Search

The coalescence of binary neutron stars (BNS) is a prolific source of gravitational waves (GWs) and electromagnetic (EM) radiation, offering a dual observational window into the Universe. Lowering the signal-to-noise ratio (S/N) threshold is a simple and cost-effective way to enhance the detection probability of GWs from BNS mergers. In this study, we introduce a metric of the purity of joint GW and EM detections |$P_{\rm joint}$|⁠ , which is in analogue to |$P_{\rm astro}$| in GW-only observations. By simulating BNS merger GWs jointly detected by the Handford, LIGO Livingston, and Virgo network and EM counterparts (kilonovae and short gamma-ray bursts, sGRBs) with an assumed merger rate density of BNS, we generate catalogues of GW events and EM counterparts. Through this simulation, we analyse joint detection pairs, both correct and misidentified. We find the following: (1) For kilonovae, requiring |$P_{\rm joint}\gt $| 95 per cent instead of |$P_{\rm astro}\gt 95~{{\ \rm per\ cent}}$| reduces the S/N from 9.2 to 8.5–8.8, allowing 5–13 additional joint detections per year and increasing the GW detection volume by 9–17 per cent. (2) For sGRBs, requiring |$P_{\rm joint}\gt $| 95 per cent instead of |$P_{\rm astro}$| reduces the S/N from 9.2 to 8.1–8.5. (3) Increasing kilonova or sGRB detection capability does not improve |$P_{\rm joint}$| due to a higher rate of misidentifications. We also show that sub-threshold GW and kilonova detections can reduce the uncertainty in measuring the Hubble constant to 89–92 per cent of its original value, and sub-threshold GW and sGRB observations can enhance the precision of constraining the speed of GWs to 88 per cent of previously established values. [ABSTRACT FROM AUTHOR]

The behaviour of a short gamma-ray burst (sGRB) afterglow light curve can reveal the angular structure of the relativistic jet and constrain the observer's viewing angle |$\theta _\textrm {obs}$|⁠. The observed deceleration time of the jet, and, therefore, the time of the afterglow peak, depends on the observer's viewing angle. A larger viewing angle leads to a later peak of the afterglow and a lower flux at peak. We utilize the earliest afterglow detections of 58 sGRBs detected with the Neil Gehrels Swift Observatory X-ray Telescope to constrain the ratio of the viewing angle |$\theta _\textrm {obs}$| to the jet's core |$\theta _\textrm {c}$|⁠. We adopt a power-law angular jet structure in both energy |$E(\theta)\propto \theta ^{-a}$| and Lorentz factor |$\Gamma (\theta)\propto \theta ^{-b}$| beyond the core. We find that either sGRBs are viewed within |$\theta _\textrm {obs}/\theta _\textrm {c}\lt 1$| or the initial Lorentz factor of material in their jet's core is extremely high (⁠|$\Gamma _0\gt 500$|⁠). If we consider tophat jets, we constrain 90 per cent of our sample to be viewed within |$\theta _\textrm {obs}/\theta _\textrm {c}\lt 1.06$| and 1.15 for our canonical and conservative afterglow scenarios. For a subset of events with measurements of the jet break, we can constrain |$\Gamma _0\theta _\textrm {c}\gtrsim 30$|⁠. This confirmation that cosmological sGRBs are viewed either on-axis or very close to their jet's core has significant implications for the nature of the prompt gamma-ray production mechanism and for the rate of future sGRB detections coincident with gravitational waves, implying that they are extremely rare. [ABSTRACT FROM AUTHOR]

According to the hypothesis that strange quark matter may be the true ground state of matter at extremely high densities, strange quark stars should be stable and could exist in the Universe. It is possible that pulsars may actually be strange stars, but not neutron stars. Here we present a short review on recent progresses in the field of strange quark stars. First, three popular phenomenological models widely used to describe strange quark matter are introduced, with special attention being paid on the corresponding equation of state in each model. Combining the equation of state with the Tolman-Oppenheimer-Volkov equations, the inner structure and mass-radius relation can be obtained for the whole sequence of strange stars. Tidal deformability and oscillations (both radial and non-radial oscillations), which are sensitive to the composition and the equations of state, are then described. Hybrid stars as a special kind of quark stars are discussed. Several other interesting aspects of strange stars are also included. For example, strong gravitational wave emissions may be generated by strange stars through various mechanisms, which may help identify strange stars via observations. Especially, close-in strange quark planets with respect to their hosts may provide a unique test for the existence of strange quark objects. Fierce electromagnetic bursts could also be generated by strange stars. The energy may come from the phase transition of neutron stars to strange stars, or from the merger of binary strange stars. The collapse of the strange star crust can also release a huge amount of energy. It is shown that strange quark stars may be involved in short gamma-ray bursts and fast radio bursts. [ABSTRACT FROM AUTHOR]

The milestone discovery of GW170817-GRB 170817A-AT 2017gfo has shown that gravitational waves (GWs) could be produced during the merger of a neutron star–neutron star/black hole and that in electromagnetic (EM) waves, a gamma-ray burst (GRB) and a kilonova (KN) are generated in sequence after the merger. Observationally, however, EM properties before the merger phase are still unclear. Here we report a peculiar precursor in a KN-associated long-duration GRB 211211A, providing evidence of the EM before the merger. This precursor lasts ∼0.2 s, and the waiting time between the precursor and the main burst is ∼1 s, comparable to that between GW170817 and GRB 170817A. The spectrum of the precursor could be well fit with a nonthermal cutoff power-law model instead of a blackbody. In particular, a ∼22 Hz quasiperiodic oscillation candidate (∼3 σ) is detected in the precursor. These temporal and spectral properties indicate that this precursor is probably produced by a catastrophic flare accompanied with magnetoelastic or crustal oscillations of a magnetar in a binary compact merger. The strong magnetic field of the magnetar can also account for the prolonged duration of GRB 211211A. However, it poses a challenge to reconcile the rather short lifetime of a magnetar with the rather long spiraling time of a binary neutron star system only by the GW radiation before the merger. [ABSTRACT FROM AUTHOR]

Fulfilling the rich promise of rapid advances in time-domain astronomy is only possible through confronting our observations with physical models and extracting the parameters that best describe what we see. Here, we introduce redback ; a Bayesian inference software package for electromagnetic transients. redback provides an object-orientated python interface to over 12 different samplers and over 100 different models for kilonovae, supernovae, gamma-ray burst afterglows, tidal disruption events, engine-driven transients among other explosive transients. The models range in complexity from simple analytical and semi-analytical models to surrogates built upon numerical simulations accelerated via machine learning. redback also provides a simple interface for downloading and processing data from various catalogues such as Swift and FINK. The software can also serve as an engine to simulate transients for telescopes such as the Zwicky Transient Facility and Vera Rubin with realistic cadences, limiting magnitudes, and sky coverage or a hypothetical user-constructed survey or a generic transient for target-of-opportunity observations with different telescopes. As a demonstration of its capabilities, we show how redback can be used to jointly fit the spectrum and photometry of a kilonova, enabling a more powerful, holistic probe into the properties of a transient. We also showcase general examples of how redback can be used as a tool to simulate transients for realistic surveys, fit models to real, simulated, or private data, multimessenger inference with gravitational waves, and serve as an end-to-end software toolkit for parameter estimation and interpreting the nature of electromagnetic transients. [ABSTRACT FROM AUTHOR]

