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THE FOLLOWING RELEASE WAS RECEIVED FROM EUROPEAN SOUTHERN OBSERVATORY
HEADQUARTERS IN GARCHING, GERMANY, AND IS FORWARDED FOR YOUR
INFORMATION. (FORWARDING DOES NOT IMPLY ENDORSEMENT BY THE
AMERICAN ASTRONOMICAL SOCIETY.) Steve Maran, AAS
Contact: Dr. Richard M. West
Phone 49-89 320-06276
rwest@eso.org
Text with all links is available on the ESO Website at URL:
http://www.eso.org/outreach/press-rel/pr-1999/pr-08-99.html
ESO Press Release 08/99
18 May 1999
For immediate release
Southern Fireworks above ESO Telescopes
New Insights from Observations of Mysterious Gamma-Ray Burst
International teams of astronomers are now busy working on new and exciting
data obtained during the last week with telescopes at the European Southern
Observatory (ESO).
Their object of study is the remnant of a mysterious cosmic explosion far
out in space, first detected as a gigantic outburst of gamma rays on May 10.
Gamma-Ray Bursters (GRBs) are brief flashes of very energetic radiation -
they represent by far the most powerful type of explosion known in the
Universe and their afterglow in optical light can be 10 million times
brighter than the brightest supernovae [1]. The May 10 event ranks among the
brightest one hundred of the over 2500 GRB's detected in the last decade.
The new observations include detailed images and spectra from the VLT 8.2-m
ANTU (UT1) telescope at Paranal, obtained at short notice during a special
Target of Opportunity programme. This happened just over one month after
that powerful telescope entered into regular service and demonstrates its
great potential for exciting science. In particular, in an observational
first, the VLT measured linear polarization of the light from the optical
counterpart, indicating for the first time that synchrotron radiation is
involved. It also determined a staggering distance of more than 7,000
million light-years to this GRB.
The astronomers are optimistic that the extensive observations will help
them to better understand the true nature of such a dramatic event and thus
to bring them nearer to the solution of one of the greatest riddles of
modern astrophysics.
A prime example of international collaboration
The present story is about important new results at the front-line of
current research. At the same time, it is also a fine illustration of a
successful collaboration among several international teams of astronomers
and the very effective way modern science functions.
It began on May 10, at 08:49 hrs Universal Time (UT), when the Burst And
Transient Source Experiment (BATSE) onboard NASA's Compton Gamma-Ray
Observatory (CGRO) high in orbit around the Earth, suddenly registered an
intense burst of gamma-ray radiation from a direction less than 100 from the
celestial south pole. Independently, the Gamma-Ray Burst Monitor (GRBM) on
board the Italian-Dutch BeppoSAX satellite also detected the event (see GCN
GRB Observation Report 304 [2]). Following the BATSE alert, the BeppoSAX
Wide-Field Cameras (WFC) quickly localized the sky position of the burst
within a circle of 3 arcmin radius in the southern constellation Chamaeleon.
It was also detected by other satellites, including the ESA/NASA Ulysses
spacecraft, since some years in a wide orbit around the Sun.
The event was designated GRB 990510 and the measured position was
immediately distributed by BeppoSAX Mission Scientist Luigi Piro to a
network of astronomers. It was also published on Circular No. 7160 of the
International Astronomical Union (IAU).
>From Amsterdam (The Netherlands), Paul Vreeswijk, Titus Galama, and Evert
Rol of the Amsterdam/Huntsville GRB follow-up team (led by Jan van Paradijs)
immediately contacted astronomers at the 1-meter telescope of the South
African Astronomical Observatory (SAAO) (Sutherland, South Africa) of the
PLANET network microlensing team, an international network led by Penny
Sackett in Groningen (The Netherlands). There, John Menzies of SAAO and
Karen Pollard (University of Canterbury, New Zealand) were about to begin
the last of their 14 nights of observations, part of a continuous world-wide
monitoring program looking for evidence of planets around other stars. Other
PLANET sites in Australia and Tasmania where it was still nighttime were
unfortunately clouded out (some observations were in fact made that night at
the Mount Stromlo observatory in Australia, but they were only announced one
day later).
