Gravitational waves (GWs) are one of the most remarkable predictions of Einstein’s general theory of relativity. These ripples in the curvature of space-time are caused by some of the most violent and energetic processes in the Universe, such as coalescing compact stellar objects (black holes, neutron stars) and supernovae. With further improvements planned for the current GW detectors (LIGO, Virgo, and KAGRA) and other GW detectors (LIGO India) coming online, a large number of GW events are expected to be discovered in the coming years (the new observing run O4 is expected to start late 2022). Of particular interest are neutron star - neutron star (NS-NS) and black hole - neutron star (BH-NS) mergers. These mergers are leading candidates for short Gamma-Ray Burst (GRB) progenitors and the primary targets for the ground-based GW observatories.
The binary neutron star merger GW 170817 was the first multi-messenger event observed in both gravitational and electromagnetic (EM) waves. The GW detection in coincidence with the short duration GRB 170817A led to a major follow-up campaign across the EM spectrum. The gamma-ray coincidence shows that at least a subset of short GRBs is produced by binary NS mergers, although the peak isotropic luminosity of GRB 170817A is abnormally low. Short GRBs are caused by relativistic jets associated with a BH formation during the merger of compact stellar objects. Since the high luminosity and the rapid variability of GRBs require ultra-relativistic motion in the production mechanism, gamma-ray triggered events are always associated with relativistic jets, and the gamma-rays are detected only when the highly collimated jet points towards us. However, GW observatories are able to detect all mergers which occur within their detection horizon. Considering that GW radiation is semi-isotropic, the jets are seen off-axis in most cases if jets actually emerge from the mergers. Gravitational and electromagnetic wave measurements open a completely new parameter space for the study of the outflow from the relativistic objects, and upcoming observations will allow us to address interesting open questions.
Some of the most important ones are: Do all NS-NS and BH-NS mergers produce a relativistic jet? Are outflows from NS-NS and BH-NS similar? Are magnetic fields involved in the formation of merger jets? Considering the abnormally low gamma-ray luminosity, it is also not clear yet whether GRB 170817A is really a GRB or we have found a new class of transients. One of the long-standing problems in BH astrophysics is the formation, collimation and acceleration process of jets. Astrophysical jets are observed on many different scales from proto-stars and X-ray binaries within our Galaxy to radio galaxies, blazars and GRBs at cosmological distances. Relativistic jets are known to emerge from catastrophic BH formation events. According to the currently favored scenario, AGN jets launch in the vicinity of the central massive BH by magnetic fields extracting energy from the spinning BH or the accretion disk. We recently detected highly polarized optical emission in the early afterglow of long GRBs by using the Liverpool telescope and other telescopes. Our results imply that GRB jets are also accelerated by a magnetic process associated with BHs. We will investigate the properties of GRB and GW merger outflows in a unified manner by using numerical models and hydrodynamics simulations. Outflows from mergers can radiate extremely bright EM signals in radio, optical, X-ray and gamma-ray, in the time domain right after the binary coalescence to more than a few months. We will analyse GW and EM signals in a coherent framework to understand the properties and populations of merger outflows.