DCU astronomers measure the speed of the most powerful explosions in the Universe

GRBs are the most powerful energy releases since the Big Bang and are so bright that they rival the whole Universe in luminosity. They last for a very short time, from less than a second to a few minutes. In order to emit such incredible power in so little time, the exploding material must be moving at a speed comparable with that of light, which is 300,000 km per second.

The initial readings for two such GRBs were recorded by a satellite gamma-ray telescope orbiting the Earth, on 18 April and on 7 June 2006. In a matter of a few seconds, their position was transmitted to the ground, and the robotic 'REM' Telescope at the European Southern Observatory (ESO; La Silla, Chile) began to observe these GRB fields, detecting near-infrared afterglows and monitoring the evolution of their luminosity as a function of time (the light curve).

The two gamma-ray bursts were located in far-away galaxies, at 9.3 and 11.5 billion light-years respectively. For both events, the light curve was initially rising, then reached a peak, and eventually started to decline, as it is expected for GRB afterglows. The peak is, however, only rarely detected. Its determination is very important and very exciting, since it allows a direct measurement of the expansion velocity of the explosion.

“The burst of 7 June 2006 exploded when we were at a GRB Conference in Venice. When we looked at the first data and saw that the light curve was rising, we were so excited that we performed the data reduction in real time. As a result, the afterglow light curve could be shown at the conference just a few hours later” recalls Susanna Vergani (postgraduate student, DCU-DIAS).

For both bursts, the velocity turns out to be very close to the speed of light, to be precise 99.9997% of this value. Astronomers use a special number, called Lorentz factor, to express these high velocities. Objects moving much slower than light have a Lorentz factor of about 1, while for these two GRBs it is about 400.

“This is an important result, which confirms the 'fireball' theory that has been put forward to explain these exceptional explosions and, interestingly, has been achieved with a small-size telescope” says Prof. Evert Meurs, Director of Dunsink Observatory and leader of the Irish team involved in this project.