Relativistic jets are generally found to occur in accreting black hole systems. Recently, however, jets have also been seen in a few neutron star systems, implying that a relativistic jet needs only a compact relativistic star, and not necessarily a black hole. This project consists of numerical simulations of relativistic jets from black holes and neutron stars using a general relativistic magnetohydrodynamics code, in order to identify similarities and differences in the two kinds of system. Very recent preliminary work on this problem by Professor Narayan's finishing graduate student has led to very interesting results, and an opportunity has arisen at short notice to keep that student working on this problem before taking up his post-thesis appointment elsewhere. There is clear intrinsic value to the research being done by the person most familiar with it, and this extension of his thesis research will provide this junior researcher with an opportunity to expand into a new area before moving on to the next stage of his career.
This award is made under the Grants for Rapid Response Research (RAPID) program.
For several decades astrophysicists have puzzled over the phenomenon of relativistic jets. These jets are often found when gas flows into a black hole. Part of the inflowing gas gets turned around and is ejected from the system. The ejected gas, called a jet, moves at nearly the speed of light and carries away a large amount of energy. The puzzle with jets is the following. Everyone has got used to the idea that a black hole eats stuff and that whatever falls in disappear forever. The jet phenomenon shows just the opposite. It shows that a black hole can throw stuff out. In fact, in some cases the black hole can throw out more energy in the outflowing jet than all the energy flowing into the black hole. In other words, far from being a monster that eats everything, a black hole can sometimes behave like a benevolent source of energy! The PI, Professor Ramesh Narayan, postdoctoral associate, Dr. Alexander Tchekhovskoy, and other collaborators used this NSF award to study how jets work. In one set of investigations, the nature of the jet fluid was investigated. It is widely believed that jets are highly magnetized and that it is the magnetic field that allows the jet phenomenon to operate. The present investigators compared the predictions of the magnetized jet model with observational data and showed that certain popular ideas are ruled out. Although this is a negative result, it is progress since it allows us to focus on the remaining possibilities. Hopefully it will enable us one day to achieve a full understanding of the composition of a jet. In a second set of investigations, the connection between the inflowing gas and the jet was studied. The aim was to understand why some of the gas flows out in a jet and under what conditions the jet becomes very powerful. The bulk of this work consisted of numerical simulations carried out on NSF-operated TeraGrid supercomputers. It was found that the energy flowing out in the jet is largest when the inflowing gas has a lot of magnetic field. Remarkably, the simulations showed it is possible to have more energy flowing out of the black hole than flows in, exactly as seen in observations. This is an extremely promising result.