We consider the final evolutionary stages of a neutron star–black hole pair. According to the current paradigm, such systems eventually coalesce, which in some cases is accompanied by neutron-star tidal disruption. Using analytical methods, we show that the scenario of slow (of the order of several seconds) neutron-star stripping by the black hole is also possible, depending on the system parameters (the initial masses and intrinsic angular momenta of the components, the equation of state for the neutron star). Reaching the lower mass limit (about one tenth of the solar mass), the neutron star explodes to produce a comparatively powerful electromagnetic transient. Our population calculations show that the stripping mechanism is possible in 50–90 of the cases among all coalescing neutron star–black hole pairs, depending on the model assumptions about the evolution of close binary systems (the common-envelope efficiency parameter, the supernova explosion mechanism) and the initial metallicity of the stellar population. Because of the large mass of the ejected material, the kilonova emission in this scenario has good prospects of detection. [ABSTRACT FROM AUTHOR]

Since their first discovery in the late 1960s, gamma-ray bursts have attracted an exponentially growing interest from the international community due to their central role in the most highly debated open questions of the modern research of astronomy, astrophysics, cosmology, and fundamental physics. These range from the intimate nuclear composition of high-density material within the core of ultra-dense neuron stars, to stellar evolution via the collapse of massive stars, the production and propagation of gravitational waves, as well as the exploration of the early universe by unveiling the first stars and galaxies (assessing also their evolution and cosmic re-ionization). GRBs in the past ∼50 years have stimulated the development of cutting-edge technological instruments for observations of high-energy celestial sources from space, leading to the launch and successful operations of many different scientific missions (several of them still in data-taking mode currently). In this review, we provide a brief description of the GRB-dedicated missions from space being designed and developed for the future. The list of these projects, not meant to be exhaustive, shall serve as a reference to interested readers to understand what is likely to come next to lead the further development of GRB research and the associated phenomenology. [ABSTRACT FROM AUTHOR]

According to the hypothesis that strange quark matter may be the true ground state of matter at extremely high densities, strange quark stars should be stable and could exist in the Universe. It is possible that pulsars may actually be strange stars, but not neutron stars. Here we present a short review on recent progresses in the field of strange quark stars. First, three popular phenomenological models widely used to describe strange quark matter are introduced, with special attention being paid on the corresponding equation of state in each model. Combining the equation of state with the Tolman-Oppenheimer-Volkov equations, the inner structure and mass-radius relation can be obtained for the whole sequence of strange stars. Tidal deformability and oscillations (both radial and non-radial oscillations), which are sensitive to the composition and the equations of state, are then described. Hybrid stars as a special kind of quark stars are discussed. Several other interesting aspects of strange stars are also included. For example, strong gravitational wave emissions may be generated by strange stars through various mechanisms, which may help identify strange stars via observations. Especially, close-in strange quark planets with respect to their hosts may provide a unique test for the existence of strange quark objects. Fierce electromagnetic bursts could also be generated by strange stars. The energy may come from the phase transition of neutron stars to strange stars, or from the merger of binary strange stars. The collapse of the strange star crust can also release a huge amount of energy. It is shown that strange quark stars may be involved in short gamma-ray bursts and fast radio bursts.

Based on different neutron star–white dwarf (NS–WD) population models, we investigate the prospects of gravitational-wave (GW) detections for NS–WD mergers, with the help of early warnings from two space-borne decihertz GW observatories, DO-Optimal and DECIGO. We not only give quick assessments of the GW detection rates for NS–WD mergers with the two decihertz GW detectors, but also report systematic analyses on the characteristics of GW-detectable merger events using the method of Fisher matrix. With a sufficient 1-d early-warning time, the yearly GW detection number for DO-Optimal is in the range of (1.5–1.9) × 103, while it is (3.3–4.6) × 104 for DECIGO. More importantly, our results show that most NS–WD mergers can be localized with an uncertainty of |$\mathcal {O}(10^{-2})\, \mathrm{deg}^2$|⁠. Given the NS–WD merger as a possible origin for a peculiar long-duration gamma-ray burst, GRB 211211A, followed with kilonova-like emissions, we further suggest that the GW early-warning detection would allow future electromagnetic telescopes to get prepared to follow up transients after some special NS–WD mergers. Based on our analyses, we emphasize that such a feasible 'wait-for' pattern can help to firmly identify the origin of GRB 211211A-like events in the future and bring excellent opportunities for the multimessenger astronomy. [ABSTRACT FROM AUTHOR]

GW170817 is a binary neutron star merger that exhibited a gravitational wave (GW) and a gamma-ray burst, followed by an afterglow. In this work, we estimate the Hubble constant (H 0) using broad-band afterglow emission and relativistic jet motion from the Very Long Baseline Interferometry and HST images of GW170817. Compared to previous attempts, we combine these messengers with GW in a simultaneous Bayesian fit. We probe the H 0 measurement robustness depending on the data set used, the assumed jet model, the possible presence of a late time flux excess. Using the sole GW leads to a 20 per cent error (⁠|$77^{+21}_{-10}$|   |$\rm km\, s^{-1}\, Mpc^{-1}$|⁠ , medians, 16th–84th percentiles), because of the degeneracy between viewing angle (θv) and luminosity distance (d L). The latter is reduced by the inclusion in the fit of the afterglow light curve, leading to |$H_0=96^{+13}_{-10}$|   |$\rm km\, s^{-1}\, Mpc^{-1}$|⁠ , a large value, caused by the fit preference for high viewing angles due to the possible presence of a late-time excess in the afterglow flux. Accounting for the latter by including a constant flux component at late times brings |$H_0=78.5^{+7.9}_{-6.4}$| |$\rm km\, s^{-1}\, Mpc^{-1}$|⁠. Adding the centroid motion in the analysis efficiently breaks, the d L − θv degeneracy and overcome the late-time deviations, giving |$H_0 = 69.0^{+4.4}_{-4.3}$|   |$\rm km\, s^{-1}\, Mpc^{-1}$| (in agreement with Planck and SH0ES measurements) and |$\theta _{\rm v} = 18.2^{+1.2}_{-1.5}$| °. This is valid regardless of the jet structure assumption. Our simulations show that for next GW runs radio observations are expected to provide at most few other similar events. [ABSTRACT FROM AUTHOR]

The discovery of a short gamma‐ray burst (GRB), GRB170817A, in association with a gravitational wave (GW) and a bright kilonova started a new era in the high‐energy astrophysics. The observation of GRB170817A and more recently, GRB200826A and GRB211211A, a short and a long burst, respectively, with a possible kilonova, reinforce the concern about new ways of classification. For this reason, a new machine learning technique is applied to Swift‐BAT data, searching for morphological similarities in the light curves. The resulting map is characterized by two distinct groups, although still correlated with standard T90 duration. Since a jet viewed off‐axis could explain the emission from GRB170817A, the modeling of this kind of sources is of great importance. A public code called JetFit, based on the "boosted fireball" model, is applied to fit Swift‐XRT afterglow light curves of short and long GRBs, with known red‐shift, from 2005 to 2021. JetFit does not model the flaring activity. For this purpose, a new technique to remove the time flaring phases, is developed. This analysis provides a comprehensive study of the prompt and of the afterglow phase, trough the study of the best‐fit parameters. [ABSTRACT FROM AUTHOR]

