Electromagnetic Signatures from the Tidal Tail of a Black Hole—Neutron Star Merger

• Published in: The Astrophysical journal Vol. 915; no. 1; pp. 69 - 89
• Main Authors: Darbha, Siva, Kasen, Daniel, Foucart, Francois, Price, Daniel J.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.07.2021; IOP Publishing
• Subjects: Asphericity; ASTRONOMY AND ASTROPHYSICS; Astrophysics; Black holes; Computational fluid dynamics; Ejecta; Emission; Fluid flow; Gravitational waves; Ground-based observation; Heating; Heavy elements; Hydrodynamical simulations; Hydrodynamics; Light curve; Luminosity; Neutron stars; Nuclear fusion; Numerical relativity; Observatories; Opacity; Parameterization; R-process; Radiative transfer; Radiative transfer simulations; Relativity; Signatures; Star mergers; Stars & galaxies; Stellar mass black holes; Transient sources; Viewing
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abff5d

Abstract Black hole—neutron star (BH–NS) mergers are a major target for ground-based gravitational wave observatories. A merger can also produce an electromagnetic counterpart (a kilonova) if it ejects neutron-rich matter that assembles into heavy elements through r -process nucleosynthesis. We study the kilonova signatures of the unbound dynamical ejecta of a BH–NS merger. We take as our initial state the results from a numerical relativity simulation and then use a general relativistic hydrodynamics code to study the evolution of the ejecta with parameterized r -process heating models. The unbound dynamical ejecta is initially a flattened, directed tidal tail largely confined to a plane. Heating from the r -process inflates the ejecta into a more spherical shape and smooths its small-scale structure, though the ejecta retains its bulk directed motion. We calculate the electromagnetic signatures using a 3D radiative transfer code and a parameterized opacity model for lanthanide-rich matter. The light curve varies with viewing angle because of two effects: asphericity results in brighter emission for orientations with larger projected areas, while Doppler boosting results in brighter emission for viewing angles more aligned with the direction of bulk motion. For typical r -process heating rates, the peak bolometric luminosity varies by a factor of ∼3 with orientation while the peak in the optical bands varies by ∼3 magnitudes. The spectrum is blueshifted at viewing angles along the bulk motion, which increases the V -band peak magnitude to ∼−14 despite the lanthanide-rich composition.