Love relation has been verified. We also have found strong correlations among the tidal deformability of the canonical neutron star, its radius, and the derivatives of the nuclear symmetry energy at the saturation density. Most of the obtained results are located within the constraints of the tidal deformabilities extracted from the GW170817 detection.

Search for event bursts in XMASS-I correlated with gravitational-wave events

A large single-phase liquid xenon detector, XMASS-I, is a multi-purpose detector suitable for detecting particles with energy deposition in the wide energy range from keV to MeV under a low-background environment and was stably operating for more than 5 years since November 2013. We conducted a search for event bursts in the XMASS-I detector associated with gravitational-wave events. In this paper, we present a preliminary result on the search for event bursts in coincidence with the binary neutron star merger GW170817.

Jet Propagation in Neutron Star Mergers and GW170817

Abstract The gravitational wave event from the binary neutron star (BNS) merger GW170817 and the following multi-messenger observations present strong evidence for i) merger ejecta expanding with substantial velocities and ii) a relativistic jet which had to propagate through the merger ejecta. The ejecta’s expansion velocity is not negligible for the jet head motion, which is a fundamental difference from the other systems like collapsars and active galactic nuclei. Here we present an analytic model of the jet propagation in an expanding medium. In particular, we notice a new term in the expression of the breakout time and velocity. In parallel, we perform a series of over a hundred 2D numerical simulations of jet propagation. The BNS merger ejecta is prepared based on numerical relativity simulations of a BNS merger with the highest-resolution to date. We show that our analytic results agree with numerical simulations over a wide parameter space. Then we apply our analytic model to GW170817, and obtain two solid constraints on: i) the central engine luminosity as Liso, 0 ∼ 3 × 1049 − 2.5 × 1052 erg s−1, and on ii) the delay time between the merger and engine activation t0 − tm < 1.3 s. The engine power implies that the apparently-faint short gamma-ray burst (sGRB) sGRB 170817A is similar to typical sGRBs if observed on-axis.

Two Years of Nonthermal Emission from the Binary Neutron Star Merger GW170817: Rapid Fading of the Jet Afterglow and First Constraints on the Kilonova Fastest Ejecta

Abstract We present Chandra and Very Large Array observations of GW170817 at ∼521–743 days post-merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time nonthermal emission follows the expected evolution of an off-axis relativistic jet, with a steep temporal decay F ν ∝ t − 1.95 ± 0.15 and power-law spectrum F ν ∝ ν − 0.575 ± 0.007 . We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and find n ≤ 9.6 × 10 − 3 cm − 3 . This measurement is independent from inferences based on jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy E 0 = 1.5 − 1.1 + 3.6 × 10 49 erg ( E iso = 2.1 − 1.5 + 6.4 × 10 52 erg ) and jet opening angle θ 0 = 5.9 − 0.7 + 1.0 deg with characteristic Lorentz factor Γ j = 163 − 43 + 23 , expanding in a low-density medium with n 0 = 2.5 − 1.9 + 4.1 × 10 − 3 cm − 3 and viewed θ obs = 30.4 − 3.4 + 4.0 deg off-axis. The synchrotron emission originates from a power-law distribution of electrons with index p = 2.15 − 0.02 + 0.01 . The shock microphysics parameters are constrained to ϵ e = 0.18 − 0.13 + 0.30 and ϵ B = 2.3 − 2.2 + 16.0 × 10 − 3 . Furthermore, we investigate the presence of X-ray flares and find no statistically significant evidence of ≥2.5σ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy E k KN ∝ ( Γ β ) − α into the environment, finding that shallow stratification indexes α ≤ 6 are disfavored. Future radio and X-ray observations will refine our inferences on the fastest kilonova ejecta properties.

Echoes from the abyss: a highly spinning black hole remnant for the binary neutron star merger GW170817

