Relativistic Jet Instabilities

Astrophysical jets are collimated outflows of plasma. Jets originating from compact objects (black holes and neutron stars) reach relativistic speeds, and become exceptionally bright creating spectacular plumes and cocoons. These jets are extremely stable: their initial radius is comparable to the horizon of the supemassive black hole where they come from, but they manage to trave undisrupted through their host galaxy. Once they get beyond the galaxy, they typically lose their coherence and become turbulent. We have explored their behaviour through three dimensional simulations in Gourgouliatos and Komissarov 2018a and we found that jet remains stable up to the reconfiment point, but beyond this point it becomes susceptible to the so-called centrifugal instability Gourgouliatos and Komissarov 2018b.

Neutron Stars and their Magnetic Fields

Neutron stars are end points of stellar evolution, created following the gravitational collapse of massive stars after supernova explosions. Their radius is about 10km and their mass approximately 1.5 solar masses. Neutron stars host the strongest magnetic fields in the Universe, reaching up to 1015 G, which is orders of magnitude above any magnetic field we have ever created artificially on Earth, and rotate very fast, the fastest that we know of has a period of 0.0014 s.

A key point in our understanding of neutron stars is the structure and the evolution of their magentic field. The radiation we receive from neutron stars comes in pulses because of the combined effect of rotation and magnetic field. Moreover, neutron star magnetic field is responsible for heating and explosive behaviour, especially in the so called magnetars hosting very strong magnetic fields. Motivated by this question we have developed a numerical simulation which traces the evolution of the magnetic field in the crust. The evolution of the magnetic field in the crust is because of the Hall Effect: the electric current is carried exclusively by electrons, leading to a special mathematical description.

We have found, after running a large ensemble of simulations, that the evolution of the magnetic field goes through three basic stages: The early evolution of the magnetic field is sensitive to the initial conditions, with the magnetic field structure changing drastically in a short time. Then the magnetic field forms whistler waves and oscillates. Eventually it relaxes to a state where the electron fluid isorotates with the magnetic field lines -an equivalent of Ferraro’s law from Solar Physics. The latter state is reached for a variety of initial conditions and is an attractor towards which the field is trying to settle.

In this animation you can see the evolution of the magnetic field in the crust of a neutron star. The black contours are the poloidal field lines, the colour scale in the left panel shows the toroidal field while in the right panel it shows the angular velocity of the electrons.

In this animation you can see the evolution towards the attractor. The horizontal axis is the poloidal flux which also corresponds to the poloidal field lines and the vertical axis denotes the angular velocity of the electrons. The system is trying to relax towards a state of isorotation, minimizing the spread of angular velocity on a given field line.

For a more technical description of the simulations and the results you may read these papers: Gourgouliatos and Cumming 2014aGourgouliatos and Cumming 2014b and Gourgouliatos et al. 2013.

Relativistic Magnetic Fields

Some astrophysical systems, such as AGN jets and gamma-ray bursts, involve dynamically significant magnetic fields and very high velocities. To model those, one needs to get into the realms of Relativistic MHD. We have found analytical and semi-analytical solutions of relativistically expanding magnetic fields, which can be applied to conical and cylindrical jets. The details of this work can be found in Gourgouliatos and Lynden-Bell 2008Gourgouliatos and Vlahakis 2010 and Gourgouliatos et al. 2012.

Magnetic Bubbles

Some clusters of galaxies with AGNs in their centres contain gigantic X-ray bubbles. As these bubbles are long-lived entities they must be in some sort stable configuration. Motivated by this question, we have explored solutions of magnetic field and hot plasma in stable equilibrium (Gourgouliatos et al. 2010). In our solutions the magnetic field drops smoothly to zero near the surface, thus there are no surface currents, which would lead to resistive instabilities. We have extended our model to expanding bubbles (Gourgouliatos and Lyutikov 2012). Depending on the adiabatic index of the plasma content of the bubble, the expansion can lead to a magnetically dominated structure, where current sheets form. Given the large available electric potential, we suggest, that it is possible to accelerate cosmic rays to ultra high energies in these current sheets. Recent observations of the Pierre Auger observatory, indicate a corellation between the sources of the cosmic rays with AGN sources.

Coronal Mass Ejections

Magnetic activity leads to the ejection of matter from the Sun in the form of Coronal Mass Ejections (CMEs). CMEs are thought to exist either in the form of twisted magnetic flux tubes, or disconected magnetic clouds, that expand while propagating away from the Sun. I have found a class of analytical solutions describing the structure and the evolution of a magnetic arcade up to the point of the formation of current sheets (Gourgouliatos 2008). We have also worked on the evolution of a CME while it propagates in the solar system describing the expansion and the twisted magnetic field through an analytical model (Lyutikov and Gourgouliatos 2011).