The project explores several avenues in parallel. Below, I give a broad overview of the main avenues and their results.
(i) The physics of binary neutron star and neutron star black hole mergers:
We wrote a comprehensive review of the subject in Physics Reports. This review summarizes the current knowledge on the subject and expands it in some areas, focusing on the light emitted following the merger. The review was followed by a series of novel studies on the subject. For example, the material that is ejected during the merger glows in optical and IR due to the power of radioactive decay of the very heavy elements synthesized during the merger. We carried out a detailed calculation of the heat deposited by this radioactive decay and derived an improved method to calculate the resulting emitted light. Another example is a study of the detectability of the relativistic jet that is launched following the merger, where we found that the emission from the jet is expected to be detected only if it is pointing not too far away from us.
(ii) The propagation of relativistic jets in a dense medium
We carried out a large set of simulations that follow the propagation of various types of jets in dense media, as expected in cases of exploding stars or compact binary mergers. In one set of studies, we investigated the structure that the jets attain due to this interaction after emerging from the dense medium. We found this structure in a range of different jet and media conditions and discussed the effect of this structure on the observations. In another study, we developed the theory of the propagation of a jet in an expanding medium, such as the ejecta of a binary neutron star merger.
(iii) Measuring the jet viewing angle and constraining the Hubble constant
The main source of error in measuring the Hubble constant from a binary neutron star merger with observed gravitational wave signal and an electromagnetic counterpart is the angle between the binary orbital plane and the line-of-sight. This angle can be measured by finding the angle between the jet axis and the line-of-sight from the electromagnetic observations. We carried out a systematic study on the errors that arise in such measurement and used it to obtain the best measure to date of the angle between the line-of-sight and the jet in GW170817. This provided an improved constraint using this method on the Hubble constant.
(iv) Constraining the iron yield and energy sources of stripped-envelope supernovae
We carried out a careful study of a large sample of well-observed stripped-envelope supernovae (core-collapse explosions of stars that lost most or all of their hydrogen envelope). This is a legacy project, since the iron yield is a fundamental quantity in any chemical evolution study. Additionally, iron is mostly a decay product of Nickle, which is the main power source of the light seen in stripped-envelope supernovae. The accurate measurement of Nickle in the observed supernova enabled us to show that the observed light is brighter than expected from Nickle alone, implying that an additional, unexpected energy source is at work in these supernovae.
(v) The role of jets in exploding stars
We have shown that in long gamma-ray bursts, the relativistic jets cannot be the energy source of the associated supernovae. Instead, there should be two types of energy sources, one relativistic and narrowly collimated, and the second wide and possibly non-relativistic. We have also shown that a similar scenario is likely to occur in some, and possibly all, stripped-envelope supernovae that are not associated with gamma-ray bursts.
