Black Hole & Neutron Star JetsTim Lemon

Gravity Group

www.southampton.ac.uk/stag

1. Introduction

Black holes and neutron stars are formedfrom massive stars at the end of their life-time. If the star’s mass exceeds a certainthreshold, then when that star goes super-nova it will collapse to form a black hole.However if it is below the threshold the starwill collapse to form a neutron star. A typ-ical neutron star has a radius of about tenkilometres, whilst having between one andtwo times the mass of our own Sun.

Astrophysical jets are spectacular and pow-erful phenomena. They are believed to beformed when matter flows onto a compactstar, such as a black hole or neutron star.This matter is then ejected at the poles ofthe star at velocities close to the speed oflight. They are thought to be the cause ofgamma-ray bursts, one of the most power-ful phenomena ever discovered in the Uni-verse.

2. X-Ray Binaries

X-ray Binaries (below) consist of an ordinary star transferring stellar matter ontoits companion compact star, as they rotate around each other. This accretedmatter then forms an accretion disk around the compact star which then releasesits potential energy in the form of X-rays.

Within X-ray Binaries, a link has been established between X-ray observations ofthe accretion disk and the radio emission from the jet. This suggests that jetsare powered by this accretion process.

Differences in jet observationsindicate that different typesof stars also have an impor-tant effect on the structureand formation of jets. A ma-jor issue is whether the mech-anism, which causes the jet, isthe same for all of these ob-jects. A cross comparison ofthese different systems is vitalto understanding the mecha-nisms behind jet formation.

3. Simulations

To study the formation of jets, I needto simulate how a jet interacts withits environment. However the envi-ronment where jets are formed is verycomplicated and is not well under-stood. For this reason the initial simu-lations were basic and just focused onhow a relativistic jet travels through adenser material.

Later the more advanced simulationsincluded an accretion disk with a mag-netic field around a compact star. Mycross-section image (right) shows theinitial density of the accretion discaround a black hole. The high density areas, such as the disk, are colouredred, whilst the low density ambient medium is blue.

4. Jet Modelling

The first jet simulations were achieved by taking someinitial properties of relativistic jets within an ambientmedium and then letting the simulation evolve overtime. With the jet material travelling at close to thespeed of light this produced a turbulent flow of ma-terial, with shocks occurring inside the high densitystructure around the jet itself.

This model was then improved upon by including thegravitational field from a compact star at the origin ofthe jet structure. This introduced strong gravitationaleffects from the star pulling most of the matter fromthe jet back to the origin, creating a high density areanear the base. My figure to the right shows such an example of jet originatingfrom a rotating black hole. The actual black hole cannot be seen in the figure,as it is just a single pixel in the simulation.

5. Accretion Disks

The next set of simulations were designedto try and create a jet directly from a com-pact star and its accretion disk. This timeI would need to take into account the ef-fects of magnetic fields as well as generalrelativity. My figure (left) shows a 3D den-sity plot of an accretion disk around a blackhole, viewed in the initial state of the sim-ulation. In the figure it is also possible tosee the field lines of the magnetic field looparound the disk.

During the simulation the matter in the ac-cretion disk rotates around the star, falling more and more inwards. The ro-tation in the disk twists the magnetic field lines around with it, as they areanchored into the disk. The matter will eventually reach the innermost stablecircular orbit. Past this point the matter should flow along the magnetic fieldlines to the black hole, and then be ejected outwards at the poles.

By the end of the simulation the systemhas ended up in a very complex state asseen in my figure (right). Simulations so farhave been carried out with both black holesand neutron stars, that were either rotatingor non-rotating. These simulations also in-cluded magnetic fields of varying strengths.The results from these do show matter flow-ing along the field lines and accumulatingat the poles. However this matter is notbeing accelerated to high enough velocitiesto escape the gravitational attraction of thecompact object and launch a jet.

6. The Future

The simulations now include all the main features associated with launching arelativistic jet from a compact object. This gives me a good starting model toimprove on and to match a more realistic jet launching environment.

The next step is to fine-tune the initial con-ditions to more closely resemble those foundwithin X-ray binaries. This would include us-ing a different structure for the magnetic fieldwithin the accretion disk, shown right.

The way the simulation evolve the system overtime could also be altered to use the computerresources more efficiently. This should lead tomore accurate results.

Images courtesy of NASA. The author acknowledges support from the University of Southampton.

Supervisor: Ian Hawke Tim.Lemon@soton.ac.uk STAG Website: