The missing link: how short gamma-ray bursts gain their jet

	During the merger of two neutron stars a strong magnetic field is formed along the rotational axis, which creates a jet.
	© L. Rezzolla (AEI), M. Koppitz (AEI/ZIB).

Gamma-ray bursts are among the most powerful explosions in the Universe, releasing in few seconds the same amount of energy that our entire Galaxy releases in one year. Because of the huge difficulties in observing it directly, the "central-engine", that is the ultimate source of these powerful explosions, is still essentially unknown. This is particularly true for the so-called short gamma-ray bursts (SGRBs), which last between few tens of ms to a couple of sec, and which have been observed by satellites such as Swift and Fermi.

Among the many models proposed to explain the observations, one suggested that the merger of two stellar-size compact objects, eg two neutron stars or a black hole and a neutron star, would have a sufficient energy reservoir to lead to such an explosion. The idea is that after the merger a rotating black hole surrounded by a massive and extremely hot torus would form and if a sufficiently strong magnetic field can be produced, a magnetic jet could be generated, launching an ultra-relativistic flow and eventually gamma rays. The presence of a highly collimated outflow jet is a basic feature of any gamma-ray-burst observation. Yet, while numerical simulations in general relativity performed at the AEI and elsewhere have shown that the merger of two compact objects leads to the formation of a torus surrounding a rapidly rotating black hole, the generation of a magnetic jet remained a mystery.

Led by Prof. L. Rezzolla at the AEI, an international group of researchers in Japan, Spain, UK and USA has confirmed the viability of such scenario and shown for the first time that the formation of a jet-like structure of an ultra-strong magnetic field is the natural outcome of the merger of magnetized neutron stars. More specifically, the team has considered two equal-mass magnetized neutron stars, with an initial poloidal magnetic field of 10¹² Gauss, during the last three orbits of their inspiral. After the merger and when a black hole has been produced, the magnetic field in the torus is amplified of several orders of magnitudes and up to 10¹⁵ Gauss as a result of magnetohydrodynamical (MHD) instabilities. Furthermore, the initially chaotic magnetic field grows and organizes itself, becoming ordered on a large scale and predominantly poloidal, i.e. jet-like, around the black-hole spin axis. Because the simulations had to be carried out on timescales much longer than any previous ones, they were able to reveal the role played by MHD instabilities in the generation of the magnetic jet.

The importance of this breakthrough is twofold. Firstly, it shows that a jet, which is an essential ingredient in any gamma-ray-burst model and whose generation baffled theorists for years, can be produced via the nonlinear development of magnetohydrodynamical instabilities. Secondly, the broad agreement of the results of the simulations with current astrophysical observations provides further and convincing support to the idea that the merger of binary neutron stars could be behind the central-engine of SGRBs.

More information on the group, as well as images and animations of the simulations can be found on the group web page:

	Numerical Relativity Group
	http://numrel.aei.mpg.de

Bibliography

NASA video ‘When Neutron Stars Collide’

Caption
Snapshots at representative times of the evolution of the binary and of the formation of a large-scale ordered magnetic field. Shown with a color-code map is the density, over which the magnetic-field lines are superposed. The panels in the upper row refer to the binary during the merger (t = 7.4 ms) and before the collapse to BH (t = 13.8 ms), while those in the lower row to the evolution after the formation of the BH (t = 15.26 ms, t = 26.5 ms). Green lines sample the magnetic field in the torus and on the equatorial plane, while white lines show the magnetic field outside the torus and near the BH spin axis. The inner/outer part of the torus has a size of ∼ 90/170 km, while the horizon has a diameter of ≃ 9 km.