Gamma-Ray Burst Afterglow That Challenges Gamma-Ray Burst Theory
15 Apr, 2007 05:57 pm
On July 29th, 2006, the Swift Gamma-Ray Burst Explorer Mission detected GRB 060729, a burst that challenges the standard theory of Gamma-Ray Bursts (GRBs). While the standard fireball model of GRBs (see e.g. Meszaros 2006, Zhang 2007) predicts the presence of a jet and a transition from a phase where the GRB afterglow fades moderately to a phase where it rapidly fades, the X-ray afterglow of GRB 060729 does not show any signs of such a rapid decay, even more than four months after its
Gamma-Ray Bursts (GRBs) are the most energetic transient objects in the Universe. Within seconds they release more energy than our Sun in its entire 10 billion year lifetime. As we know today, GRBs are explosions of very massive stars, in many ways similar to a supernova. However, their total energy seem to be a hundred or a thousand times larger than that of a typical supernova and this energy is released within only about tens of seconds. Theorist found a solution for this problem by having the emission from the GRB concentrated in a well-collimated light beam, or a jet. Like looking into the beam of a flash light we are looking into the jet of a GRB when we observe it. The crucial point here is the opening angle of that light beam or for the GRB, the jet. The smaller the opening angle the smaller the total amount of energy is needed to explain the measured flux from the source. In our example of the flash light, assuming an opening angle of the light beam of 5 degrees and a 4W light bulb, we would need a power of about 2 kW if the light we see from the light source is emitted equally into all directions (isotropic) and not collimated, way too much than what you can get out of two C batteries that power your flash light. The same applies for GRBs. Because the light we see is like a light beam from a flash light, the total energy needed is much less than what is expected if the emission is isotropic.
A GRB detected by the Swift BAT on July 29th, 2006, GRB 060729, seems to challenge the standard theory of GRBs. Swift was able to detect the X-ray afterglow of this burst even four months after the explosion, the current record for an afterglow follow-up by Swift. During that time no sign of a transition to a more rapid decay in the X-ray flux was detected. These rapid decays in the light curves are a sign of when the jet interacts with the surrounding medium and decelerates to such a less relativistic velocity that the jet edge becomes visible to us. The time when this 'jet break' happens is a measure for the jet opening angle. The later the jet break the wider is the opening angle. For GRB 060729 there was no jet break for at least 125 days after the burst. This time indicates that the opening angle of the jet has to be at least 28 degrees. Because the opening angle is so wide this burst requires an enormous energy reservoir. One possible solution of this dilemma is that there is a continuous energy injection from a central magnetar, a neutron star with a very powerful magnetic field. This magnetic field works like an electromagnetic brake, like a dynamic break on a locomotive. Like in a locomotive, the energy of motion is converted into heat which for the GRB means that the magnetic field takes energy from the rotation of the magnetar (and slows it down) and converts it into electromagnetic energy that is injected continuously into the initial blast wave. The result is that this keeps the afterglow going longer than usual.
The results of the Swift observations of GRB 060729 will be published by Penn State's astronomer Dirk Grupe as the lead author in the June 20th, 2007 edition of the Astrophysical Journal and are already available as a preprint on astro-ph: (http://arxiv.org/abs/astro-ph/0611240). Further observations of the X-ray afterglow by NASA's Chandra X-ray satellite are planned to extend the light curve of this extraordinary X-ray afterglow beyond any GRB afterglow has ever been observed and detected. These observations will be by far the best constrains ever made on a wide opening angle of a GRB.
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