This is the first in a collection of three papers introducing the science with an ultra-violet (UV) space telescope on an approximately 130 kg small satellite with a moderately fast re-pointing capability and a real-time alert communication system approved for a Czech national space mission. The mission, called Quick Ultra-Violet Kilonova surveyor—QUVIK, will provide key follow-up capabilities to increase the discovery potential of gravitational wave observatories and future wide-field multi-wavelength surveys. The primary objective of the mission is the measurement of the UV brightness evolution of kilonovae, resulting from mergers of neutron stars, to distinguish between different explosion scenarios. The mission, which is designed to be complementary to the Ultraviolet Transient Astronomy Satellite—ULTRASAT, will also provide unique follow-up capabilities for other transients both in the near- and far-UV bands. Between the observations of transients, the satellite will target other objects described in this collection of papers, which demonstrates that a small and relatively affordable dedicated UV-space telescope can be transformative for many fields of astrophysics. [ABSTRACT FROM AUTHOR]

In some cases, the merger of two neutron stars can produce a rapidly rotating and highly magnetized millisecond magnetar. A significant proportion of the rotational energy deposited to the emerging ejecta can produce a late-time radio brightening from interacting with the ambient medium. Detection of this late-time radio emission from short GRBs can have profound implications for understanding the physics of the progenitor. We report the radio observations of five short GRBs – 050709, 061210, 100625A, 140903A, and 160821B using the legacy Giant Metrewave Radio Telescope (GMRT) at 1250, 610, and 325 MHz frequencies and the upgraded-GMRT (uGMRT) at band 5 (1050–1450 MHz) and band 4 (550–900 MHz) after ∼2–11 yr from the time of the burst. The GMRT observations at low frequencies are particularly important to detect the signature of merger ejecta emission at the peak. These observations are the most delayed searches associated with some GRBs for any late-time low-frequency emission. We find no evidence for such an emission. We find that none of these GRBs is consistent with maximally rotating magnetar with a rotational energy of |$\sim 10^{53}\, {\rm erg}$|⁠. However, magnetars with lower rotational energies cannot be completely ruled out. Despite the non-detection, our study underscores the power of radio observations in the search for magnetar signatures associated with short GRBs. However, only future radio observatories may be able to detect these signatures or put more stringent constraints on the model. [ABSTRACT FROM AUTHOR]

Precursors have been observed seconds to minutes before some short gamma-ray bursts. While the precursor origins remain unknown, one explanation relies on the resonance of neutron star pulsational modes with the tidal forces during the inspiral phase of a compact binary merger. In this paper, we present a model for short gamma-ray burst precursors that relies on tidally resonant neutron star oceans. In this scenario, the onset of tidal resonance in the crust–ocean interface mode ignites the precursor flare, possibly through the interaction between the excited neutron star ocean and the surface magnetic fields. From just the precursor total energy, the time before the main event, and a detected quasi-periodic oscillation frequency, we may constrain the binary parameters and neutron star ocean properties. Our model can immediately distinguish neutron star–black hole mergers from binary neutron star mergers without gravitational wave detection. We apply our model to GRB 211211A, the recently detected long duration short gamma-ray burst with a quasi-periodic precursor, and explore the parameters of this system. The precursor of GRB 211211A is consistent with a tidally resonant neutron star ocean explanation that requires an extreme mass ratio neutron star–black hole merger and a high-mass neutron star. While difficult to reconcile with the main gamma-ray burst and associated kilonova, our results constrain the possible precursor mechanisms in this system. A systematic study of short gamma-ray burst precursors with the model presented here can test precursor origin and probe the possible connection between gamma-ray bursts and neutron star–black hole mergers. [ABSTRACT FROM AUTHOR]

The results of a study of the short gamma-ray burst GRB 231115A in the X-ray and gamma-ray ranges are presented, based on data from the INTEGRAL and Fermi space observatories. The source of the burst is localized by the IBIS/ISGRI telescope of INTEGRAL observatory with an accuracy of , it is located in the Cigar Galaxy (M 82). Early follow-up observations of the burst localization region were carried out in the optical range with the 36-cm telescope of the ISON-Kitab observatory and the 70-cm telescope AS-32 of the Abastumani Astrophysical Observatory. The optical emission has not been detected. The proximity of the host galaxy ( Mpc) significantly limits energetics of the event ( erg) and allows us to interpret the burst as a giant flare of a previously unknown soft gamma repeater (SGR) which is an extreme manifestation of the activity of a highly magnetized neutron star (magnetar). This conclusion is confirmed by the energy spectrum atypically hard for cosmological gamma-ray bursts, as well as the absence of optical afterglow and gravitational wave signal, which should have been detected in the LIGO/Virgo/KAGRA experiments if the burst was caused by a merger of binary neutron stars. The location of the burst in the and diagrams also suggests that GRB 231115A was a magnetar giant flare. This is the first well-localized giant flare of an extragalactic SGR. [ABSTRACT FROM AUTHOR]

Before the Neil Gehrels Swift Observatory, we knew little about short-duration Gamma-ray bursts (sGRBs). Their briefness led to the suspicion that they resulted from mergers of compact objects, e.g., two neutron stars or a neutron star and a black hole. However, proof was lacking. sGRB post-prompt emission, or afterglow, was undetected; thus, we could not apply essential investigation tools. Swift was the first to pinpoint sGRB afterglows. sGRBs were found to differ from long GRBs in terms of host galaxies, offset from host, environment, energy and progenitors. The Swift UV/Optical Telescope (UVOT) has greatly contributed to these discoveries with its unique combination of fast repointing capabilities and UV sensitivity. But the long-sought proof of the sGRB–merger connection arrived in 2017. The gravitational signal GW 170817A caused by two NSs collision was associated with sGRB 170817A. Swift/UVOT discovered that its early optical emission was—unusually for GRB afterglows—thermal. It was interpreted as an emission from the merger's hot debris: the kilonova. Kilonovae have seemingly been found in other sGRBs and—puzzingly—in long GRBs. Over almost 20 years, Swift/UVOT observations have also been pivotal to understanding peculiar events. In this review, I will summarize UVOT's major contributions in the fields highlighted. [ABSTRACT FROM AUTHOR]

