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Abstract This study investigates the origins of GW230529, delving into its formation from massive stars within isolated binary systems. Utilizing population-synthesis models, we present compelling evidence that the neutron star component forms second. However, the event’s low signal-to-noise ratio introduces complexities in identifying the underlying physical mechanisms driving its formation. Augmenting our analysis with insights from numerical relativity, we estimate the final black hole mass and spin to be approximately 5.3M_⊙and 0.53, respectively. Furthermore, we employ the obtained posterior samples to calculate the ejecta mass and kilonova light curves resulting fromr-process nucleosynthesis. We find the ejecta mass to be within 0–0.06M_⊙, contingent on the neutron star equation of state. The peak brightness of the kilonova light curves indicates that targeted follow-up observations with a Rubin-like observatory may have detected this emission. 

Abstract We study the ringdown signal of black holes formed in prompt-collapse binary neutron star mergers. We analyze data from 47 numerical relativity simulations. We show that the $(\ell =2,m=2)$and $(\ell =2,m=1)$multipoles of the gravitational wave signal are well fitted by decaying damped exponentials, as predicted by black-hole perturbation theory. We show that the ratio of the amplitude in the two modes depends on the progenitor binary mass ratioqand reduced tidal parameter $\tilde{\mathrm{\Lambda }}$. Unfortunately, the numerical uncertainty in our data is too large to fully quantify this dependency. If confirmed, these results will enable novel tests of general relativity in the presence of matter with next-generation gravitational-wave observatories. 

A direct detection of black hole formation in neutron star mergers would provide invaluable information about matter in neutron star cores and finite temperature effects on the nuclear equation of state. We study black hole formation in neutron star mergers using a set of 190 numerical relativity simulations consisting of long-lived and black-hole-forming remnants. The postmerger gravitational-wave spectrum of a long-lived remnant has greatly reduced power at a frequency f greater than fpeak, for f ≳ 4 kHz, with fpeak in [2.5, 4] kHz. On the other hand, black-hole-forming remnants exhibit excess power in the same large f region and manifest exponential damping in the time domain characteristic of a quasinormal mode. We demonstrate that the gravitational-wave signal from a collapsed remnant is indeed a quasinormal ringing. We report on the opportunity for direct detections of black hole formation with next-generation gravitational-wave detectors such as Cosmic Explorer and Einstein Telescope and set forth the tantalizing prospect of such observations up to a distance of 100 Mpc for an optimally oriented and located source with an SNR of 4. 

General relativity (GR) has proven to be a highly successful theory of gravity since its inception. The theory has thrivingly passed numerous experimental tests, predominantly in weak gravity, low relative speeds, and linear regimes, but also in the strong-field and very low-speed regimes with binary pulsars. Observable gravitational waves (GWs) originate from regions of spacetime where gravity is extremely strong, making them a unique tool for testing GR, in previously inaccessible regions of large curvature, relativistic speeds, and strong gravity. Since their first detection, GWs have been extensively used to test GR, but no deviations have been found so far. Given GR’s tremendous success in explaining current astronomical observations and laboratory experiments, accepting any deviation from it requires a very high level of statistical confidence and consistency of the deviation across GW sources. In this paper, we compile a comprehensive list of potential causes that can lead to a false identification of a GR violation in standard tests of GR on data from current and future ground-based GW detectors. These causes include detector noise, signal overlaps, gaps in the data, detector calibration, source model inaccuracy, missing physics in the source and in the underlying environment model, source misidentification, and mismodeling of the astrophysical population. We also provide a rough estimate of when each of these causes will become important for tests of GR for different detector sensitivities. We argue that each of these causes should be thoroughly investigated, quantified, and ruled out before claiming a GR violation in GW observations. 

Abstract 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 3–5M_⊙. 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 timet₀, Swift-BAT and Fermi-GBM together covered 100% of the sky. Performing a targeted search in a time window [t₀− 20 s,t₀+ 20 s], we report no detection by the Swift-BAT and Fermi-GBM instruments. Combining the position-dependentγ-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 atop-hatjet structure, we exclude at 90% credibility the presence of a jet that has at the same time an on-axis isotropic luminosity ≳10⁴⁸erg s⁻¹in 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.