New Weyl Law for Quasinormal Modes of Schwarzschild Black Holes

Researchers have developed a new Weyl law to quantify the quasinormal modes (QNM) of Schwarzschild black holes. These QNMs are the "fingerprints" of black hole perturbations, analogous to the vibrations of a bell, and their study is crucial for understanding the stability and dynamics of these astrophysical objects. The advance focuses on QNMs with energies near the threshold and high angular momentum, providing a more complete description of their spectral distribution. To achieve this, a new pseudodifferential operator calculus has been introduced, specifically designed for semiclassical spectral problems near threshold energies. This formalism allows for the combination of elliptic theory with the complex scaling method, leading to uniform resolvent estimates near zero energy. These estimates are applicable to operators that behave, at infinity, like a semiclassical Schrödinger operator with a repulsive inverse-square potential. Applying these methods to the Regge-Wheeler potential, which describes perturbations of Schwarzschild black holes, the results indicate the absence of high angular momentum QNMs from a disk whose radius grows linearly with angular momentum. Combined with previous asymptotic descriptions of Schwarzschild QNMs, this work shows that the number of QNMs contained in a small sector below the real axis and with modulus bounded by λ grows as Cλ³. Furthermore, the study explored the effect of cutting off the Schwarzschild resolvent away from the event horizon, concluding that such a cutoff does not lead to any pole cancellations. This theoretical development is fundamental for gravitational wave astrophysics, as a precise understanding of QNMs is essential for interpreting signals from black hole coalescences detected by observatories like LIGO and Virgo. The ability to predict and characterize these modes with greater accuracy enhances our capacity to test general relativity in strong-field environments and to explore the nature of quantum gravity.

New Swift-BAT Software Localizes Gamma-Ray Bursts During Slew Maneuvers

Scientists have developed BAT-GLIMPSE, a new open-source software tool that enables the Burst Alert Telescope (BAT) aboard the Swift observatory to localize gamma-ray bursts (GRBs) even when the spacecraft is performing slew maneuvers. Previously, Swift-BAT's onboard triggering system was disabled during these movements, preventing autonomous detection and localization of transients during these intervals. BAT-GLIMPSE employs coded-mask imaging and mosaic techniques, automatically selecting the appropriate analysis based on spacecraft attitude, thereby enabling searches for transient sources during both pointing observations and spacecraft slews. The performance of BAT-GLIMPSE was validated on a sample of 66 previously reported GRBs. The software successfully recovered arcminute positions for 43 of these events, with typical offsets of approximately 5 arcminutes, consistent with published localizations. Notably, approximately 88% of the GRBs that occurred during spacecraft slews were recovered through BAT-GLIMPSE's imaging or mosaic analyses. This demonstrates the new system's ability to fill a critical gap in GRB detection. BAT-GLIMPSE played a crucial role during the fourth LIGO-Virgo-KAGRA observing run (O4) in the search for gamma-ray counterparts to gravitational waves, particularly in response to pre-merger alerts that triggered Swift spacecraft slews with extremely low latency. Operating synergistically with NITRATES, another analysis tool, BAT-GLIMPSE is estimated to double Swift-BAT's onboard arcminute-localization rate, unlocking the full potential of the Swift mission for time-domain and multi-messenger astrophysics.