As soon as possible - immediately after sundown and less than 9 hours after
the initial burst was recorded - the PLANET observers turned their telescope
and quickly obtained a series of CCD images in visual light of the sky
region where the gamma-ray burst was detected, then shipped them off
electronically to their Dutch colleagues [3]. Comparing the new photos with
earlier ones in the digital sky archive, Vreeswijk, Galama and Rol almost
immediately discovered a new, relatively bright visual source in the region
of the gamma-ray burst, which they proposed as the optical counterpart of
the burst, cf. their dedicated webpage at
http://www.astro.uva.nl/~titus/grb990510/ .
The team then placed a message on the international Gamma-Ray Burster
web-noteboard (GCN Circular 310), thereby alerting their colleagues all over
the world. One hour later, the narrow-field instruments on BeppoSax
identified a new X-Ray source at the same location (GCN Circular 311), thus
confirming the optical identification.
All in all, a remarkable synergy of human and satellite resources!
Observations of GRB 990510 at ESO
Vreeswijk, Galama and Rol, in collaboration with Nicola Masetti, Eliana
Palazzi and Elena Pian of the BeppoSAX GRB optical follow-up team (led by
Filippo Frontera) and the Huntsville optical follow-up team (led by Chryssa
Kouveliotou), also contacted the European Southern Observatory (ESO).
Astronomers at the this Organization's observatories in Chile were quick to
exploit this opportunity and crucial data were soon obtained with several of
the main telescopes at La Silla and Paranal, less than 14 hours after the
first detection of this event by the satellite.
[ESO PR Photo 22a/99] ESO PR Photo [ESO PR Photo 22b/99] ESO PR Photo
22a/99 22b/99
[Preview - JPEG: 211 x 400 pix - [Preview - JPEG: 400 x 437 pix -
72k] 297k]
[Normal - JPEG: 422 x 800 pix - [Normal - JPEG: 800 x 873 pix -
212k] 1.1M]
[High-Res - JPEG: 1582 x 3000 pix - [High-Res - JPEG: 2300 x 2509 pix -
2.6M] 5.9M]

Caption to PR Photo 22a/99: This wide-field photo was obtained with the
Wide-Field Imager (WFI) at the MPG/ESO 2.2-m telescope at La Silla on May
11, 1999, at 08:42 UT, under inferior observing conditions (seeing = 1.9
arcsec). The exposure time was 450 sec in a B(lue) filter. The optical
image of the afterglow of GRB 990510 is indicated with an arrow in the
upper part of the field that measures about 8 x 16 arcmin2. The original
scale is 0.24 pix/arcsec and there are 2k x 4k pixels in the original
frame. North is up and East is left.
Caption to PR Photo 22b/99: This is a (false-)colour composite of the area
around the optical image of the afterglow of GRB 990510, based on three
near-infrared exposures with the SOFI multi-mode instrument at the 3.6-m
ESO New Technology Telescope (NTT) at La Silla, obtained on May 10, 1999,
between 23:15 and 23:45 UT. The exposure times were 10 min each in the J-
(1.2 5m; here rendered in blue), H- (1.6 5m; green) and K-bands (2.2 5m;
red); the image quality is excellent (0.6 arcsec). The field measures
about 5 x 5 arcmin2; the original pixel size is 0.29 arcsec. North is up
and East is left.
