Mergers Of Binary Compact Objects Research Articles

Exploring Gamma-Ray Burst Diversity: Clustering Analysis of the Emission Characteristics of Fermi- and BATSE-detected Gamma-Ray Bursts

Gamma-ray bursts (GRBs), often attributed to massive star collapse or binary compact object mergers, exhibit diverse emission characteristics hinting at multiple GRB classes based on various factors like progenitors, radiation mechanisms, and central engines. This study employs unsupervised clustering using the nested Gaussian mixture model algorithm to analyze data from Fermi and BATSE, identifying four classes (A–D) based on duration, spectral peak, and spectral index of time-integrated spectra of GRBs. Class proportions are approximately 70%, 10%, 3%, and 17%, respectively, with A and B comprising mostly long GRBs, C mainly short GRBs, and D encompassing both types. The classes are further assessed based on spectral index α, indicating distinct radiation mechanisms: α > −0.67 for photospheric emission, α ≤ −1.5 for fast-cooling synchrotron, and −1.5 < α ≤ −0.67 for slow-cooling synchrotron. Classes B and C align with photospheric emission, while A and D predominantly exhibit synchrotron radiation. Short GRBs are predominantly photospheric emission, whereas long GRBs tend to favor synchrotron emission. Overall, 63% of the total bursts exhibit α profiles indicative of synchrotron emission, with the remaining 37% associated with photospheric emission. Considering the limited data of kilonova and supernova associated with GRBs, classes are examined for progenitor origins, suggesting a hybrid nature for A and D, and collapsar and merger origins for B and C, respectively. This clustering analysis results in four GRB classes, which, upon investigation, reveal the diverse and complex nature of GRBs in terms of their radiation, duration, and progenitor.

Inferring the astrophysical population of gravitational wave sources in the presence of noise transients

ABSTRACT The global network of interferometric gravitational wave (GW) observatories (LIGO, Virgo, KAGRA) has detected and characterized nearly 100 mergers of binary compact objects. However, many more real GWs are lurking sub-threshold, which need to be sifted from terrestrial-origin noise triggers (known as glitches). Because glitches are not due to astrophysical phenomena, inference on the glitch under the assumption it has an astrophysical source (e.g. binary black hole coalescence) results in source parameters that are inconsistent with what is known about the astrophysical population. In this work, we show how one can extract unbiased population constraints from a catalogue of both real GW events and glitch contaminants by performing Bayesian inference on their source populations simultaneously. In this paper, we assume glitches come from a specific class with a well-characterized effective population (blip glitches). We also calculate posteriors on the probability of each event in the catalogue belonging to the astrophysical or glitch class, and obtain posteriors on the number of astrophysical events in the catalogue, finding it to be consistent with the actual number of events included.

Mergers of neutron stars and black holes with cores of giant stars: a population synthesis study

ABSTRACT We perform population synthesis of massive binaries to study the mergers of neutron stars (NSs) and black holes (BHs) with the cores of their giant secondaries during common envelope evolution (CEE). We use different values of the efficiency parameter αCE in the framework of the energy formalism for traditional CEE (αCE ≤ 1) and including additional energy sources to unbind the envelope (αCE &amp;gt; 1). We constrain the possible values of αCE by comparing the results of our simulations with local rate densities of binary compact object mergers as inferred from gravitational-wave observations. We find two main evolutionary pathways of binary systems that result in NS–core mergers, while only one of them can also lead to the merger of a BH with the core of the giant star. We explore the zero-age main-sequence statistical properties of systems that result in NS/BH–core mergers and find that the two evolutionary channels correspond to a bimodal distribution of orbital separations. We estimate the percentage of the mergers’ event rates relative to core-collapse supernovae. We include the effect of mass accreted by the NS/BH during CEE in a separate set of simulations and find that it does not affect the mergers’ event rates.