SACRA-K: New Numerical Relativity Code for Astrophysical Simulations

Researchers have developed SACRA-K, a new numerical relativity code designed to simulate extreme astrophysical events such as the merger of black holes and neutron stars. This code, an adaptation to C++ with the Kokkos library from the previous Fortran code SACRA-MPI, retains the original physics and numerical methods, including BSSN spacetime evolution with Z4c constraint propagation and Berger-Oliger adaptive mesh refinement. The main novelty of SACRA-K is its "performance portability" across different hardware architectures, allowing it to leverage the power of graphics processing units (GPUs) and accelerated processing units (APUs). SACRA-K was validated by comparing it against SACRA-MPI across various configurations, such as binary black hole systems, black hole-neutron star systems, and binary neutron star systems. The results show that discrepancies in the generated gravitational waveforms are well below the variability observed among independent codes and resolution-dependent variations within a given code. Furthermore, these differences remain at or below the distinguishability threshold of current gravitational-wave detectors. The code also preserves π symmetry at the bitwise level and exhibits second-order convergence in the gravitational wave phase during neutron star mergers. In the smallest test configurations, SACRA-K proved to be approximately an order of magnitude faster on NVIDIA A100 GPU clusters or AMD MI300A APU clusters than Fortran SACRA-MPI on CPU clusters. The team has successfully scaled SACRA-K's performance up to 256 accelerator devices, highlighting its ability to exploit the massive parallelism of modern architectures. This advancement is crucial for numerical astrophysics, enabling faster and more efficient simulations of gravitational-wave-generating phenomena, thus facilitating a better interpretation of observational data from detectors like LIGO and Virgo.

New Gravitational Wave Model Improves Orbital Eccentricity Detection

A new waveform model, SEOBNRv6EHM, has been developed to more accurately analyze gravitational waves from eccentric compact binaries. Orbital eccentricity is a key indicator of the formation channels and astrophysical environments of these systems, making its correct inference crucial. This advancement overcomes the limitations of previous models such as SEOBNRv5EHM and TEOBResumS-Dalí, which showed biases in estimating eccentricity, masses, and spins in complex configurations. The research team applied SEOBNRv6EHM to 26 gravitational wave events detected by the LIGO-Virgo-KAGRA collaboration during their O1-O4 observing runs, including binary black hole mergers, neutron star-black hole systems, and binary neutron stars. They identified five events with moderate support for eccentricity over the quasi-circular precessing spin hypothesis, with Bayes factors $\log_{10} \mathcal{B}^{\text{EAS}}_{\text{QCP}} > 0.5$. Furthermore, the model is applicable to generic planar binaries, allowing for the re-analysis of five high-mass events under the consideration of unbound initial conditions. For three of these events, including GW190521 (previously suggested as a dynamical capture), a direct capture configuration was found to be comparable or marginally favored over the eccentric aligned-spin and quasi-circular precessing-spin hypotheses, with Bayes factors $\log_{10}\mathcal{B}^{\rm unbound}_{\rm QCP} \approx 0.2-0.6$ for GW190521. However, the recovered configurations are not astrophysically realistic and cannot be confidently distinguished from highly eccentric bound orbits, thus these results do not confirm an unbound origin. SEOBNRv6EHM is approximately three times faster in parameter estimation analyses than its predecessor, SEOBNRv5EHM, while also improving waveform accuracy, facilitating efficient and large-scale inferences with eccentric waveforms.

Gravitational waves from binary black holes could reveal dark matter

Scientists have proposed a new model that would allow for the detection of dark matter from gravitational waves emitted by merging black holes. This approach suggests that the characteristics of these waves, detectable by observatories such as LIGO and Virgo, could contain distinctive "fingerprints" of the interaction between black holes and the surrounding dark matter. Dark matter, which constitutes approximately 27% of the universe, does not interact with light or other forms of electromagnetic radiation, making it extremely difficult to detect directly. Therefore, its study relies primarily on its gravitational effects. The model focuses on how dark matter could alter the orbital dynamics of black holes before their merger. If black holes are immersed in a dense halo of dark matter, it could exert a frictional force on them, subtly modifying the phase and amplitude of the emitted gravitational waves. These modifications would be small but, in principle, detectable with current and future detector sensitivity. The proposal opens a new window for the search for dark matter, complementing traditional methods based on direct particle detection or the observation of large-scale gravitational effects in galaxies and clusters. The ability to discern these small perturbations in gravitational wave signals will require very precise data analysis and comparison with detailed theoretical models of black hole mergers in the absence of dark matter. If such signatures were detected, it would not only confirm the existence of dark matter but also provide crucial information about its properties, such as its local density and its interaction with gravity in extreme environments. This method could offer a unique perspective on the nature of one of the greatest unknowns in modern physics.