We introduce a new capability of the Neil Gehrels Swift Observatory, dubbed “continuous commanding,” that achieves 10 s latency response time on orbit to unscheduled target-of-opportunity requests received on the ground. We show that this will allow Swift to respond to premerger (early-warning) gravitational-wave (GW) detections, rapidly slewing the Burst Alert Telescope (BAT) across the sky to place the GW origin in the BAT field of view at or before merger time. This will dramatically increase the GW/gamma-ray burst (GRB) codetection rate and enable prompt arcminute localization of a neutron star merger. We simulate the full Swift response to a GW early-warning alert, including input sky maps produced at different early-warning times, a complete model of the Swift attitude control system, and a full accounting of the latency between the GW detectors and the spacecraft. 60 s of early warning can double the rate of a prompt GRB detection with arcminute localization, and 140 s guarantees observation anywhere on the unocculted sky, even with localization areas ≫1000 deg ^2 . While 140 s is beyond current GW detector sensitivities, 30–70 s is achievable today. We show that the detection yield is now limited by the latency of LIGO/Virgo cyberinfrastructure and motivate a focus on its reduction. Continuous commanding has been integrated as a general capability of Swift, significantly increasing its versatility in response to the growing demands of time-domain astrophysics. We demonstrate this potential on an externally triggered fast radio burst (FRB), slewing 81° across the sky, and collecting X-ray and UV photons from the source position

The recent gamma-ray burst (GRB) GRB 211211A provides the earliest (∼5 hr) data of a kilonova (KN) event, displaying bright (∼10 ^42 erg s ^−1 ) and blue early emission. Previously, this KN was explained using simplistic multicomponent fitting methods. Here, in order to understand the physical origin of the KN emission in GRB 211211A, we employ an analytic multizone model for r -process-powered KNe. We find that r -process-powered KN models alone cannot explain the fast temporal evolution and the spectral energy distribution (SED) of the observed emission. Specifically, (i) r -process models require high ejecta mass to match early luminosity, which overpredicts late-time emission, while (ii) red KN models that reproduce late emission underpredict early luminosity. We propose an alternative scenario involving early contributions from the GRB central engine via a late low-power jet, consistent with plateau emission in short GRBs and GeV emission detected by Fermi-LAT at ∼10 ^4 s after GRB 211211A. Such late central engine activity, with an energy budget of ∼a few percent of that of the prompt jet, combined with a single red KN ejecta component, can naturally explain the light curve and SED of the observed emission, with the late-jet–ejecta interaction reproducing the early blue emission and r -process heating reproducing the late red emission. This supports claims that late low-power engine activity after prompt emission may be common. We encourage early follow-up observations of future nearby GRBs and compact binary merger events to reveal more about the central engine of GRBs and r -process events.

GW230529 is the first compact binary coalescence detected by the LIGO–Virgo–KAGRA collaboration with at least one component mass confidently in the lower mass gap, corresponding to the range 3–5 M _⊙ . If interpreted as a neutron star–black hole merger, this event has the most symmetric mass ratio detected so far and therefore has a relatively high probability of producing electromagnetic (EM) emission. However, no EM counterpart has been reported. At the merger time t _0 , Swift-BAT and Fermi-GBM together covered 100% of the sky. Performing a targeted search in a time window [ t _0 − 20 s, t _0 + 20 s], we report no detection by the Swift-BAT and Fermi-GBM instruments. Combining the position-dependent γ -ray flux upper limits and the gravitational-wave posterior distribution of luminosity distance, sky localization, and inclination angle of the binary, we derive constraints on the characteristic luminosity and structure of the jet possibly launched during the merger. Assuming a top-hat jet structure, we exclude at 90% credibility the presence of a jet that has at the same time an on-axis isotropic luminosity ≳10 ^48 erg s ^−1 in the bolometric band 1 keV–10 MeV and a jet opening angle ≳15°. Similar constraints are derived by testing other assumptions about the jet structure profile. Excluding GRB 170817A, the luminosity upper limits derived here are below the luminosity of any GRB observed so far.

The bimodal distribution of the observed duration of gamma-ray bursts (GRBs) has led to the identification of two distinct progenitors; compact star mergers, comprising either two neutron stars (NSs) or an NS and a black hole, for short GRBs (SGRBs), and the so-called collapsars for long GRBs (LGRBs). It is therefore expected that formation rate (FR) of LGRBs should be similar to the cosmic star formation rate (SFR), while that of SGRBs to be delayed relative to the SFR. The localization of some LGRBs in and around the star-forming regions of host galaxies and some SGRBs away from such regions support this expectation. Another distinct feature of SGRBs is their association with gravitational-wave (GW) sources and kilonovae. However, several independent investigations of the FRs of long and short bursts, using the Efron–Petrosian non-parametric method, have shown the presence of a mild luminosity evolution, and an LGRB FR that is significantly larger than SFR at low redshift, and similar to the FR of SGRBs. In addition, the recent discovery of association of two low-redshift LGRB 211211A and LGRB 230307A with a kilonova cast doubt about their collapsar origin. In this Letter we review these results and show that our results predict that about 60% ± 5% of LGRBs with redshift less than 2 could have compact star merger as progenitors increasing the expected rate of the GW sources and kilonovae significantly. The remaining 40% ± 5% have collapsars as progenitors, with some having associated supernovae.

Long-lived massive magnetars are expected to be remnants of some binary neutron star (BNS) mergers. In this paper, we argue that the magnetic powered flaring activities of these merged magnetars would occur dominantly in their early millisecond-period-spin phase, which is in the timescale of days. Such flares endure significant absorption by the ejecta from the BNS collision, and their detectable energy range is from 0.1 to 10 MeV, in a time lag of approximately days after the merger events indicated by the gravitational wave chirps. We estimate the rate of such flares in different energy ranges, and find that there could have been 0.1–10 cases detected by Fermi/GBM. A careful search for ∼10 ms spin-period modulation in weak short gamma-ray bursts (GRBs) may identify them from the archival data. The next-generation megaelectronvolt detectors could detect them at a mildly higher rate. The recent report on the Quasi-Period-Oscillation found in two BASTE GRBs should not be considered as cases of such flares, for they were detected in a lower energy range and with a much shorter period spin modulation. [ABSTRACT FROM AUTHOR]

GRB 170817A, the first short gamma-ray burst (sGRB) to be detected in coincidence with a gravitational wave signal, demonstrated that merging binary neutron star (BNS) systems can power collimated ultra-relativistic jets, and in turn, produce sGRBs. Moreover, it revealed that sGRB jets possess an intrinsic angular structure that is imprinted in the observable prompt and afterglow emission. Advanced numerical simulations represent the leading approach to investigate the physical processes underlying the evolution of sGRB jets breaking out of post-merger environments, and thus connect the final angular structure and energetics with specific jet launching conditions. In a previous paper, we carried out the first 3D special-relativistic hydrodynamic simulations of incipient (top-hat) sGRB jets propagating across the realistic environment resulting from a general-relativistic (GR) hydrodynamic BNS merger simulation. While the earlier work marked an important step toward a consistent end-to-end description of sGRB jets from BNS mergers, those simulations did not account for the presence of magnetic fields, which are expected to play a key role. Here, we overcome this limitation, reporting the first 3D special-relativistic magnetohydrodynamic (MHD) simulation of a magnetized (structured and rotating) sGRB jet piercing through a realistic magnetized post-merger environment, wherein the initial conditions of the latter are directly imported from the outcome of a previous GRMHD BNS merger simulation. [ABSTRACT FROM AUTHOR]

