Optimal Follow-up of Gravitational-wave Events with the UltraViolet EXplorer (UVEX)

Finding the electromagnetic (EM) counterpart of binary compact star merger, especially the binary neutron star (BNS) merger, is critically important for gravitational wave (GW) astronomy, cosmology and fundamental physics. On Aug. 17, 2017, Advanced LIGO and \textit{Fermi}/GBM independently triggered the first BNS merger, GW170817, and its high energy EM counterpart, GRB 170817A, respectively, resulting in a global observation campaign covering gamma-ray, X-ray, UV, optical, IR, radio as well as neutrinos. The High Energy X-ray telescope (HE) onboard \textit{Insight}-HXMT (Hard X-ray Modulation Telescope) is the unique high-energy gamma-ray telescope that monitored the entire GW localization area and especially the optical counterpart (SSS17a/AT2017gfo) with very large collection area ($\sim$1000 cm$^2$) and microsecond time resolution in 0.2-5 MeV. In addition, \textit{Insight}-HXMT quickly implemented a Target of Opportunity (ToO) observation to scan the GW localization area for potential X-ray emission from the GW source. Although it did not detect any significant high energy (0.2-5 MeV) radiation from GW170817, its observation helped to confirm the unexpected weak and soft nature of GRB 170817A. Meanwhile, \textit{Insight}-HXMT/HE provides one of the most stringent constraints (~10$^{-7}$ to 10$^{-6}$ erg/cm$^2$/s) for both GRB170817A and any other possible precursor or extended emissions in 0.2-5 MeV, which help us to better understand the properties of EM radiation from this BNS merger. Therefore the observation of \textit{Insight}-HXMT constitutes an important chapter in the full context of multi-wavelength and multi-messenger observation of this historical GW event.

Context. Next-generation gravitational wave (GW) observatories, such as the Einstein Telescope (ET) and Cosmic Explorer, will observe binary neutron star (BNS) mergers across cosmic history, providing precise parameter estimates for the closest ones. Innovative wide-field observatories, such as the Vera Rubin Observatory, will quickly cover large portions of the sky with unprecedented sensitivity to detect faint transients. Aims. This study aims to assess the prospects for detecting optical emissions from BNS mergers with next-generation detectors, considering how uncertainties in neutron star (NS) population properties and microphysics may affect detection rates, while developing realistic observational strategies by ET operating with the Rubin Observatory. Methods. Starting from BNS merger populations exploiting different NS mass distributions and equations of state (EOSs), we modelled the GW and kilonova (KN) signals based on source properties. We modelled KNe ejecta through numerical-relativity informed fits, considering the effect of prompt collapse of the remnant to black hole and new fitting formulas appropriate for more massive BNS systems, such as GW190425. We included optical afterglow emission from relativistic jets consistent with observed short gamma-ray bursts. We evaluated the detected mergers and the source parameter estimations for different geometries of ET, operating alone or in network of current or next-generation GW detectors. Finally, we developed target-of-opportunity strategies to follow up on these events using Rubin and evaluated the joint detection capabilities. Results. ET as a single observatory enables the detection of about ten to a hundred KNe per year by the Rubin Observatory. This improves by a factor of ∼10 already when operating in network with current GW detectors. Detection rate uncertainties are dominated by the poorly constrained local BNS merger rate, and depend to a lesser extent on the NS mass distribution and EOS.

The Ultraviolet Explorer (UVEX) is expected to fly in 2030 and will have the opportunity—and the rapid near/far-ultraviolet (UV) capabilities—to glean unprecedented insight into the bright UV emission present in kilonovae like that of AT 170817gfo, the electromagnetic counterpart to binary neutron star merger GW170817. To do so, it will need to perform prompt target-of-opportunity observations following detection of binary neutron star mergers by the LIGO–Virgo–KAGRA gravitational observatories. We present initial simulations to develop UVEX target-of-opportunity strategies for such events and provide the community with detailed initial estimates of the prospects for and characteristics of UVEX target-of-opportunity observations following gravitational-wave events, considering fiducial scenarios for the fifth and sixth LIGO–Virgo–KAGRA observing runs. Additionally, in light of the relatively few binary neutron star mergers observed since GW170817, we consider variant target-of-opportunity strategies for UVEX to maximize scientific gain in the case of a lowered binary neutron star merger rate.

