* Astronomy

Title: The GRB afterglow onset observed by REM: fireball Lorentz factor and afterglow fluence
Authors: Daniele Malesani (DARK), Emilio Molinari (INAF/Brera), Susanna Vergani (DIAS and DCU), Stefano Covino (INAF/Brera), for the REM team

We report observations of the early light curves of GRB 060418 and GRB 060607A, carried out with the pink robotic telescope REM. A clear peak is detected for both events, which is interpreted as the onset of the afterglow, that is the time at which the fireball starts decelerating. This detection allows to directly measure the initial fireball Lorentz factor, which was found to be Gamma_0 ~ 400 for both events, fully confirming the ultrarelativistic nature of gamma-ray burst fireballs. Sampling the light curve before the peak also allows to compute the bolometric fluence of the afterglow, which is 16% of the prompt one in the case of GRB 060418.


X-ray and NIR/optical light curves of GRB 060418 and GRB060607A. The REM data have been complemented by GCN and VLT data.

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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" - 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. 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.

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U.S. astronomers have determined flares seen after a gamma-ray burst are apparently a continuation of the burst itself.
Gamma-ray bursts release in seconds the same amount of energy the sun will emit during its expected 10 billion-year lifetime. That energy comes from the core of a massive star collapsing to form a black hole or neutron star.
Early in its mission, the National Aeronautics and Space Administration's Swift satellite's X-ray Telescope discovered the initial pulse of gamma-rays -- known as prompt emission -- is often followed by short-lived but powerful, X-ray flares, suggesting a GRB's central engines remain active long after the prompt emission.
In the latest study, Hans Krimm and colleagues at the Universities Space Research Association and the Goddard Space Flight Centre demonstrated X-ray flares are indeed a continuation of the prompt emission.

"This pattern points to a continuous injection of energy from the central engine, perhaps fuelled by sporadic in-fall of material onto a black hole. The black hole just keeps gobbling up gas and the engine keeps spewing out energy" - Hans Krimm.

The research is to appear in the Aug. 10 issue of the Astrophysical Journal.

Source UPI

VLT Automatically Takes Detailed Spectra of Gamma-Ray Burst Afterglows Only Minutes After Discovery
A time-series of high-resolution spectra in the optical and ultraviolet has twice been obtained just a few minutes after the detection of a gamma-ray bust explosion in a distant galaxy. The international team of astronomers responsible for these observations derived new conclusive evidence about the nature of the surroundings of these powerful explosions linked to the death of massive stars.
At 11:08 pm on 17 April 2006, an alarm rang in the Control Room of ESO's Very Large Telescope on Paranal, Chile. Fortunately, it did not announce any catastrophe on the mountain, nor with one of the world's largest telescopes. Instead, it signalled the doom of a massive star, 9.3 billion light-years away, whose final scream of agony - a powerful burst of gamma rays - had been recorded by the Swift satellite only two minutes earlier. The alarm was triggered by the activation of the VLT Rapid Response Mode, a novel system that allows for robotic observations without any human intervention, except for the alignment of the spectrograph slit.
Starting less than 10 minutes after the Swift detection, a series of spectra of increasing integration times (3, 5, 10, 20, 40 and 80 minutes) were taken with the Ultraviolet and Visual Echelle Spectrograph (UVES), mounted on Kueyen, the second Unit Telescope of the VLT.

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