Joaquin Pelle

Black hole impacts on accretion disks in galactic nuclei can power luminous transients, but predicting their observable signatures is challenging because the post-collision flow is highly time-dependent and inhomogeneous. We present a radiative post-processing framework for relativistic hydrodynamics simulations of black hole-disk collisions. Using physically motivated prescriptions for shock heating, optical depth via an eikonal solver, and photon escape fractions that account for advection trapping and diffusion, we predict light curves and spectral energy distributions over a range of disk densities and collision velocities. Our results indicate that the emission is dominated by the long-lived, highly super-Eddington accretion flow onto the secondary black hole, rather than by cooling of the unbound ejecta. In the parameter range explored, the luminosity can reach several times the Eddington luminosity of the secondary, and the emission is generically dominated by soft X-rays. We find that lower velocity collisions produce brighter flares, while the disk surface density mainly controls spectral evolution: low-density disks typically produce keV-peaked flares with weak spectral evolution, whereas high-density disks show softer early emission and late-time hardening. A depletion-time estimate calibrated to our results suggests characteristic durations of hours to days for intermediate-mass secondaries, and yields t_\mathrmflare ∝P_\mathrmQPE. We discuss implications for QPE-like transients and for the SMBH-binary candidate OJ 287.

To reproduce the observed spectra and light curves originated in the neighborhood of compact objects requires accurate relativistic ray-tracing codes. In this work we present \textttSkylight, a new numerical code for general-relativistic ray tracing and radiative transfer in arbitrary space-time geometries and coordinate systems. The code is capable of producing images, spectra and light curves from astrophysical models of compact objects as seen by distant observers. We incorporate two different schemes, namely Monte Carlo radiative transfer integrating geodesics from the astrophysical region to distant observers, and camera techniques with backwards integration from the observer to the emission region. The code is validated by successfully passing several test cases, among them: thin accretion disks and neutron stars hot spot emission.