The neutron star binary merger is one of the most energetic phenomena in our Universe. Based on the calculation of gravitational radiation (GR) with the separate consideration of revolution and stellar rotation, and electromagnetic radiation (ER) with unipolar induction DC circuit model and magnetic dipole model, the results are compared to analyse the extent to which the stellar rotation will affect the total gravitational radiation power. Besides, the relationships between radiation power and the related parameters (e.g., orbital radius and stellar mass) are investigated. Furthermore, the feasibility of different types of binary star merger as the progenitor of fast radio bursts and gamma-ray bursts were studied based on the two models of ER, obtaining that binary neutron star and neutron star-white dwarf systems are among the possible progenitors. Finally, the radiation power of both GR and ER are compared under the same conditions. According to the results, the energy dissipation of the system is dominated by gravitational radiation. These results shed lights on further studies on the radiation processes of binary neutron star mergers and binary star systems.

The first multimessenger observation of a binary neutron star (BNS) merger in August 2017 demonstrated the huge scientific potential of these extraordinary events. This breakthrough led to a number of discoveries and provided the best evidence that BNS mergers can launch short gamma-ray burst (SGRB) jets and are responsible for a copious production of heavy r-process elements. On the other hand, the details of the merger and post-merger dynamics remain only poorly constrained, leaving behind important open questions. Numerical relativity simulations are a powerful tool to unveil the physical processes at work in a BNS merger and as such they offer the best chance to improve our ability to interpret the corresponding gravitational wave (GW) and electromagnetic emission. Here, we review the current theoretical investigation on BNS mergers based on general relativistic magnetohydrodynamics simulations, paying special attention to the magnetic field as a crucial ingredient. First, we discuss the evolution, amplification, and emerging structure of magnetic fields in BNS mergers. Then, we consider their impact on various critical aspects: (i) jet formation and the connection with SGRBs, (ii) matter ejection, r-process nucleosynthesis, and radiocatively-powered kilonova transients, and (iii) post-merger GW emission.

Gravitational wave (GW) observations of binary neutron star (BNS) mergers can be used to measure luminosity distances and hence, when coupled with estimates for the mergers' host redshifts, infer the Hubble constant, $H_0$. These observations are, however, affected by GW measurement noise, uncertainties in host redshifts and peculiar velocities, and are potentially biased by selection effects and the mis-specification of the cosmological model or the BNS population. The estimation of $H_0$ from samples of BNS mergers with optical counterparts is tested here by using a phenomenological model for the GW strains that captures both the data-driven event selection and the distance-inclination degeneracy, while being simple enough to facilitate large numbers of simulations. A rigorous Bayesian approach to analyzing the data from such simulated BNS merger samples is shown to yield results that are unbiased, have the appropriate uncertainties, and are robust to model mis-specification. Applying such methods to a sample of $N \simeq 50$ BNS merger events, as LIGO+Virgo could produce in the next $\sim 5$ years, should yield robust and accurate Hubble constant estimates that are precise to a level of $\sim 2$ km s$^{-1}$ Mpc$^{-1}$, sufficient to reliably resolve the current tension between local and cosmological measurements of $H_0$.

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.

Gravitational Waves (GWs) from coalescing binaries carry crucial information about their component sources, like mass, spin and tidal effects. This implies that the analysis of GW signals from binary neutron star mergers can offer unique opportunities to extract information about the tidal properties of NSs, thereby adding constraints to the NS equation of state. In this work, we use Deep Learning (DL) techniques to overcome the computational challenges confronted in conventional methods of matched-filtering and Bayesian analyses for signal-detection and parameter-estimation. We devise a DL approach to classify GW signals from binary black hole and binary neutron star mergers. We further employ DL to analyze simulated GWs from binary neutron star merger events for parameter estimation, in particular, the regression of mass and tidal deformability of the component objects. The results presented in this work demonstrate the promising potential of DL techniques in GW analysis, paving the way for further advancement in this rapidly evolving field. The proposed approach is an efficient alternative to explore the wealth of information contained within GW signals of binary neutron star mergers, which can further help constrain the NS EoS.