Modelling the host galaxies of binary compact object mergers with observational scaling relations

ABSTRACT The merger rate density evolution of binary compact objects and the properties of their host galaxies carry crucial information to understand the sources of gravitational waves. Here, we present galaxy$\mathcal {R}$ate, a new code that estimates the merger rate density of binary compact objects and the properties of their host galaxies, based on observational scaling relations. We generate our synthetic galaxies according to the galaxy stellar mass function. We estimate the metallicity according to both the mass–metallicity relation (MZR) and the fundamental metallicity relation (FMR). Also, we take into account galaxy–galaxy mergers and the evolution of the galaxy properties from the formation to the merger of the binary compact object. We find that the merger rate density changes dramatically depending on the choice of the star-forming galaxy main sequence, especially in the case of binary black holes (BBHs) and black hole neutron star systems (BHNSs). The slope of the merger rate density of BBHs and BHNSs is steeper if we assume the MZR with respect to the FMR, because the latter predicts a shallower decrease of metallicity with redshift. In contrast, binary neutron stars (BNSs) are only mildly affected by both the galaxy main sequence and metallicity relation. Overall, BBHs and BHNSs tend to form in low-mass metal-poor galaxies and merge in high-mass metal-rich galaxies, while BNSs form and merge in massive galaxies. We predict that passive galaxies host at least ∼5–10 per cent, ∼15–25 per cent, and ∼15–35 per cent of all BNS, BHNS, and BBH mergers in the local Universe.

Properties of rotating neutron stars in light of binary compact object mergers

Properties of rotating neutron stars in light of binary compact object mergers

Anisotropic Multimessenger Signals from Black Hole Neutrino-dominated Accretion Flows with Outflows in Binary Compact Object Mergers

Abstract A black hole (BH) hyperaccretion system might be born after the merger of a BH and a neutron star (NS) or a binary NS (BNS). In the case of a high mass accretion rate, the hyperaccretion disk is in a state of neutrino-dominated accretion flow (NDAF) and emits numerous anisotropic MeV neutrinos. Only a small fraction of these neutrinos annihilates in the space outside of the disk and then launches ultrarelativistic jets that break away from the merger ejecta to power gamma-ray bursts. Mergers and their remnants are generally considered sources of gravitational waves (GWs), neutrinos, and kilonovae. Anisotropic neutrino emission and anisotropic high-velocity material outflows from central BH–NDAF systems can also trigger strong GWs and luminous disk-outflow-driven (DOD) kilonovae, respectively. In this paper, the anisotropic multimessenger signals from NDAFs with outflows, including DOD kilonovae, MeV neutrinos, and GWs, are presented. According to the results, the typical AB magnitude of the DOD kilonovae is lower than that of astronomical transient AT 2017gfo at the same distance, and it decreases with increasing viewing angles and its anisotropy is not sensitive to the outflow mass distribution but mainly determined by the velocity distribution. Since neutrinos with ≳10 MeV are mainly produced in the inner region of the disk, they will be dramatically deflected to a large viewing angle by relativity effects. Moreover, the strains of GWs induced by anisotropic neutrinos increase with increasing viewing angles. The accumulation of multimessenger detection of the BNS/BH–NS mergers with different viewing angles might further verify the existence of NDAFs with outflows.

Optimized localization for gravitational waves from merging binaries

ABSTRACT The Advanced LIGO and Virgo gravitational-wave observatories have opened a new window with which to study the inspiral and mergers of binary compact objects. These observations are most powerful when coordinated with multimessenger observations. This was underlined by the first observation of a binary neutron star merger GW170817, coincident with a short gamma-ray burst, GRB170817A, and the identification of the host galaxy NGC 4993 from the optical counterpart AT2017gfo. Finding the fast-fading optical counterpart critically depends on the rapid production of a sky map based on LIGO/Virgo data. Currently, a rapid initial sky map is produced, followed by a more accurate, high-latency, ${\gtrsim}{12}\, {\rm h}$ sky map. We study optimization choices of the Bayesian prior and signal model, which can be used alongside other approaches such as reduced order quadrature. We find these yield up to a $60{{\ \rm per\ cent}}$ reduction in the time required to produce the high-latency localization for binary neutron star mergers.

