Prompt Electromagnetic Transients from Binary Black Hole Mergers

TL;DR

This study investigates electromagnetic signals from binary black hole mergers in plasma environments, using simulations to understand jet-like emissions and synchrotron radiation, revealing that certain EM signatures are relatively insensitive to initial conditions.

Contribution

The paper introduces magnetohydrodynamic simulations of BBH mergers in plasma, showing that EM luminosity is regulated by gas flow and remains consistent across various initial magnetic field strengths.

Findings

Poynting luminosity is in the range 10^{45}-10^{46} erg/s.

Synchrotron flux varies little before merger despite lensing effects.

Magnetic field strength is regulated by gas flow, leading to stable EM emissions.

Abstract

Binary black hole (BBH) mergers provide a prime source for current and future interferometric GW observatories. Massive BBH mergers may often take place in plasma-rich environments, leading to the exciting possibility of a concurrent electromagnetic (EM) signal observable by traditional astronomical facilities. However, many critical questions about the generation of such counterparts remain unanswered. We explore mechanisms that may drive EM counterparts with magnetohydrodynamic simulations treating a range of scenarios involving equal-mass black-hole binaries immersed in an initially homogeneous fluid with uniform, orbitally aligned magnetic fields. We find that the time development of Poynting luminosity, which may drive jet-like emissions, is relatively insensitive to aspects of the initial configuration. In particular, over a significant range of initial values, the central magnetic field strength is effectively regulated by the gas flow to yield a Poynting luminosity of $10^{45}-10^{46} \rho_{-13} M_8^2 \, {\rm erg}\,{\rm s}^{-1}$, with BBH mass scaled to $M_8 \equiv M/(10^8 M_{\odot})$ and ambient density $\rho_{-13} \equiv \rho/(10^{-13} \, {\rm g} \, {\rm cm}^{-3})$. We also calculate the direct plasma synchrotron emissions processed through geodesic ray-tracing. Despite lensing effects and dynamics, we find the observed synchrotron flux varies little leading up to merger.