With no binary neutron star (BNS) merger detected yet during the fourth observing run (O4) of the LIGO-Virgo-KAGRA (LVK) gravitational wave (GW) detector network, despite the time volume (VT) surveyed with respect to the end of O3 having increased by more than a factor of three, a pressing question is how likely the detection of at least one BNS merger is in the remainder of the run. I present here a simple and general method of addressing such a question, which constitutes the basis for the predictions that have been presented in the LVK Public Alerts User Guide during the hiatus between the O4a and O4b parts of the run. The method, which can be applied to neutron star–black hole (NSBH) mergers as well, is based on simple Poisson statistics and on an estimate of the ratio of the VT span by the future run to that span by previous runs. An attractive advantage of this method is that its predictions are independent of the mass distribution of the merging compact binaries, which is very uncertain at the present moment. The results, not surprisingly, show that the most likely outcome of the final part of O4 is the absence of any BNS merger detection. Still, the probability of a non-zero number of detections is 34−46%. For NSBH mergers, the probability of at least one additional detection is 64−71%. The prospects for the next observing run, O5, are more promising, with predicted numbers of NBNS,O5 = 2.8−21+44, and NSBH detections of NNSBH,O5 = 65−38+61 (median and 90% symmetric credible range), based on the current LVK detector target sensitivities for the run. The calculations presented here also lead to an update of the LVK local BNS merger rate density estimate that accounts for the absence of BNS merger detections in O4 so far, which reads 2.8 Gpc−3 yr−1 ≤ R0 ≤ 480 Gpc−3 yr−1.

The joint detection of GW170817 and GRB 170817A opened the era of multimessenger astronomy with gravitational waves (GWs) and provided the first direct probe that at least some binary neutron star (BNS) mergers are progenitors of short gamma-ray bursts (S-GRBs). In the next years, we expect to have more multimessenger detections of BNS mergers, thanks to the increasing sensitivity of GW detectors. Here, we present a comprehensive study on the prospects for joint GW and electromagnetic observations of merging BNSs in the fourth Laser Interferometer Gravitational-wave Observatory (LIGO)–Virgo–Kamioka Gravitational Wave Detector (KAGRA) observing run with Fermi Gamma-ray Space Telescope (Fermi), Neil Gehrels Swift Observatory (Swift), INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), and Space Variable Objects Monitor (SVOM). This work combines accurate population synthesis models with simulations of the expected GW signals and the associated S-GRBs, considering different assumptions about the gamma-ray burst (GRB) jet structure. We show that the expected rate of joint GW and electromagnetic detections could be up to ∼6 yr−1 when Fermi/Gamma-ray Burst Monitor (GBM) is considered. Future joint observations will help us to better constrain the association between BNS mergers and S-GRBs, as well as the geometry of the GRB jets.

Gravitational Wave (GW) signals from binary neutron star (BNS) mergers provide critical insights into the properties of matter under extreme conditions. Due to the scarcity of observational data, numerical relativity (NR) simulations are indispensable for exploring these phenomena, without replacing the need for observational confirmation. However, simulating BNS mergers is a formidable challenge, and ensuring the consistency, reliability or convergence, especially in the post-merger, remains a work in progress. In this paper we assess the performance of current BNS merger simulations by analyzing open-source GW waveforms from five leading NR codes – SACRA, BAM, THC, Whisky, and SpEC. We focus on the accuracy of these simulations and on the effect of the equation of state on waveform predictions. We first check if different codes give similar results for similar initial data, then apply two methods to calculate convergence and quantify discretization errors. Lastly, we perform a thorough investigation into the effect of tidal interactions on key frequencies in the GW spectrum. We introduce a novel quasi-universal relation for the transient post-merger time, enhancing our understanding of remnant dynamics in this region. This detailed analysis clarifies agreements and discrepancies between these leading NR codes, and highlights necessary improvements for the advanced accuracy requirements of future GW detectors.