We present an analytic model for measuring the jet core angle (θ c ) and viewing angle (θ obs ) of off-axis gamma-ray bursts independently of the jet angular structure outside of the core. We model the images of off-axis jets, and using this model, we show that θ obs and θ c can be measured using any two of the three following observables: the afterglow light curve, the flux-centroid motion, and the image width. The model is calibrated using 2D relativistic hydrodynamic simulations with a broad range of jet angular structures. We study the systematic errors due to the uncertainty in the jet structure and find that when using the light curve and centroid motion to determine θ obs and θ c , our formulae can be accurate to a level of 5–10 per cent and 30 per cent, respectively. In light of the Hubble tension, the systematic error in cos θ obs in GRBs originating in a binary compact object merger is of special interest. We find that the systematic uncertainty on the measurement of cos θ obs due to the unknown jet structure is smaller than 1.5 per cent for well-observed events. A similar error is expected if the microphysical parameters evolve at a level that is not easily detected by the light curve. Our result implies that this type of systematic uncertainty will not prevent measurement of H 0 to a level of 2 per cent with a sample of well-observed GW events with resolved afterglow image motion. Applying our model to the light curve and centroid motion observations of GW170817, we find |$\theta _{obs}=19.2\pm 2~\deg$| (1σ) and |$\theta _c=1.5-4~\deg$|⁠. [ABSTRACT FROM AUTHOR]

We investigate the impact of forthcoming nuclear data on the predictions of the neutron star (NS) stripping model for short gamma-ray bursts. The main area to which we pay attention is the NS crust. We show that the uncertain properties of the NS equation of state can significantly influence the stripping time t str , the main dynamical parameter of the model. Based on the known time delay ( t str ≈ 1.7 s) between the peak of the gravitational wave signal GW170817 and the detection of gamma photons from GRB170817A, we obtain new restrictions on the nuclear matter parameters, in particular, the symmetry energy slope parameter: L < 114.5 MeV. In addition, we study the process of nucleosynthesis in the outer and inner crusts of a low-mass NS. We show that the nucleosynthesis is strongly influenced by both the forthcoming nuclear data and the equation of state of the NS matter. [ABSTRACT FROM AUTHOR]

We present an analytic model for measuring the jet core angle (θ c ) and viewing angle (θ obs ) of off-axis gamma-ray bursts independently of the jet angular structure outside of the core. We model the images of off-axis jets, and using this model, we show that θ obs and θ c can be measured using any two of the three following observables: the afterglow light curve, the flux-centroid motion, and the image width. The model is calibrated using 2D relativistic hydrodynamic simulations with a broad range of jet angular structures. We study the systematic errors due to the uncertainty in the jet structure and find that when using the light curve and centroid motion to determine θ obs and θ c , our formulae can be accurate to a level of 5–10 per cent and 30 per cent, respectively. In light of the Hubble tension, the systematic error in cos θ obs in GRBs originating in a binary compact object merger is of special interest. We find that the systematic uncertainty on the measurement of cos θ obs due to the unknown jet structure is smaller than 1.5 per cent for well-observed events. A similar error is expected if the microphysical parameters evolve at a level that is not easily detected by the light curve. Our result implies that this type of systematic uncertainty will not prevent measurement of H 0 to a level of 2 per cent with a sample of well-observed GW events with resolved afterglow image motion. Applying our model to the light curve and centroid motion observations of GW170817, we find |$\theta _{obs}=19.2\pm 2~\deg$| (1σ) and |$\theta _c=1.5-4~\deg$|⁠. [ABSTRACT FROM AUTHOR]

On 2017 August 17, Advanced LIGO and Virgo observed GW170817, the first gravitational-wave (GW) signal from a binary neutron star merger. It was followed by a short-duration gamma-ray burst, GRB 170817A, and by a non-thermal afterglow emission. In this work, a combined simultaneous fit of the electromagnetic (EM, specifically, afterglow) and GW domains is implemented, both using the posterior distribution of a GW standalone analysis as prior distribution to separately process the EM data, and fitting the EM and GW domains simultaneously. These approaches coincide mathematically, as long as the actual posterior of the GW analysis, and not an approximation, is used as prior for the EM analysis. We treat the viewing angle, θ v , as shared parameter across the two domains. In the afterglow modelling with a Gaussian structured jet this parameter and the jet core angle, θc, are correlated, leading to high uncertainties on their values. The joint EM + GW analysis relaxes this degeneracy, reducing the uncertainty compared to an EM-only fit. We also apply our methodology to hypothetical GW170817-like events occurring in the next GW observing run at ∼140 and 70 Mpc. At 70 Mpc the existing EM degeneracy is broken, thanks to the inclusion of the GW domain in the analysis. At 140 Mpc, the EM-only fit cannot constrain θ v nor θc because of the lack of detections in the afterglow rising phase. Folding the GW data into the analysis leads to tighter constraints on θ v , still leaving θc unconstrained, requiring instruments with higher sensitivities, such as Athena. [ABSTRACT FROM AUTHOR]

Upcoming LIGO–Virgo–KAGRA (LVK) observational runs offer new opportunities to probe the central engines of extreme transient events. Cosmological gamma-ray bursts (GRBs) and core-collapse supernovae (CC-SNe), in particular, are believed to be powered by compact objects, i.e., a neutron star (NS) or black hole (BH). A principal distinction between an NS and BH is the energy reservoir in the angular momentum E J . Per unit mass, this reaches a few percent in a rapidly rotating NS and tens of percent in a Kerr BH, respectively. Calorimetry by E G W on a descending chirp may break the degeneracy between the two. We review this approach, anticipating new observational opportunities for planned LVK runs. GRB170817A is the first event revealing its central engine by a descending chirp in gravitational radiation. An accompanying energy output E G W ≃ 3.5 % M ⊙ c 2 is observed during GRB170817A in the aftermath of the double neutron star merger GW170817. The progenitors of normal long GRBs, on the other hand, are the rare offspring of CC-SNe of type Ib/c. Yet, the extended emission to SGRBs (SGRBEEs) shares similar durations and the same Amati-relation of the prompt GRB emission of LGRBs, pointing to a common central engine. The central engine of these extreme transient events has, hitherto, eluded EM observations alone, even when including neutrino observations, as in SN1987A. The trigger signaling the birth of the compact object and the evolution powering these events is expected to be revealed by an accompanying GW signal, perhaps similar to that of GRB170817A. For GRB170817A, E G W exceeds E J in the initial hyper-massive neutron star (HMNS) produced in the immediate aftermath of GW170817. It identifies the spin-down of a Kerr BH of mass ∼ 2.4 M ⊙ defined by the total mass of GW170817. This observation is realized in spectrograms generated by Butterfly matched filtering, a time-symmetric analysis with equal sensitivity to ascending and descending chirps, calibrated by signal injection experiments. It is implemented on a heterogeneous computing platform with synaptic parallel processing in F90/C++/C99 under bash. A statistical significance of 5.5 σ is derived from multi-messenger event timing, based on a probability of false alarm (PFA) factored over a probability p 1 = 8.3 × 10 − 4 by causality and a p-value p 2 = 4.9 × 10 − 5 of consistency between H1 and L1 observations. For upcoming observations, this approach may be applied to similar emissions from SNIb/c and GRBs in the Local Universe, upon the mass-scaling of present results by the mass of their putative black hole-central engines. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Starting with models for the compact object merger event rate, the short-duration gamma-ray burst (sGRB) luminosity function, and the Swift-BAT detector, we calculate the observed Swift sGRB rate and its uncertainty. Our probabilistic sGRB world model reproduces the observed number distributions in redshift and flux for 123 Swift-BAT detected sGRBs and can be used to predict joint sGRB/LIGO detection rates. We discuss the dependence of the rate predictions on the model parameters and explore how they vary with increasing experimental sensitivity. In particular, the number of bursts in the LIGO volume depends strongly on the parameters that govern sGRB beaming. Our results suggest that nearby sGRBs should be observed to have broader jets on average (θ jet ≳ 30°), as compared to the narrowly beamed appearance of cosmological sGRBs due to the detection-selection effect driving the observed jet angle. Assuming all sGRBs are due to compact object mergers, within a D < 200 Mpc aLIGO volume, we predict 0.18 − 0.08 + 0.19 sGRB–gravitational wave (GW) associations all sky per year for on-axis events at Swift sensitivities, increasing to 1.2 − 0.6 + 1.9 with the inclusion of off-axis events. We explore the consistency of our model with GW170817/GRB 170817A in the context of structured jets. Predictions for future experiments are made. [ABSTRACT FROM AUTHOR]