The first direct observation of a binary neutron star (BNS) merger was a watershed moment in multi-messenger astronomy. However, gravitational waves from GW170817 have only been observed prior to the BNS merger, but electromagnetic observations all follow the merger event. While post-merger gravitational wave signal in general relativity is too faint (given current detector sensitivities), here we present the first tentative detection of post-merger gravitational wave “echoes” from a highly spinning “black hole” remnant. The echoes may be expected in different models of quantum black holes that replace event horizons by exotic Planck-scale structure and tentative evidence for them has been found in binary black hole merger events. The fact that the echo frequency is suppressed by log M (in Planck units) puts it squarely in the LIGO sensitivity window, allowing us to build an optimal model-agnostic search strategy via cross-correlating the two detectors in frequency/time. We find a tentative detection of echoes at fecho ≃ 72 Hz, around 1.0 sec after the BNS merger, consistent with a 2.6–2.7 M⊙ “black hole” remnant with dimensionless spin 0.84–0.87. Accounting for all the “look-elsewhere” effects, we find a significance of 4.2 σ, or a false alarm probability of 1.6× 10−5, i.e. a similar cross-correlation within the expected frequency/time window after the merger cannot be found more than 4 times in 3 days. If confirmed, this finding will have significant consequences for both physics of quantum black holes and astrophysics of binary neutron star mergers.

A Galaxy-targeted Search for the Optical Counterpart of the Candidate NS–BH Merger S190814bv with Magellan

Abstract On 2019 August 14 the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo gravitational wave interferometer announced the detection of a binary merger, S190814bv, with a low false alarm rate of about 1 in 1.6 × 1025 yr, a distance of 267 ± 52 Mpc, a 90% (50%) localization region of about 23 (5) deg2, and a probability of being a neutron star–black hole (NS–BH) merger of >99%. The LIGO/Virgo Collaboration (LVC) defines NS–BH such that the lighter binary member has a mass of <3 M ⊙ and the more massive one has >5 M ⊙, and this classification is in principle consistent with a BH–BH merger depending on the actual upper mass cutoff for neutron stars. Additionally, the LVC designated a probability that the merger led to matter outside the final BH remnant of <1%, suggesting that an electromagnetic (EM) counterpart is unlikely. Here we report our optical follow-up observations of S190814bv using the Magellan Baade 6.5 m telescope to target all 96 galaxies in the Galaxy List for the Advanced Detector Era catalog within the 50% localization volume (representing about 70% of the integrated luminosity within this region). No counterpart was identified to a median 3σ limiting magnitude of i = 22.2 (M i ≈ −14.9 mag), comparable to the brightness of the optical counterpart of the binary neutron star merger GW170817 at the distance of S190814bv; similarly, we can rule out an on-axis jet typical of short GRBs. However, we cannot rule out other realistic models, such as a kilonova with only ∼0.01 M ⊙ of lanthanide-rich material, or an off-axis jet with a viewing angle of θ obs ≳ 15°.

Gravitational-wave luminosity distance in quantum gravity

Dimensional flow, the scale dependence of the dimensionality of spacetime, is a feature shared by many theories of quantum gravity (QG). We present the first study of the consequences of QG dimensional flow for the luminosity distance scaling of gravitational waves in the frequency ranges of LIGO and LISA. We find generic modifications with respect to the standard general-relativistic scaling, largely independent of specific QG proposals. We constrain these effects using two examples of multimessenger standard sirens, the binary neutron-star merger GW170817 and a simulated supermassive black-hole merger event detectable with LISA. We apply these constraints to various QG candidates, finding that the quantum geometries of group field theory, spin foams and loop quantum gravity can give rise to observable signals in the gravitational-wave spin-2 sector. Our results complement and improve GW propagation-speed bounds on modified dispersion relations. Under more model-dependent assumptions, we also show that bounds on quantum geometry can be strengthened by solar-system tests.

The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin

Abstract We present a revised and complete optical afterglow light curve of the binary neutron star merger GW170817, enabled by deep Hubble Space Telescope (HST) F606W observations at ≈584 days post-merger, which provide a robust optical template. The light curve spans ≈110–362 days, and is fully consistent with emission from a relativistic structured jet viewed off-axis, as previously indicated by radio and X-ray data. Combined with contemporaneous radio and X-ray observations, we find no spectral evolution, with a weighted average spectral index of 〈 β 〉 = − 0.583 ± 0.013 , demonstrating that no synchrotron break frequencies evolve between the radio and X-ray bands over these timescales. We find that an extrapolation of the post-peak temporal slope of GW170817 to the luminosities of cosmological short gamma-ray bursts matches their observed jet break times, suggesting that their explosion properties are similar, and that the primary difference in GW170817 is viewing angle. Additionally, we place a deep limit on the luminosity and mass of an underlying globular cluster (GC) of L ≲ 6.7 × 103 L ⊙, or M ≲ 1.3 × 104 M ⊙, at least 4 standard deviations below the peak of the GC mass function of the host galaxy, NGC 4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a GC. As highlighted here, HST (and soon the James Webb Space Telescope) enables critical observations of the optical emission from neutron star merger jets and outflows.

