Matter Flashed at Ultra Speed (Forwarded)

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Contact

Emilio Molinari
INAF / Osservatorio Astronomico di Brera
Merate, Italy
Phone: +39 039 9991158

ESO Science Release 26/07

For Immediate Release: 12 June 2007

Matter Flashed at Ultra Speed

Robotic Telescope Measures Speed of Material Ejected in Cosmic Death

Using a robotic telescope at the ESO La Silla Observatory, astronomers
have for the first time measured the velocity of the explosions known as
gamma-ray bursts. 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.

"With the development of fast-slewing ground-based telescopes such as the
0.6-m REM telescope at ESO La Silla, we can now study in great detail the
very first moments following these cosmic catastrophes," says Emilio
Molinari, leader of the team that made the discovery.

Gamma-ray bursts (GRBs) are powerful explosions occurring in distant
galaxies, that often signal the death of stars. They are so bright that,
for a brief moment, they almost rival the whole Universe in luminosity.
They last, however, for only a very short time, from less than a second to
a few minutes. Astronomers have long known that, 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, namely 300 000 km per second. By
studying the temporal evolution of the burst luminosity, it has now been
possible for the first time to precisely measure this velocity.

Gamma-ray bursts, which are unseen by our eyes, are discovered by
artificial satellites. The collision of the gamma-ray burst jets into the
surrounding gas generates an afterglow visible in the optical and
near-infrared that can radiate for several weeks. An array of robotic
telescopes were built on the ground, ready to catch this vanishing
emission (see e.g. ESO 17/07). 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 small size
of the telescope is compensated by its rapidity of slewing, which allowed
astronomers to begin observations very soon after each GRB's detection (39
and 41 seconds after the alert, respectively), and to monitor the very
early stages of their light curve.

The two 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.

"Matter is thus moving with a speed that is only different from that of
light by three parts in a million," says Stefano Covino, co-author of the
study. "While single particles in the Universe can be accelerated to still
larger velocities -- i.e. much larger Lorentz factors -- 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," adds team member Susanna
Vergani.

The measurement of the Lorentz factor is an important step in
understanding gamma-ray burst explosions. This is in fact one of the
fundamental parameters of the theory which tries to explain these gigantic
explosions, and up to now it was only poorly determined.

"The next question is which kind of 'engine' can accelerate matter to such
enormous speeds," says Covino.

More Information

"REM observations of GRB060418 and GRB060607A: the onset of the afterglow
and the initial fireball Lorentz factor determination", by E. Molinari, S.
D. Vergani, D. Malesani, S. Covino, et al. The paper is available at
http://dx.doi.org/10.1051/0004-6361:20077388 (A&A, 469, L13-L16, 2007)

The REM team is formed by G. Chincarini, E. Molinari, F.M. Zerbi, L.A.
Antonelli, S. Covino, P. Conconi, L. Nicastro, E. Palazzi, M. Stefanon, V.
Testa, G. Tosti, F. Vitali, A. Monfardini, F. D'Alessio, P. D'Avanzo, D.
Fugazza, G. Malaspina, S. Piranomonte, S.D. Vergani, P.A. Ward, S.
Campana, P. Goldoni, D. Guetta, D. Malesani, N. Masetti, E.J.A. Meurs, L.
Norci, E. Pian, A. Fernandez-Soto, L. Stella, G. Tagliaferri, G. Ihle, L.
Gonzalez, A. Pizarro, P. Sinclair, and J. Valenzuela.

Notes

Gamma-ray bursts (GRBs) 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 gamma-ray bursts are linked with the ultimate explosion of
massive stars (hypernovae; see e.g. ESO PR 16/03) while the short ones
most likely originate from the violent collision of neutron stars and/or
black holes (see e.g. ESO PR 26/05 and 32/05). 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.

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. For
more information, see
http://www.rem.inaf.it and Chincarini et al. (ESO Messenger, 113, 40,
2003)

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