Inference of neutrino flavor evolution through data assimilation and neural differential equations

The evolution of neutrino flavor in dense environments such as core-collapse supernovae and binary compact object mergers constitutes an important and unsolved problem. Its solution has potential implications for the dynamics and heavy-element nucleosynthesis in these environments. In this paper, we build upon recent work to explore inference-based techniques for estimation of model parameters and neutrino flavor evolution histories. We combine data assimilation, ordinary differential equation solvers, and neural networks to craft an inference approach tailored for non-linear dynamical systems. Using this architecture, and a simple two-neutrino, two-flavor model, we test various optimization algorithms with the help of four experimental setups. We find that employing this new architecture, together with evolutionary optimization algorithms, accurately captures flavor histories in the four experiments. This work provides more options for extending inference techniques to large numbers of neutrinos.

Precursors in Short Gamma-Ray Bursts as a Possible Probe of Progenitors

Abstract We extract 18 candidate short gamma-ray bursts (SGRBs) with precursors from 660 SGRBs observed by the Fermi and Swift satellites, and carry out a comprehensive analysis of their temporal and spectral features. We obtain the following results: (1) for a large fraction of candidates, the main burst durations are longer than their precursor durations, comparable to their quiescent times from the end of precursors to the beginning of their main bursts. (2) The average flux of precursors tends to increase as their main bursts brighten. (3) As seen from the distributions of hardness ratio and spectral fitting, the precursors are slightly spectrally softer with respect to the main bursts. Moreover, a significant portion of precursors and all main bursts favor a non-thermal spectrum. (4) The precursors might be a probe of the progenitor properties of SGRBs such as the magnetic field strength and the crustal equation of state if they arise from some processes before mergers of binary compact objects rather than post-merger processes.

How to understand high-energy gamma-ray bursts?

Gamma-ray bursts (GRBs) are short-duration flashes of gamma rays occurring at cosmological distances and the most violent explosive phenomena since the cosmic big bang. GRBs were accidentally discovered by Vela military satellites of United States in 1967. The BATSE instrument onboard the Compton Space Gamma-Ray Observatory found two types of GRBs, long-duration ($T_{90}>$ 2s) and short-duration ($T_{90} Science as one of the top ten scientific breakthroughs of the year. In 1999, long GRBs were found to be associated with the birth of stellar mass black holes. In 2003, the HETE-II satellite discovered the first direct association of a long GRB with a type Ic supernova, confirming that long GRBs are linked to the core collapse of massive stars. Science ranked these two major advances as one of the worlds top ten scientific breakthroughs of the year in 1999 and 2003, respectively. In 2005, the Swift satellite made the first accurate localization of short GRBs, leading to the discoveries of afterglows, host galaxies and redshifts of short GRBs. These observations provide indirect evidence that short GRBs originate from mergers of binary systems of compact objects (at least including one neutron star) at cosmological distances. In particular, on 2017 August 17, the LIGO/Virgo gravitational wave (GW) detectors, for the first time, discovered a GW event from a binary neutron star (BNS) merger, GW170817. About 1.74 s after the merger, Fermi/GBM detected a short gamma-ray burst (named GRB170817A). Subsequently, many ground-based and space-based telescopes detected X-ray, ultraviolet, optical, nearly infrared, and radio counterparts to GW170817, especially including a multi-wavelength kilonova (named AT2017gfo). These discoveries mark the beginning of a new era of multi-messenger astronomy. To summarize, all the observations have shown that long bursts originate from the core collapse of massive stars and short bursts originate from the mergers of binary compact objects (at least including one neutron star); besides prompt gamma-ray emission, the sources of GRBs produce X-ray, optical and radio afterglows in timescales of weeks, months and years after the burst trigger, respectively. Theoretically, prompt gamma-ray emissions of GRBs are thought to arise from some energy dissipation processes in the interiors of relativistic jets and multi-wavelength afterglows arise from forward shocks due to collisions between the jets and their ambient media. Therefore, GRBs are not only astronomical laboratories of studying extremely physical phenomena (e.g., newborn compact objects including stellar-mass black holes and neutron stars, gravitational waves, ultra-high-energy cosmic rays, and high-energy neutrinos) and of testing the basic physical principles with high accuracy, but also become an important probe of the star formation and evolution in the early universe, high-redshift galaxies, and cosmology. GRBs now are a multidisciplinary field (including astronomy, cosmology, and physics) and thus one of the most competitive fundamental research fields. In this paper, we review recent researches of GRBs and electromagnetic counterparts to gravitational waves, by focusing on the relevant key scientific issues, and discuss how to seize the opportunity to plan the interdisciplinary strategy based on the development trend and the research foundation in China, to maximize domestic scientific equipment achievements, and to enhance Chinese international influence in this field.