Lessons from the light of a neutron star merger

The discovery by Advanced LIGO/Virgo of gravitational waves from the binary neutron star merger GW170817, and subsequently by astronomers of transient emission across the electromagnetic spectrum, has initiated a new era of multi-messenger gravitational wave astronomy. Here I summarize the electromagnetic discoveries in the context of theoretical counterpart models and present personal views on the major take-away lessons and outstanding new questions from this watershed event, focused on the implications for nuclear physics. The luminosity and colors of the early optical emission discovered within a day of the merger agree well with predictions for “kilonova” emission, powered by the radioactive decay of light r-process nuclei (atomic mass number A≲140). The transition of the spectral energy distribution to near-infrared wavelengths on timescales of days indicates that inner portions of the ejecta contain heavy r-process nuclei with high UV/optical opacity due to the presence of at least some lanthanide elements (A≳140). The “blue” and “red” ejecta components likely possess distinct origins (e.g. dynamical ejecta, magnetar-powered wind, or accretion disk outflow), with implications for the merger process (e.g. the lifetime of the remnant prior to black hole formation) and fundamental properties of neutron stars. I outline the predicted diversity in the electromagnetic emission of future mergers−observed with different ingoing binary masses and/or viewing angles−discovered in the years ahead as LIGO/Virgo reach design sensitivity.

Follow-up of the Neutron Star Bearing Gravitational-wave Candidate Events S190425z and S190426c with MMT and SOAR

Abstract On 2019 April 25.346 and 26.640 UT the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational-wave (GW) observatory announced the detection of the first candidate events in Observing Run 3 that contained at least one neutron star (NS). S190425z is a likely binary neutron star (BNS) merger at d L = 156 ± 41 Mpc, while S190426c is possibly the first NS–black hole (BH) merger ever detected, at d L = 377 ± 100 Mpc, although with marginal statistical significance. Here we report our optical follow-up observations for both events using the MMT 6.5 m telescope, as well as our spectroscopic follow-up of candidate counterparts (which turned out to be unrelated) with the 4.1 m SOAR telescope. We compare to publicly reported searches, explore the overall areal coverage and depth, and evaluate those in relation to the optical/near-infrared (NIR) kilonova emission from the BNS merger GW170817, to theoretical kilonova models, and to short gamma-ray burst (SGRB) afterglows. We find that for a GW170817-like kilonova, the partial volume covered spans up to about 40% for S190425z and 60% for S190426c. For an on-axis jet typical of SGRBs, the search effective volume is larger, but such a configuration is expected in at most a few percent of mergers. We further find that wide-field γ-ray and X-ray limits rule out luminous on-axis SGRBs, for a large fraction of the localization regions, although these searches are not sufficiently deep in the context of the γ-ray emission from GW170817 or off-axis SGRB afterglows. The results indicate that some optical follow-up searches are sufficiently deep for counterpart identification to about 300 Mpc, but that localizations better than 1000 deg2 are likely essential.

Improved analytic modeling of neutron star interiors

Studies of neutron stars are extremely timely given the recent detection of gravitational waves from a binary neutron star merger GW170817, and an International Space Station payload NICER currently in operation that aims to determine radii of neutron stars to a precision better than 5%. In many cases, neutron star solutions are constructed numerically due to the complexity of the field equations with realistic equations of state. However, in order to relate observables like the neutron star mass and radius to interior quantities like central density and pressure, it would be useful to provide an accurate, analytic modeling of a neutron star interior. One such solution for static and isolated neutron stars is the Tolman VII solution characterized only by two parameters (e.g. mass and radius), though its agreement with numerical solutions is not perfect. We here introduce an improved analytic model based on the Tolman VII solution by introducing an additional parameter to make the analytic density profile agree better with the numerically obtained one. This additional parameter can be fitted in terms of the stellar mass, radius and central density in an equation-of-state-insensitive way. In most cases, we find that the new model more accurately describes realistic profiles than the original Tolman VII solution by a factor of 2-5. Our results are first-step calculations towards constructing analytic interior solutions for more realistic neutron stars under rotation or tidal deformation.

