Constraints on dark matter in rotating black hole halos

Researchers have explored how the presence of a Hernquist-type dark matter halo affects the optical properties of a rotating Kerr black hole. The study focused on the spacetime geometry generated by this configuration, deriving the null geodesic equations and effective potentials. This approach allowed for the analysis of three-dimensional photon trajectories around the event horizon and ergoregion, as well as the calculation of critical impact parameters for unstable spherical photon orbits. The team constructed the black hole shadow contours for a distant observer, finding that the rotation parameter primarily shifts and distorts the shadow. However, the presence of the Hernquist dark matter halo significantly increases the photon capture region and, consequently, the apparent size of the shadow. By comparing the area-equivalent shadow diameter with Event Horizon Telescope (EHT) measurements for Sgr A* and M87*, they were able to establish constraints on the dimensionless halo parameter, $\hat{\rho}=M^2\rho$. The strongest restrictions come from Sgr A*, with values of $\hat{\rho}\sim(2.7-3.8)\times10^{-3}$ at $1\sigma$ and $\hat{\rho}\sim(4.1-5.2)\times10^{-3}$ at $2\sigma$. In addition to the shadow analysis, the study examined gravitational lensing in both the strong-field and weak-field regimes. In the strong-field regime, the halo shifts the unstable photon orbit and critical impact parameter, influencing the logarithmic deflection angle and the position of relativistic images. In the weak-field regime, the halo contributes to the leading bending angle and amplifies deviations from the Kerr metric as $\rho$ increases. Using the Einstein ring of ESO325-G004, further constraints were obtained for the parameter $\hat{\rho}$: $0\leq\hat{\rho}\lesssim0.00939$ at $1\sigma$ and $0\leq\hat{\rho}\lesssim0.01963$ at $2\sigma$.

Tilted Accretion Disks Modeled in Full Kerr Spacetime

Researchers have developed an equation to describe the dynamics of thin, viscous accretion disks around compact objects within the full Kerr spacetime. This formulation is valid for all values of the Kerr parameter $a$, enabling the study of both Kerr black holes ($0 < a \le 1$) and Kerr naked singularities ($a > 1$). The model incorporates exact Keplerian and Lense-Thirring precession frequencies, analytically deriving radial disk tilt profiles without recourse to slow-spin or weak-field approximations. Numerical solutions of these equations, obtained under realistic boundary conditions, reveal significant deviations from slow-spin approximations, particularly in the inner disk where relativistic effects dominate. In the diffusive regime, the study finds that for Kerr naked singularities, the tilt profile exhibits distinct inner hump(s) near the radius where the specific angular momentum vanishes—a feature absent in Kerr black holes. Considering the tilt in the inner disk could significantly influence the interpretation of observed X-ray spectral, timing, and polarization features. These observations are crucial for probing the strong gravity regime and inferring the spin of the central object. While the distinct hump feature alone does not uniquely distinguish Kerr black holes from Kerr naked singularities, its interpretation in conjunction with disk regime constraints may provide an observational handle on the nature of the accreting collapsed object.

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.