The effect of cooling on the accretion of circumprimary discs in merging supermassive black hole binaries

ABSTRACT At the final stages of a supermassive black hole coalescence, the emission of gravitational waves will efficiently remove energy, and angular momentum from the binary orbit, allowing the separation between the compact objects to shrink. In the scenario where a circumprimary disc is present, a squeezing phase will develop, in which the tidal interaction between the disc and the secondary black hole could push the gas inwards, enhancing the accretion rate on to the primary and producing what is known as an electromagnetic precursor. In this context, using 3D hydrodynamic simulations, we study how an adiabatic circumprimary accretion disc responds to the varying gravitational potential as the secondary falls on to the more massive object. We included a cooling prescription controlled by the parameter β = Ωtcool, which will determine how strong the final accretion rate is: a hotter disc is thicker, and the tidal interaction is suppressed for the gas outside the binary plane. Our main results are that for scenarios where the gas cannot cool fast enough (β ≥ 30), the disc becomes thick and renders the system invisible, while for β ≤ 10 the strong cooling blocks any leakage on to the secondary’s orbit, allowing an enhancement in the accretion rate of two orders of magnitude stronger than the average through the rest of the simulation.

Interaction of the electromagnetic precursor from a relativistic shock with the upstream flow – II. Induced scattering of strong electromagnetic waves

ABSTRACT This is the second in the series of papers aiming to study interaction of the electromagnetic precursor waves from relativistic shocks with the upstream flow. Here, I consider the induced scattering of strong waves. In such a wave, the electrons oscillate with relativistic velocities therefore, the scattering generally occurs in harmonics of the incident wave. I show that the induced scattering occurs predominantly in the first harmonics. I also show that even though in the weak case regime, the induced scattering rate is proportional to the intensity of the incident wave, in the strong wave case, the rate decreases as the wave amplitude grows.

Kinetic modeling of the electromagnetic precursor from an axisymmetric binary pulsar coalescence

Context. The recent detection of gravitational waves associated with a binary neutron star merger revives interest in interacting pulsar magnetospheres. Current models predict that a significant amount of magnetic energy should be released prior to the merger, leading to electromagnetic precursor emission. Aims. In this paper, we revisit this problem in the light of the recent progress in kinetic modeling of pulsar magnetospheres. We limit our work to the case of aligned magnetic moments and rotation axes, and thus neglect the orbital motion. Methods. We perform global two-dimensional axisymmetric particle-in-cell simulations of two pulsar magnetospheres merging at a rate consistent with the emission of gravitational waves. Both symmetric and asymmetric systems are investigated. Results. Simulations show a significant enhancement of magnetic dissipation within the magnetospheres as the two stars approach one another. Even though the magnetospheric configuration depends on the relative orientations of the pulsar spins and magnetic axes, all configurations present nearly the same radiative signature, indicating that a common dissipation mechanism is at work. The relative motion of both pulsars drives magnetic reconnection at the boundary between the two magnetospheres, leading to efficient particle acceleration and high-energy synchrotron emission. Polar-cap discharge is also strongly enhanced in asymmetric configurations, resulting in vigorous pair production and potentially additional high-energy radiation. Conclusions. We observe an increase in the pulsar radiative efficiency by two orders of magnitude over the last orbit before the merger, exceeding the spindown power of an isolated pulsar. The expected signal is too weak to be detected at high energies even in the nearby universe. However, if a small fraction of this energy is channeled into radio waves, it could be observed as a non-repeating fast radio burst.