Application of hidden Markov model tracking to the search for long-duration transient gravitational waves from the remnant of the binary neutron star merger GW170817

The nature of the post-merger remnant of the first binary neutron star coalescence observed by the Advanced Laser Interferometer Gravitational Wave Observatory (Advanced LIGO) and Advanced Virgo, GW170817, is unknown. Searches have been carried out for short ($\lesssim 1$ s), intermediate ($\lesssim 500$ s), and long ($\sim$ days) signals using various algorithms without yielding a detection. We describe an efficient frequency tracking scheme based on a hidden Markov model to search for long-duration transient signals from a neutron star remnant with spin-down time-scale in the range $\sim 10^2$s-$10^4$s. It was one of the methods used in the search for signals from a long-lived remnant of GW170817. We validate the method and estimate its sensitivity through Monte-Carlo simulations on the same data set as used in the GW170817 search. We describe the search configuration, procedure, and follow-up step by step. The methodology of the hidden Markov model is described fully to ensure that future analyses of this kind can be reproduced by an independent party.

A First Search for Prompt Radio Emission from a Gravitational-wave Event

Abstract Multimessenger observations of the binary neutron star merger GW170817 have enabled the discovery of a diverse array of electromagnetic counterparts to compact binary mergers, including an unambiguous kilonova, a short gamma-ray burst, and a late-time radio jet. Beyond these counterparts, compact binary mergers are additionally predicted to be accompanied by prompt low-frequency radio emission. The successful observation of a prompt radio counterpart would be immensely valuable, but is made difficult by the short delay between the gravitational-wave and prompt electromagnetic signals, as well as by the poor localization of gravitational-wave sources. Here, we present the first search for prompt radio emission accompanying a gravitational-wave event, targeting the binary black hole merger GW170104 detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo during their second (O2) observing run. Using the Owens Valley Radio Observatory Long Wavelength Array, we search a ∼900 deg2 region for transient radio emission within approximately one hour of GW170104, obtaining an upper limit of 2.5 × 1041 erg s−1 on its equivalent isotropic luminosity between 27 and 84 MHz. We additionally discuss plans to target binary neutron star mergers in Advanced LIGO and Virgo’s upcoming O3 observing run.

Constraining the Inclinations of Binary Mergers from Gravitational-wave Observations

Abstract Much of the information we hope to extract from the gravitational-wave signatures of compact binaries is only obtainable when we can accurately constrain the inclination of the orbital plane relative to the line of sight. In this paper, we discuss in detail a degeneracy between the measurement of the binary distance and inclination that limits our ability to accurately measure the inclination using gravitational waves alone. This degeneracy is exacerbated by the expected distribution of events in the universe, which leads us to prefer face-on systems at a greater distance. We use a simplified model that only considers the binary distance and orientation and show that this gives comparable results to the full parameter estimates obtained from the binary neutron star merger GW170817. For the advanced LIGO-Virgo network, it is only binaries that are close to edge-on, i.e., with inclinations ι ≳ 75°, that will be distinguishable from face-on systems. Extended networks that have good sensitivity to both gravitational-wave polarizations will only be able to constrain the inclination of a face-on binary at a signal-to-noise ratio of 20 to ι ≲ 45°. Even for loud signals with signal-to-noise ratios of 100, face-on signals will only be constrained to have inclinations . In the absence of observable higher modes or orbital precession, this degeneracy will dominate the mass measurements of binary black hole mergers at cosmological distances.

All-sky search for long-duration gravitational-wave transients in the second Advanced LIGO observing run

We present the results of a search for long-duration gravitational-wave transients in the data from the Advanced LIGO second observation run; we search for gravitational-wave transients of $2~\text{--}~ 500$~s duration in the $24 - 2048$\,Hz frequency band with minimal assumptions about signal properties such as waveform morphologies, polarization, sky location or time of occurrence. Targeted signal models include fallback accretion onto neutron stars, broadband chirps from innermost stable circular orbit waves around rotating black holes, eccentric inspiral-merger-ringdown compact binary coalescence waveforms, and other models. The second observation run totals about \otwoduration~days of coincident data between November 2016 and August 2017. We find no significant events within the parameter space that we searched, apart from the already-reported binary neutron star merger GW170817. We thus report sensitivity limits on the root-sum-square strain amplitude $h_{\mathrm{rss}}$ at $50\%$ efficiency. These sensitivity estimates are an improvement relative to the first observing run and also done with an enlarged set of gravitational-wave transient waveforms. Overall, the best search sensitivity is $h_{\mathrm{rss}}^{50\%}$=$2.7\times10^{-22}$~$\mathrm{Hz^{-1/2}}$ for a millisecond magnetar model. For eccentric compact binary coalescence signals, the search sensitivity reaches $h_{\mathrm{rss}}^{50\%}$=$9.6\times10^{-22}$~$\mathrm{Hz^{-1/2}}$.