Gamma-Ray Burst Optical Afterglows with Two-component Jets: Polarization Evolution Revisited

Abstract Gamma-ray bursts have been widely argued to originate from binary compact object mergers or core collapse of massive stars. Jets from these systems may have two components: an inner, narrow sub-jet and an outer, wider sub-jet. Such a jet subsequently interacts with its ambient gas, leading to a reverse shock (RS) and a forward shock. The magnetic field in the narrow sub-jet is very likely to be mixed by an ordered component and a random component during the afterglow phase. In this paper, we calculate light curves and polarization evolution of optical afterglows with this mixed magnetic field in the RS region of the narrow sub-jet in a two-component jet model. The resultant light curve has two peaks: an early peak arising from the narrow sub-jet and a late-time rebrightening due to the wider sub-jet. We find the polarization degree (PD) evolution under such a mixed magnetic field confined in the shock plane is very similar to that under the purely ordered magnetic field condition. The two-dimensional “mixed” magnetic fields confined in the shock plane are essentially the ordered magnetic fields only with different configurations. The position angle (PA) of the two-component jet can change gradually or abruptly by 90°. In particular, an abrupt 90° change of the PA occurs when the PD changes from its decline phase to the rise phase.

Detecting Binary Compact-object Mergers with Gravitational Waves: Understanding and Improving the Sensitivity of the PyCBC Search

Abstract We present an improved search for binary compact-object mergers using a network of ground-based gravitational-wave detectors. We model a volumetric, isotropic source population and incorporate the resulting distribution over signal amplitude, time delay, and coalescence phase into the ranking of candidate events. We describe an improved modeling of the background distribution, and demonstrate incorporating a prior model of the binary mass distribution in the ranking of candidate events. We find an ∼ 10 % and ∼ 20 % increase in detection volume for simulated binary neutron star and neutron star black hole systems, respectively, corresponding to a reduction of the false alarm rates assigned to signals by between one and two orders of magnitude.

Possible Correlations between the Emission Properties of SGRBs and Their Offsets from the Host Galaxies

Abstract Short gamma-ray bursts (SGRBs) are widely believed to be from mergers of binary compact objects involving at least one neutron star and hence have a broad range of spatial offsets from their host galaxies. In this work, we search for possible correlations between the emission properties of 18 SGRBs and their offsets from the host galaxies. The SGRBs with and without extended emission do not show significant differences between their offset distributions, in agreement with some previous works. There are, however, possible correlations between the optical and X-ray afterglow emission and the offsets. The underlying physical origins are examined.

Numerical relativity simulations of thick accretion disks around tilted Kerr black holes

In this work we present 3D numerical relativity simulations of thick accretion disks around tilted Kerr BH. We investigate the evolution of three different initial disk models with a range of initial black hole spin magnitudes and tilt angles. For all the disk-to-black hole mass ratios considered (0.044-0.16) we observe significant black hole precession and nutation during the evolution. This indicates that for such mass ratios, neglecting the self-gravity of the disks by evolving them in a fixed background black hole spacetime is not justified. We find that the two more massive models are unstable against the Papaloizou-Pringle (PP) instability and that those PP-unstable models remain unstable for all initial spins and tilt angles considered, showing that the development of the instability is a very robust feature of such PP-unstable disks. Our lightest model, which is the most astrophysically favorable outcome of mergers of binary compact objects, is stable. The tilt between the black hole spin and the disk is strongly modulated during the growth of the PP instability, causing a partial global realignment of black hole spin and disk angular momentum in the most massive model with constant specific angular momentum l. For the model with non-constant l-profile we observe a long-lived m=1 non-axisymmetric structure which shows strong oscillations of the tilt angle in the inner regions of the disk. This effect might be connected to the development of Kozai-Lidov oscillations. Our simulations also confirm earlier findings that the development of the PP instability causes the long-term emission of large amplitude gravitational waves, predominantly for the l=m=2 multipole mode. The imprint of the BH precession on the gravitational waves from tilted BH-torus systems remains an interesting open issue that would require significantly longer simulations than those presented in this work.