[ESO PR Photo 22c/99] ESO PR Photo [ESO PR Photo 22d/99] ESO PR Photo
22c/99 22d/99
[Preview - JPEG: 400 x 235 pix - [Preview - JPEG: 400 x 441 pix -
81k] 154k]
[Normal - JPEG: 800 x 469 pix - [Normal - JPEG: 800 x 887 pix -
244k] 561k]
[High-Res - JPEG: 2732 x 1603 pix - [High-Res - JPEG: 2300 x 2537 pix -
2.6M] 2.3M]

Caption to PR Photo 22c/99: To the left is a reproduction of a short (30
sec) centering exposure in the V-band (green-yellow light), obtained with
VLT ANTU and the multi-mode FORS1 instrument on May 11, 1999, at 03:48 UT
under mediocre observing conditions (image quality 1.0 arcsec).The optical
image of the afterglow of GRB 990510 is easily seen in the box, by
comparison with an exposure of the same sky field before the explosion,
made with the ESO Schmidt Telescope in 1986 (right).The exposure time was
120 min on IIIa-F emulsion behind a R(ed) filter. The field shown measures
about 6.2 x 6.2 arcmin2. North is up and East is left.
Caption to PR Photo 22d/99: Enlargment from the 30 sec V-exposure by the
VLT, shown in Photo 22c/99. The field is about 1.9 x 1.9 arcmin2. North is
up and East is left.
The data from Chile were sent to Europe where, by quick comparison of images
from the Wide-Field Imager (WFI) at the MPG/ESO 2.2-m telescope at La Silla
with those from SAAO, the Dutch and Italian astronomers found that the
brightness of the suspected optical counterpart was fading rapidly; this was
a clear sign that the identification was correct (GCN Circular 313).
With the precise sky position of GRB 990510 now available, the ESO observers
at the VLT were informed and, setting other programmes aside under the
Target of Opportunity scheme, were then able to obtain polarimetric data as
well as a very detailed spectrum of the optical counterpart.
Comprehensive early observations of this object were also made at La Silla
with the ESO 3.6-m telescope (CCD images in the UBVRI-bands from the
ultraviolet to the near-infrared part of the spectrum) and the ESO 3.6-m New
Technology Telescope (with the SOFI multimode instrument in the infrared
JHK-bands). A series of optical images in the BVRI-bands was secured with
the Danish 1.5-m telescope, documenting the rapid fading of the object.
Observations at longer wavelengths were made with the 15-m Swedish-ESO
Submillimetre Telescope (SEST).
All of the involved astronomers concur that a fantastic amount of
observations has been obtained. They are still busy analyzing the data, and
are confident that much will be learned from this particular burst.
The VLT scores a first: Measurement of GRB polarization
[ESO PR Photo 22e/99] ESO PR Photo Caption to PR Photo 22e/99:
22e/99 Preliminary polarization measurement
of the optical image of the
[Preview - JPEG: 400 x 434 pix - afterglow of GRB 990510, as observed
92k] with the VLT 8.2-m ANTU telescope
and the multi-mode FORS1 instrument.
[Normal - JPEG: 800 x 867 pix - The abscissa represents the
228k] measurement angle; the ordinate the
corresponding intensity. The
sinusoidal curve shows the best fit
to the data points (with error
bars); the resulting degree of
polarization is 1.7 1 0.2 percent.
A group of Italian astronomers led by Stefano Covino of the Observatory of
Brera in Milan, have observed for the first time polarization (some degree
of alignment of the electric fields of emitted photons) from the optical
afterglow of a gamma-ray burst, see their dedicated webpage at
http://www.merate.mi.astro.it/~lazzati/GRB990510/ . This yielded a
polarization at a level of 1.7 10.2 percent for the optical afterglow of GRB
990510, some 18 hours after the gamma-ray burst event; the magnitude was R =
19.1 at the time of this VLT observation. Independently, the Dutch
astronomers Vreeswijk, Galama and Rol measured polarization of the order of
2 percent with another data set from the VLT ANTU and FORS1 obtained during
the same night.
This important result was made possible by the very large light-gathering
power of the 8.2-m VLT-ANTU mirror and the FORS1 imaging polarimeter.
