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Optical counterparts of Gamma Ray Bursts detected by SWIFT
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Figure
1: Optical light curve of the afterglow of GRB 060203.
Data were acquired starting just one hour after the GRB
explosion. Filled points represent Asiago data,
while open points were acquired by other telescopes.
The Asiago observatory, together with many of the
biggest telescopes in the world, partecipates to the race of chasing
the elusive optical counterparts of gamma-ray bursts (GRBs).
Observations carried out by Italian astronomers allowed to discover one
of these vanishing objects, to measure the distance of
another one, and to monitor the brightness decay of one of the farthest
of them.
Gamma-ray bursts are sudden flashes of gamma-ray radiation, which
pinpoint the death of a massive star collapsing to a black hole. Their
gamma-ray signal, that lasts at most a few minutes, is collected by
dedicated satellites, such as the US/Italy/UK mission Swift.
As soon as the satellite catches one of these
mysterious events, in a matter of seconds it slews to the burst
location, to observe its X-ray counterpart.
Figure 2:
The optical spectrum of the GRB 060124
obtained at Asiago (182cm
telescope Cima Ekar)
using AFOSC .
Then it comes the turn of ground-based optical
observatories. Optical
counterparts may be very bright in the first minutes and up to hours
after the
GRB event, but then rapidly fade. Optical astronomers have thus to
struggle to
observe the GRB position as soon as possible. Thanks to a
flexible
scheduling and TOO observations
the Asiago observatory
was among the first ones to observe three recent GRBs detected by Swift.
In one of these cases (GRB 060203), inspection of the images taken from
Asiago led to the identification of the optical counterpart.
Observations started just about 1 hour after the gamma ray explosion.
The
object could be recognized thanks to its fading behaviour (see Figure
1),
which shows the typical power-law behaviour.
The identification of the GRB counterparts is of course only the first
step to study these objects. One of the primary goals is to estabilsh
their distance, which can be obtained by measuring the
redshifts of the host galaxy.
Figure 2 shows the optical spectrum taken with AFOSC of GRB 060124,
a gamma-ray burst exploded on 2006 January 24. Absorption lines allow
to measure the GRB redshift z = 2.301, which in turns gives its
distance. The GRB thus exploded only 3 billion years after the Big
Bang, that is, when the Universe age was only 20% of its present value.
But there are even more distant and powerful events. On February 6, a
GRB event at z=4 was discovered (see Figure 3). This GRB was so
bright that its light was
observable for
several days. From the Asiago observatory, the optical counterpart was
observed at several epochs, monitoring its temporal evolution.
Figure 3:
The field of GRB 060206, as imaged by Asiago + AFOSC (2006 Feb 8) and
TNG + DOLoRes (2006 Feb 14). The optical counterpart (indicated by a
circle) has
clearly faded between the two exposures.
The observations
presented here were carried out within a large
international
program aimed
to study in detail GRB explosions. In Italy, GRB astronomers operate in
the
environment of two big collaborations, named MISTICI and GRACE.
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