Origins of short gamma-ray bursts deduced from offsets in their host galaxies revisited

The spatial distribution of short Gamma-ray bursts (GRBs) in their host galaxies provides us with an opportunity to investigate their origins. Based on the currently observed distribution of short GRBs relative to their host galaxies, we obtain the fraction of the component that traces the mergers of binary compact objects and the one that traces star formation rate (such as massive stars) in early- and late-type host galaxies. From the analysis of projected offset distribution and only based on population synthesis and massive star models, we find that the fraction of massive stars is 0.37+0.42−0.37 with an error at the 1σ level for a sample with 22 short GRBs in the literature. From these results, it is hard to accept that the origin of short GRBs with observed statistics is well described by current models using only the offset distribution. The uncertainties in observational localizations of short GRBs also strongly affect the resulting fraction.

Hydrodynamics in full general relativity with conservative adaptive mesh refinement

There is great interest in numerical relativity simulations involving matter due to the likelihood that binary compact objects involving neutron stars will be detected by gravitational wave observatories in the coming years, as well as to the possibility that binary compact object mergers could explain short-duration gamma-ray bursts. We present a code designed for simulations of hydrodynamics coupled to the Einstein field equations targeted toward such applications. This code has recently been used to study eccentric mergers of black hole-neutron star binaries. We evolve the fluid conservatively using high-resolution shock-capturing methods, while the field equations are solved in the generalized-harmonic formulation with finite differences. In order to resolve the various scales that may arise, we use adaptive mesh refinement (AMR) with grid hierarchies based on truncation error estimates. A noteworthy feature of this code is the implementation of the flux correction algorithm of Berger and Colella to ensure that the conservative nature of fluid advection is respected across AMR boundaries. We present various tests to compare the performance of different limiters and flux calculation methods, as well as to demonstrate the utility of AMR flux corrections.

Gravitational waves and non-axisymmetric oscillation modes in mergers of compact object binaries

We study the excitation of nonaxisymmetric modes in the post-merger phase of binary compact object mergers and the associated gravitational wave emission. Our analysis is based on general-relativistic simulations, in the spatial conformal flatness approximation, using smoothed-particle-hydrodynamics for the evolution of matter, and we use a set of equal and unequal mass models, described by two nonzero-temperature hadronic equations of state and by one strange star equation of state. Through Fourier transforms of the evolution of matter variables, we can identify a number of oscillation modes, as well as several nonlinear components (combination frequencies). We focus on the dominant m=2 mode, which forms a triplet with two nonlinear components that are the result of coupling to the quasiradial mode. A corresponding triplet of frequencies is identified in the gravitational wave spectrum, when the individual masses of the compact objects are in the most likely range of 1.2 to 1.35 $M_\odot$. We can thus associate, through direct analysis of the dynamics of the fluid, a specific frequency peak in the gravitational wave spectrum with the nonlinear component resulting from the difference between the m=2 mode and the quasiradial mode. Once such observations become available, both the m=2 and quasiradial mode frequencies could be extracted, allowing for the application of gravitational-wave asteroseismology to the post-merger remnant and leading to tight constraints on the equation of state of high-density matter.

Mergers of Binary Compact Objects

We work out the effects of hypercritical accretion, which transfers mass from the secondary to the primary (first-born) neutron star (NS) in a binary, showing that the mass of the primary would end up 0.6 M_sun greater than the secondary NS in contradiction with the very nearly equal masses of the two NS's in binaries measured to date. We discuss that the primary NS evolves into a low-mass black-hole (LMBH) due to the hypercritical accretion. Using a flat distribution in the mass ratio q (q = M_secondary/M_primary), we calculated a ratio of LMBH-NS and NS-NS systems to be 5, in rough agreement with Pinsonneault and Stanek (2006). These authors emphasize the importance of twins, which we discuss. The two NS's in the twins would be close in mass and further increase the number of mergings.