The multimessenger detection of GW170817 showed that binary neutron star (BNS) mergers are progenitors of (at least some) short gamma-ray bursts (GRBs), and that short GRB jets (and their afterglows) can have structures (and observational properties) more complex than predicted by the standard top-hat jet scenario. Indeed, the emission from the structured jet launched in GW170817 peaked in the radio band (centimeter wavelengths) at ≈100 days since merger—a timescale much longer than the typical time span of radio follow-up observations of short GRBs. Moreover, radio searches for a potential late-time radio flare from the fast tail of the neutron-rich debris that powered the kilonova associated with GW170817 (AT 2017gfo) have extended to even longer timescales (years after the merger). In light of this, here we present the results of an observational campaign targeting a sample of seven, years-old GRBs in the Swift/BAT sample with no redshift measurements and no promptly identified X-ray counterpart. Our goal is to assess whether this sample of short GRBs could harbor nearby BNS mergers, searching for the late-time radio emission expected from their ejecta. We found one radio candidate counterpart for one of the GRBs in our sample, GRB 111126A, though an origin related to emission from star formation or from an active galactic nucleus in its host galaxy cannot be excluded without further observations. [ABSTRACT FROM AUTHOR]

GRB 170817A (also GW170817) became the first binary neutron star merger event detected via gravitational waves and electromagnetic signals. Over the next 4 yr, various multiband observations have led to reimaging of the various short gamma-ray burst and interstellar medium interaction models. While these models successfully explain the observed afterglow until ∼900 days, a rebrightening or excess flux was observed in the 1 keV X-ray band after ∼1000 days. In this study, we reevaluate the jet parameters using new observations (until ∼1234 days) with a boosted fireball jet model. We study the observable effects of the counterjet for GRB 170817A, using our new afterglow code, firefly . Our results show that it is indeed possible for the observed excess to coincide with the emissions from a counterjet (∼800 days). We also computed an empirical scaling law between the jet and counterjet peak emission timescales and the observer angle. The firefly code can also track the simulated object through the observer’s sky and numerically model the apparent motion. The calculated apparent motion (≈2.6 c ) does not match the observed apparent motion (5.2 c −7.5 c ). Hence we conclude that the excess flux of GRB 170817A may not be associated with a counterjet; however, it is not enough to reject this hypothesis from the traditional counterjet visibility timescale, which predicts ≥5000 days. The apparent motion, combined with the multiband lightcurves, is needed to break the degeneracy between the geometrical parameters and the microphysical parameters of the afterglow.

GW150914 marked the start of the gravitational wave (GW) era with the direct detection of a binary black hole (BBH) merger by the LIGO-Virgo GW detectors. The event was tentatively associated with a signal detected by the Fermi Gamma-ray Burst Monitor (GBM), which hinted toward electromagnetic emission associated with compact object coalescence. The detection of a short gamma-ray burst (GRB) associated with GW170817, along with several multiwavelength detections, truly established that compact object mergers are indeed multimessenger events. The CadmiumZincTelluride Imager (CZTI) on board AstroSat can search for X-ray counterparts of the GW events and has detected over 650 GRBs in the 8.5 yr since it was launched. We present results from our searches for counterparts coincident with GW triggers from the first three LIGO-Virgo-KAGRA (LVK) GW Transient Catalogs. For 71 out of 90 GW events for which AstroSat-CZTI data was available, we undertook a systematic search for temporally coincident transients in the 20–200 keV band and detected no X-ray counterparts. We evaluate the upper limits on the maximum possible flux from the source in a 100 s window centered around each trigger, consistent with the GW localization of the event. Thanks to the sensitivity of CZTI, these upper limits are competitive with those from other spacecraft. We use these upper limits to constrain theoretical models that predict high-energy counterparts to BBH mergers. We also discuss the probability of nondetections of BBH mergers at different luminosities and the implications of such nondetections from the ongoing fourth observing run of the LVK detectors.

The duration of orbital decay induced by gravitational waves (GWs) is often the bottleneck of the evolutionary phases going from star formation to a merger. We show here that kicks imparted to the newly born compact object during the second collapse generically result in a GW merger time distribution behaving like ${dN}/d\mathrm{log}t\propto {t}^{2/7}$ at short durations, leading to ultrafast mergers. Namely, a nonnegligible fraction of neutron star binaries, formed in this way, will merge on a timescale as short as 10 Myr, and a small fraction will merge even on a timescale less than 10 kyr. The results can be applied to different types of compact binaries. We discuss here the implications for binary neutron star mergers. These include unique short gamma-ray bursts (GRBs), eccentric and misaligned mergers, r -process enrichment in the very early Universe and in highly star-forming regions, and possible radio precursors. Interestingly, we conclude that among the few hundred short GRBs detected so far, a few must have formed via this ultrafast channel.

Follow-up observations of short gamma-ray bursts (sGRBs) have continuously unveiled late extended/plateau emissions, attributed to jet launch due to late engine activity, the nature of which remains enigmatic. Observations of GW170817 have confirmed that sGRBs are linked to neutron star (NS) mergers, and discovered a kilonova (KN) transient. Nevertheless, the origin of the early blue KN in GW170817 remains unclear. Here, we investigate the propagation of late jets in the merger ejecta. By analytically modeling jet dynamics, we determine the properties of the jet-heated cocoon, and estimate its cooling emission. Our results reveal that late jets generate significantly brighter cocoons compared to prompt jets, primarily due to reduced energy loss by adiabatic cooling. Notably, with typical late jets, emission from the cocoon trapped inside the ejecta can reproduce the blue KN emission. We estimate that the forthcoming Einstein Probe mission will detect the early cocoon emission at a rate of $\sim {2.1}_{-1.6}^{+3.2}$ yr ^−1 , and that optical/UV follow-ups in the LIGO-Virgo-KAGRA O5 run will be able to detect $\sim {1.0}_{-0.7}^{+1.5}$ cocoon emission events. As an electromagnetic counterpart, this emission provides an independent tool to probe NS mergers in the Universe, complementing insights from sGRBs and gravitational waves.