Revisiting the Lower Bound on Tidal Deformability Derived by AT 2017gfo

Abstract We revisit the lower bound on binary tidal deformability imposed by a luminous kilonova/macronova, AT 2017gfo, by numerical-relativity simulations of models that are consistent with gravitational waves from the binary neutron star merger GW170817. Contrary to the claim made in the literature, we find that binaries with can explain the luminosity of AT 2017gfo, as long as moderate mass ejection from the remnant is assumed as had been done in previous work. The reason is that the maximum mass of a neutron star is not strongly correlated with the tidal deformability of neutron stars with a typical mass of ≈1.4 M ⊙. If the maximum mass is so large that the binary does not collapse into a black hole immediately after merger, the mass of the ejecta can be sufficiently large irrespective of the binary tidal deformability. We present models of binary mergers with down to 242 that satisfy the requirement on the mass of the ejecta from the luminosity of AT 2017gfo. We further find that the luminosity of AT 2017gfo could be explained by models that do not experience bounce after merger. We conclude that the luminosity of AT 2017gfo is not very useful for constraining the binary tidal deformability. Accurate estimation of the mass ratio will be necessary to establish a lower bound using electromagnetic counterparts in the future. We also caution that merger simulations that employ a limited class of tabulated equations of state could be severely biased due to the lack of generality.

Search for Gravitational Waves from a Long-lived Remnant of the Binary Neutron Star Merger GW170817

Abstract One unanswered question about the binary neutron star coalescence GW170817 is the nature of its post-merger remnant. A previous search for post-merger gravitational waves targeted high-frequency signals from a possible neutron star remnant with a maximum signal duration of 500 s. Here, we revisit the neutron star remnant scenario and focus on longer signal durations, up until the end of the second Advanced LIGO-Virgo observing run, which was 8.5 days after the coalescence of GW170817. The main physical scenario for this emission is the power-law spindown of a massive magnetar-like remnant. We use four independent search algorithms with varying degrees of restrictiveness on the signal waveform and different ways of dealing with noise artefacts. In agreement with theoretical estimates, we find no significant signal candidates. Through simulated signals, we quantify that with the current detector sensitivity, nowhere in the studied parameter space are we sensitive to a signal from more than 1 Mpc away, compared to the actual distance of 40 Mpc. However, this study serves as a prototype for post-merger analyses in future observing runs with expected higher sensitivity.

Multiwavelength studies of gravitational wave sources: Physics and phenomenology

The discovery of binary neutron star merger GW170817 from its gravitational wave (GW) signature, together with its accompanying electromagnetic (EM) emission spanning gamma‐ray to radio, marked the birth of GW + EM multimessenger astrophysics. The radioactively powered thermal kilonova, which dominated the ultraviolet to infrared in the hours to weeks after the merger, indicates that such mergers are the site of heavy‐element nucleosynthesis, likely extending to the third r‐process peak. The prompt gamma‐ray flash, and late‐time nonthermal (X‐ray to radio) emission, indicate that the merger also produced an ultrarelativistic jet, thus tying this event to the phenomena of short‐duration gamma‐ray bursts. In the future, observations of further mergers promise to establish their contribution to global nucleosynthesis, allow investigation of jet launching and structure, provide independent estimates of the cosmological parameters, constrain the neutron star equation of state, and address questions of fundamental physics.

Multimessenger parameter estimation of GW170817

We combine gravitational wave (GW) and electromagnetic (EM) data to perform a Bayesian parameter estimation of the binary neutron star (NS) merger GW170817. The EM likelihood is constructed from a fit to a large number of numerical relativity simulations which we combine with a lower bound on the mass of the remnant's accretion disk inferred from the modeling of the EM light curve. In comparison with previous works, our analysis yields a more precise determination of the tidal deformability of the binary, for which the EM data provide a lower bound, and of the mass ratio of the binary, with the EM data favoring a smaller mass asymmetry. The 90\% credible interval for the areal radius of a $1.4\ M_\odot$ NS is found to be $12.2^{+1.0}_{-0.8} \pm 0.2\ {\rm km}$ (statistical and systematic uncertainties).