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.

Advanced LIGO and Virgo have reported 90 confident gravitational-wave (GW) observations from compact-binary coalescences from their three observation runs. In addition, numerous subthreshold GW candidates have been identified. Binary neutron star (BNS) mergers can produce GWs and short-gamma-ray bursts, as confirmed by GW170817/GRB 170817A. There may be electromagnetic counterparts recorded in archival observations associated with subthreshold GW candidates. The CHIME/FRB Collaboration has reported the first large sample of fast radio bursts (FRBs), millisecond radio transients detected up to cosmological distances; a fraction of these may be associated with BNS mergers. This work searches for coincident GWs and FRBs from BNS mergers using candidates from the fourth Open Gravitational-wave Catalog and the first CHIME/FRB catalog. We use a ranking statistic for GW/FRB association that combines the GW detection statistic with the odds of temporal and spatial association. We analyze GW candidates and nonrepeating FRBs from 2019 April 1 to 2019 July 1, when both the Advanced LIGO/Virgo GW detectors and the CHIME radio telescope were observing. The most significant coincident candidate has a false alarm rate of 0.29 per observation time, which is consistent with a null observation. The null results imply, at most, – of FRBs are produced promptly from the BNS mergers.

We present new (3+1) dimensional numerical relativity simulations of the binary neutron star (BNS) mergers that take into account the NS spins. We consider different spin configurations, aligned or antialigned to the orbital angular momentum, for equal and unequal mass BNS and for two equations of state. All the simulations employ quasiequilibrium circular initial data in the constant rotational velocity approach, i.e. they are consistent with Einstein equations and in hydrodynamical equilibrium. We study the NS rotation effect on the energetics, the gravitational waves (GWs) and on the possible electromagnetic (EM) emission associated to dynamical mass ejecta. For dimensionless spin magnitudes of $\chi\sim0.1$ we find that spin-orbit interactions and also spin-induced-quadrupole deformations affect the late-inspiral-merger dynamics. The latter is, however, dominated by finite-size effects. Spin (tidal) effects contribute to GW phase differences up to 5 (20) radians accumulated during the last eight orbits to merger. Similarly, after merger the collapse time of the remnant and the GW spectrogram are affected by the NSs rotation. Spin effects in dynamical ejecta are clearly observed in unequal mass systems in which mass ejection originates from the tidal tail of the companion. Consequently kilonovae and other EM counterparts are affected by spins. We find that spin aligned to the orbital angular momentum leads to brighter EM counterparts than antialigned spin with luminosities up to a factor of two higher.

We present new (3+1)D numerical relativity simulations of the binary neutron star (BNS) merger and postmerger phase. We focus on a previously inaccessible region of the binary parameter space spanning the binary's mass-ratio $q\sim1.00-1.75$ for different total masses and equations of state, and up to $q\sim2$ for a stiff BNS system. We study the mass-ratio effect on the gravitational waves (GWs) and on the possible electromagnetic emission associated to dynamical mass ejecta. We compute waveforms, spectra, and spectrograms of the GW strain including all the multipoles up to $l=4$. The mass-ratio has a specific imprint on the GW multipoles in the late-inspiral-merger signal, and it affects qualitatively the spectra of the merger remnant. The multipole effect is also studied by considering the dependency of the GW spectrograms on the source's sky location. Unequal mass BNSs produce more ejecta than equal mass systems with ejecta masses and kinetic energies depending almost linearly on $q$. We estimate luminosity peaks and light curves of macronovae events associated to the mergers using a simple approach. For $q\sim2$ the luminosity peak is delayed for several days and can be up to four times larger than for the $q=1$ cases. The macronova emission associated with the $q\sim2$ BNS is more persistent in time and could be observed for weeks instead of few days ($q=1$) in the near infrared. Finally, we estimate the flux of possible radio flares produced by the interaction of relativistic outflows with the surrounding medium. Also in this case a large $q$ can significantly enhance the emission and delay the peak luminosity. Overall, our results indicate that BNS merger with large mass ratio have EM signatures distinct from the equal mass case and more similar to black hole - neutron star binaries.