Wednesday, July 25, 2007

Matter at Ultra Speed

The REM Telescope courtesy of P Aniol. ESO Release.

Matter Flashed at Ultra Speed

Astronomers using REM have for the first time measured the velocity of the explosions known as gamma-ray bursts (GRBs). The material is travelling at the extraordinary speed of more than 99.999% of the velocity of light, the maximum speed limit in the Universe.
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Gamma Ray Burts are short flashes of energetic gamma-rays lasting from less than a second to several minutes. They release a tremendous quantity of energy in this short time making them the most powerful events since the Big Bang. They come in two different flavours, long and short ones. Over the past few years, international efforts have convincingly shown that long GRBs are linked with the ultimate explosion of massive stars (hypernovae) while the short ones most likely originate from the violent collision of neutron stars and/or black holes . Irrespective of the original source of the GRB energy, the injection of so much energy into a confined volume will cause a fireball to form. Gamma-ray photons have nearly a million times more energy than the 'visual' photons the eye can see.

On 18 April and 7 June 2006, the NASA/PPARC/ASI Swift satellite detected two bright gamma-ray bursts. In a matter of a few seconds, their position was transmitted to the ground, and the REM telescope began automatically to observe the two GRB fields, detecting the near-infrared afterglows, and monitored the evolution of their luminosity as a function of time (the light curve).

The gamma-ray bursts were located 9.3 and 11.5 billion light-years away, respectively. For both events, the afterglow light curve initially rose, then reached a peak, and eventually started to decline, as is typical of GRB afterglows.

The peak is, however, only rarely detected. Its determination is very important, since it allows a direct measurement of the expansion velocity of the explosion of the material.

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

While single particles in the Universe can be accelerated to still larger velocities - one has to realise that in the present cases, it is the equivalent of about 200 times the mass of the Earth that acquired this incredible speed.

"You certainly wouldn't like to be in the way"
"The next question is which kind of 'engine' can accelerate matter to such enormous speeds," said Stefano Covino.

Notes. Strictly speaking, the Lorentz factor is the ratio between the total and rest-mass energy of the fireball.
REM (Rapid Eye Mount) is a small (60 cm mirror diameter) rapid reaction automatic telescope dedicated to monitor the prompt afterglow of Gamma Ray Burst events. It is located at the ESO La Silla Observatory in Chile.

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