The gamma ray bursts



At the beginning of the 1970's, the american spy satellites, who were looking for possible atomic explosions, detected bursts of gamma rays. Measurements by triangulation fast showed that the origine of these radiations was not terrestrial, but came from outside the solar system. And even from outside the galaxy.

BATSE satellite measurements
This is the result of the localization of 2704 measures fo the BATSE satellite.
The spatial distribution is perfectly homogeneous, what clearly indicates an extra-galactic origin of the phenomenon.

Source NASA.

Gamma Ray Bursts (so called GRB) are the most energetic events of the known universe, apart from the Big Bang itself : very short pulses of highly energetic gamma radiations, of duration included between 10 milliseconds and ten or so seconds, followed by an afterglow of radiation in all the frequencies of the spectrum which can last for weeks. The measurements of the spectrums clearly show a non-thermal origin of the phenomenon.

GRB spectrums
Some typical spectrums of GRB.

Source : G.J.Fishman

GRBs can be classified in two types :

The afterglow of these GRBs is sometimes visible by the ground observatories as a very bright supernova.


Coalescence of compact stars

One of the first ventured hypothesis implies a pair of neutron stars, or a neutron star and a black hole engaged in a binary system. These two bodies lose of their energy by emission of gravitational waves and eventually "fall" the one on the other.
At the time of the meeting - called coalescence - a shock wave propagates at nearly the speed of light in the surrounding medium. Following this coalescence, a black hole is formed, with a very high speening speed. If an intense magnetic field takes place in the surrounding accretion disk, the particles will transmit a synchrotron radiation, which appears as the GRB.

In such a case, the radiation is quite isotropic : it radiates in all the directions of space, and calculations show that the implicated energy must be around 1053 ergs. This is the amount of energy radiated by 1,000 Sun type stars in their whole life.
This energy is also about 1000 times the energy released by a nova, hence the name of 'kilonova' for these events.

Numerical simulations show that such a mechanism is compatible with the first type of GRB, the shortest ones.


The collapsar model

In a certain number of cases, the GRBs were able to be put in correlation with the visible explosion of particularly violent supernovae, called hypernovae.
These hypernovae would be the result of cataclysmic collapses of very hot and massive - at least 25 solar masses - stars called Wolf-Rayet stars.
Dr Stan Woosley of the University of California proposed the model of the "collapsar" : at the end of its life, the core of the star will collapse in the same manner that a supernova, but the peripheral layers of the stars won't be affected, because of the huge size of this star.
The collapsed core is directly transformed into a black hole and surrounds itself with an accretion disk inside the star.
By simplifying, we can say that the rotating accretion disk will produce an intense magnetic field and hence will cause a double jet of matter in a speed close to that of the light. This jet, and the shock wave which accompanies it, produce a broadcast of gamma rays in their axis.
When this shock wave comes out of the surface of the star, its collision with the gases surrounding the star will produce the typical afterglow of the GRB in the range of the X rays, then of the visible light, and finally of the radio waves, according to its decline.

Simplified progress of a collapsar, since the creation of the black hole inside the star, until the destruction of this one.
The proportions are not respected.

In this scenario, the origin of gamma rays is not clearly established : in particular are they formed inside the envelope of the star or outside?

This jet of matter and the associated shock wave propagate in the axis of the poles of the black hole, which corresponds to the axis of rotation of the star. So, the burst becomes visible if the observer is situated in this axis.
This fact has two important consequences :


Distance of the GRBs

The correlation of the observations from gamma ray detectors embarked on satellites and the observations on the ground shows that these GRBs appear in very distant galaxies, thus when these were young.

In the scenario of the coalescence of compact stars, calculations show that these phenomenon are more usual in young galaxies.

coalescence rates
Coalescence rates of binary neutron stars (NS+NS) and neutron star and black hole (NS+BH) according to the age of their galaxy in billions of years.
These events are more likely in young, hence distant, galaxies.

Source : Lipunov et al.

In the same way, hypernovae always appear in distant galaxies. It's because very massive stars are needed to produce them. We think that the first generation of stars, at the beginning of the universe, was able to produce superhuge stars up to 200 solar masses or more, much more able to end into collapsar that the stars of the current generation which rarely exceed 25 solar masses.

Anyway, it is highly desirable that this kind of phenomenon occurs far from us. If a hypernova came to burst in our galaxy, and if the Earth is in the axis of the radiations, the burst of X-rays and gamma rays would destroy the upper ozone layer, so destroying the shield which protects us from the particles from the Sun. The life on Earth would then be compromised.
Fortunately, we know only 3 stars in the galaxy which could end in hypernova by less than a million years, the closest to which is Eta Carina, at the distance of 8000 light years. But we also know that in case of explosion of this star, the Earth would not be in the axis of the GRB.


References :
La coalescence des objets compacts (R. Mochkovitch)
Iron lines from GRBs : clues toward the progenitor (Böttcher & Fryer)
The Scenario Machine: Binary Star Population Synthesis (Lipunov et al.)
Observed GRB Properties (G.J. Fishman)
Collapsars, Gamma-Ray Bursts, and Supernovae (A. I. MacFadyen)
Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked (ESO)
GRB Time Profiles and Spatial Distribution (R. Diehl)
The Collapsar model (K. Kretschmer)