The coalescence of binary neutron stars can yield the expulsion of a fast-moving, quasi-isotropic material, which may induce thermal radiation and give rise to kilonova emission. Moreover, the interaction between the ejected material and the surrounding environment generates an external shock, which can result in a long-lasting radio signal that persists for several decades following the merger. In contrast to supernova ejecta, the kilonova ejecta exhibits a relatively lesser mass and higher velocity, and its expansion may ultimately result in the ejecta density becoming so low that the medium particles can freely pass through the ejecta. It would thereby lead to a kind of incomplete sweeping on the interstellar medium. Employing a toy model, our investigation reveals that such incomplete sweeping may considerably diminish the late-time radio radiation power, irrespective of whether the binary neutron star merger results in the formation of a black hole or a neutron star. Our findings thus imply that the previously reported radio upper limits for certain short gamma-ray bursts (GRBs) may not necessarily place stringent constraints on the presence of a long-lived magnetar remnant in these short GRBs.

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 B s ≥ 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. [ABSTRACT FROM AUTHOR]

A black-hole neutron-star binary merger can lead to an electromagnetic counterpart called a kilonova if the neutron star is disrupted prior to merger. The observability of a kilonova depends on the amount of neutron star ejecta, which is sensitive to the aligned component of the black hole spin. We explore the dependence of the ejected mass on two main mechanisms that provide high black hole spin in isolated stellar binaries. When the black hole inherits a high spin from a Wolf–Rayet star that was born with least |$\sim 10{{\ \rm per\ cent}}$| of its breakup spin under weak stellar core-envelope coupling, relevant for all formation pathways, the median of the ejected mass is ≳10−2 M⊙. Though only possible for certain formation pathways, similar ejected mass results when the black hole accretes |$\gtrsim 20{{\ \rm per\ cent}}$| of its companion's envelope to gain a high spin. Together, these signatures suggest that a population analysis of black-hole neutron-star binary mergers with observed kilonovae may help distinguish between mechanisms for spin and possible formation pathways. We show that these kilonovae will be difficult to detect with current capabilities, but that future facilities, such as the Vera Rubin Observatory, can do so even if the aligned dimensionless spin of the black hole is as low as ∼0.2. Our model predicts kilonovae as bright as Mi ∼ −14.5 for an aligned black hole spin of ∼0.9 and mass ratio Q  = 3.6. [ABSTRACT FROM AUTHOR]

The binary neutron star merger observed and localized on 17 August 2017 by the LIGO and Virgo gravitational interferometers and by numerous telescopes on the ground and in orbit linked in an unambiguous way the coalescence of double neutron stars with the formation of a relativistic outflow (short gamma-ray burst GRB170817A) and of a thermal radioactive source (kilonova). The vicinity of the event (40 Mpc) made it possible to monitor the electromagnetic counterpart in detail at all wavelengths and to map its close environment in the outskirts of the lenticular galaxy NGC 4993. Radio VLBI images of GRB170817A allowed the first direct detection of superluminal motion in a GRB afterglow, pointing to a collimated ultra-relativistic jet rather than to a quasi-isotropically, mildly relativistically expanding source. The accurate spectroscopy of the kilonova at ultraviolet-to-infrared wavelengths with the X-Shooter spectrograph of the ESO Very Large Telescope showed the long-sought-after signature of rapid neutron capture process (in short: r-process) nucleosynthesis. Kilonova detection makes gravitational wave sources optimal tracers of heavy element formation sites. [ABSTRACT FROM AUTHOR]

Gamma-ray burst GRB 211211A may have been the result of a neutron star merger at ≈350 Mpc. However, none of the LIGO–Virgo detectors were operating at the time. We show that the gravitational-wave signal from a GRB 211211A-like binary neutron star inspiral in the next LIGO–Virgo–KAGRA observing run (O4) would be below the conventional detection threshold, however a coincident gamma-ray burst observation would provide necessary information to claim a statistically significant multimessenger observation. We calculate that with O4 sensitivity, approximately |$11{{\ \rm per\ cent}}$| of gamma-ray bursts within 600 Mpc will produce a confident association between the gravitational-wave binary neutron star inspiral signature and the prompt gamma-ray signature. This corresponds to a coincident detection rate of |$0.22^{+8.3}_{-0.22}\,\mathrm{yr^{-1}}$|⁠ , where the uncertainties are the 90 per cent confidence intervals arising from uncertainties in the absolute merger rate, beaming and jet-launching fractions. These increase to approximately |$34{{\ \rm per\ cent}}$| and |$0.71^{+26.8}_{-0.70}\,\mathrm{yr^{-1}}$| with proposed O5 sensitivity. We show that the above numbers do not depend significantly on the number of gravitational-wave observatories operating with the specific sensitivity. That is, the number of confident joint gamma-ray burst and gravitational-wave detections is only marginally improved with two or three detectors operating compared to a single detector. It is therefore worth considering whether one detector with sufficient sensitivity (post O4) should remain in sky-watch mode at all times to elucidate the true nature of GRB 211211A-like events, a proposal we discuss in detail. [ABSTRACT FROM AUTHOR]

Before the Neil Gehrels Swift Observatory, we knew little about short-duration Gamma-ray bursts (sGRBs). Their briefness led to the suspicion that they resulted from mergers of compact objects, e.g., two neutron stars or a neutron star and a black hole. However, proof was lacking. sGRB post-prompt emission, or afterglow, was undetected; thus, we could not apply essential investigation tools. Swift was the first to pinpoint sGRB afterglows. sGRBs were found to differ from long GRBs in terms of host galaxies, offset from host, environment, energy and progenitors. The Swift UV/Optical Telescope (UVOT) has greatly contributed to these discoveries with its unique combination of fast repointing capabilities and UV sensitivity. But the long-sought proof of the sGRB–merger connection arrived in 2017. The gravitational signal GW 170817A caused by two NSs collision was associated with sGRB 170817A. Swift/UVOT discovered that its early optical emission was—unusually for GRB afterglows—thermal. It was interpreted as an emission from the merger’s hot debris: the kilonova. Kilonovae have seemingly been found in other sGRBs and—puzzingly—in long GRBs. Over almost 20 years, Swift/UVOT observations have also been pivotal to understanding peculiar events. In this review, I will summarize UVOT’s major contributions in the fields highlighted.

Upcoming LIGO–Virgo–KAGRA (LVK) observing runs are expected to detect a variety of inspiralling gravitational-wave (GW) events that come from black hole and neutron star binary mergers. Detection of noninspiral GW sources is also anticipated. We report the discovery of a new class of noninspiral GW sources—the end states of massive stars—that can produce the brightest simulated stochastic GW burst signal in the LVK bands known to date, and could be detectable in LVK run A+. Some dying massive stars launch bipolar relativistic jets, which inflate a turbulent energetic bubble—cocoon—inside of the star. We simulate such a system using state-of-the-art 3D general relativistic magnetohydrodynamic simulations and show that these cocoons emit quasi-isotropic GW emission in the LVK band, ∼10–100 Hz, over a characteristic jet activity timescale ∼10–100 s. Our first-principles simulations show that jets exhibit a wobbling behavior, in which case cocoon-powered GWs might be detected already in LVK run A+, but it is more likely that these GWs will be detected by the third-generation GW detectors with an estimated rate of ∼10 events yr ^−1 . The detection rate drops to ∼1% of that value if all jets were to feature a traditional axisymmetric structure instead of a wobble. Accompanied by electromagnetic emission from the energetic core-collapse supernova and the cocoon, we predict that collapsars are powerful multimessenger events.

Using numerical relativity simulations with a subgrid dynamo prescription to generate strong initial magnetic fields, we investigate the possibility of launching a jet-like outflow from the hypermassive neutron star (HMNS) during the early stages of the merger, prior to the remnant’s collapse to a black hole. We demonstrate that buoyant instabilities in the strongly magnetized HMNS can lead to a periodic emission of powerful electromagnetic flares shortly after the merger. These are followed by a collimated mildly relativistic outflow. Both types of outflows feature quasiperiodic kilohertz substructure. These early-time outflows may power precursors to short-duration gamma-ray bursts (sGRBs) or in some cases the entire sGRB. While the overall temporal power spectrum we find broadly agrees with the one recently reported for quasiperiodic oscillations in the sGRB GRB910711, our simulations suggest that the periodic electromagnetic substructure is dominated by magnetohydrodynamic shearing processes rather than correlating with the corresponding postmerger gravitational-wave signal.

Particle acceleration is expected to be different between relativistic and non-relativistic collisionless shocks. We show that electromagnetic counterparts to gravitational waves (GWs), gamma-ray burst (GRB) afterglows, are ideal targets for observing trans-relativistic evolution of accelerated electron distribution because the GWs spot nearby GRBs with off-axis jets, otherwise missed in gamma-ray observations. We find that the relativistic spectral slope begins to change steeply near the peak time of the light curve and approaches the non-relativistic limit in about 10 times the peak time. The trans-relativistic evolution of the afterglow synchrotron spectrum is consistent with GRB 170817A observations within errors, and will be measurable in similar but more distant events at a GW horizon ∼200 Mpc in a denser environment. We roughly estimate that such events represent a fraction of 10–50 per cent of the GRB 170817A-like off-axis short GRBs. We also find that the spectral evolution does not depend on the jet structure if their light curves are similar to each other. [ABSTRACT FROM AUTHOR]

The collapsar model is widely accepted as one of the standard scenarios for gamma-ray bursts (GRBs). In the massive collapsar scenario, the core will collapse to a black hole (BH) surrounded by a temporary hyperaccretion disk with a very high accretion rate. The newborn BH hyperaccretion system would launch the relativistic jets via neutrino annihilation and Blandford-Znajek (BZ) mechanism. At the initial accretion stage, the accretion disk should be a neutrino-dominated accretion flow (NDAF). If the jets can break out from the envelope and circumstellar medium, then a GRB will be triggered. In this review, we summarize the theoretical progress on the multimessenger astronomy of the BH hyperaccretion in the center of collapsars. The main topics include: jet propagation in collapsar, MeV neutrinos from NDAFs and proto-neutron stars, gravitational waves from collapsars. [ABSTRACT FROM AUTHOR]

We investigate the prospects of electromagnetic follow-up observations for binary neutron star (BNS) mergers, with the help of early warnings from decihertz gravitational-wave (GW) observatories, B-DECIGO and DO-Optimal. Extending the previous work, we not only give quick assessments of joint short γ-ray burst (sGRB) detection rates for different γ-ray satellites and BNS population models, but also elaborate on the analyses and results on multiband kilonova detections for survey telescopes with different limiting magnitudes. During an assumed 4-yr mission time for decihertz GW observatories, we find that for the goals of electromagnetic follow-ups, DO-Optimal performs better than B-DECIGO as a whole on the detection rate, and has a larger detectable distance for joint sGRB/kilonova searches. Taking the lognormal population model for BNS mergers and a 1-d early-warning time as an example, we discuss the accuracy in localization and timing, as well as the redshift distributions for various synergy observations with electromagnetic facilities and decihertz GW detectors. Based on our analyses, we propose a feasible 'wait-for' pattern as a novel detecting mode for future multimessenger astrophysics. [ABSTRACT FROM AUTHOR]

Resonant shattering flares (RSFs) are bursts of gamma-rays expected to be triggered by tidal resonance of a neutron star (NS) during binary inspiral. They are strongly dependent on the magnetic field strength at the surface of the NS. By modelling these flares as being the result of multiple colliding relativistic shells launched during the resonance window, we find that the prompt non-thermal gamma-ray emission may have luminosity up to a few |$\times 10^{48}\rm{ erg\,s}^{-1}$|⁠ , and that a broad-band afterglow could be produced. We compute the expected rates of detectable RSFs using the BPASS population synthesis code, with different assumptions about the evolution of surface magnetic field strengths before merger. We find the rate of detectable RSFs to be ∼0.0001–5 per year for BHNS mergers and ∼0.0005–25 per year for NSNS mergers, with the lower bound corresponding to surface-field decay consistent with magneto-thermal evolution in purely crustal fields, while the upper bounds are for systems that have longer lived surface magnetic fields supported by flux frozen into the superconducting core. If some of the observed SGRB precursor flares are indeed RSFs, this suggests the presence of a longer lived surface field for some fraction of the NS population, and that we could expect RSFs to be the most common detectable EM counterpart to GW detections of BHNS mergers. The non-detection of an RSF prior to GRB170817A provides an upper bound on the magnetic fields of the progenitor NSs of B surf ∼ 1013.5G. [ABSTRACT FROM AUTHOR]

The detection rate of electromagnetic (EM) and gravitational wave (GW) transients is growing exponentially. As the accuracy of the transient rates will significantly improve over the coming decades, so will our understanding of their evolution through cosmic history. To this end, we present predicted rates for EM and GW transients over the age of the universe using Binary Population and Spectral Synthesis (bpass) results combined with four cosmic star formation histories (SFHs). These include a widely used empirical SFH of Madau & Dickinson and those from three cosmological simulations: MilliMillennium, EAGLE, and IllustrisTNG. We find that the choice of SFH changes our predictions: transients with short delay times are most affected by the star formation rate and change up to a factor of 2, while long delay time events tend to depend on the metallicity evolution of star formation and can change the predicted rate up to an order of magnitude. Importantly, we find that the cosmological simulations have very different metallicity evolution that cannot be reproduced by the widely used metallicity model of Langer & Norman, which impacts the binary black hole merger, stripped-envelope supernovae, and LGRBs in the local Universe most acutely. We recommend against using simple prescriptions for the metallicity evolution of the universe when predicting the rates of events that can have long delay times and that are sensitive to metallicity evolution. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]