Albeit small, the detected degree of polarization is highly significant; it
is also one of the most precise measurements of polarization ever made in an
object as faint as this one. Most importantly, it provides the strongest
evidence to date that the afterglow radiation of gamma-ray bursts is, at
least in part, produced by the synchrotron process, i.e. by relativistic
electrons spiralling in a magnetized region. This type of process is able to
imprint some linear polarization on the produced radiation, if the magnetic
field is not completely chaotic.
The spectrum
[ESO PR Photo 22f/99] ESO PR Photo Caption to PR Photo 22f/99: A
22f/99 spectrum of the afterglow of GRB
990510, obtained with VLT ANTU and
[Preview - JPEG: 400 x 485 pix - the multi-mode FORS1 instrument
112k] during the night of May 10-11, 1999.
Some of the redshifted absorption
[Normal - JPEG: 800 x 969 pix - lines are identified and the
288k] stronger bands from the terrestrial
atmosphere are also indicated.
A VLT spectrum with the multi-mode FORS1 instrument was obtained a little
later and showed a number of absorption lines, e.g. from ionized Aluminium,
Chromium and neutral Magnesium. They do not arise in the optical counterpart
itself - the gas there is so hot and turbulent that any spectral lines will
be extremely broad and hence extremely difficult to identify - but from
interstellar gas in a galaxy 'hosting' the GRB source, or from intergalactic
clouds along the line of sight. It is possible to measure the distance to
this intervening material from the redshift of the lines; astronomers
Vreeswijk, Galama and Rol found z = 1.619 1 0.002 [4]. This allows to
establish a lower limit for the distance of the explosion and also its total
power.
The numbers turn out to be truly enormous. The burst occurred at an epoch
corresponding to about one half of the present age of the Universe (at a
distance of about 7,000 million light-years [5]), and the total energy of
the explosion in gamma-rays must be higher than 1.4 1053 erg, assuming a
spherical emission. This energy corresponds to the entire optical energy
emitted by the Milky Way in more than 30 years; yet the gamma-ray burst took
less than 100 seconds.
Since the optical afterglows of gamma-ray bursts are faint, and their flux
decays quite rapidly in time, the combination of large telescopes and fast
response through suitable observing programs are crucial and, as
demonstrated here, ESO's VLT is ideally suited to this goal!
The lightcurve
Combining results from a multitude of telescopes has provided most useful
information. Interestingly, a "break" was observed in the light curve (the
way the light of the optical counterpart fades) of the afterglow. Some 1.5 -
2 days after the explosion, the brightness began to decrease more rapidly;
this is well documented with the CCD images from the Danish 1.5-m telescope
at La Silla and the corresponding diagrams are available on a dedicated
webpage at http://www.astro.ku.dk/~jens/grb990510/ at the Copenhagen
University Observatory. Complete, regularly updated lightcurves with all
published measurements, also from other observatories, may be found at
another webpage in Milan at http://www.merate.mi.astro.it/~gabriele/990510/
.
This may happen if the explosion emits radiation in a beam which is pointed
towards the Earth. Such beams are predicted by some models for the
production of gamma-ray bursts. They are also favoured by many astronomers,
because they can overcome the fundamental problem that gamma-ray bursts
simply produce too much energy. If the energy is not emitted equally in all
directions ("isotropically"), but rather in a preferred one along a beam,
less energy is needed to produce the observed phenomenon.
Such a break has been observed before, but this time it occurred at a very
favourable moment, when the source was still relatively bright so that
high-quality spectroscopic and multi-colour information could be obtained
with the ESO telescopes. Together, these observations may provide an answer
to the question whether beams exist in gamma-ray bursts and thus further
help us to understand the as yet unknown cause of these mysterious
explosions.
Latest News
[ESO PR Photo 22g/99] ESO PR Photo Caption to PR Photo 22g/99: V(isual)
22g/99 image of the sky field around GRB
990510 (here denoted "OT"), as
[Normal - JPEG: 453 x 585 pix - obtained with the VLT ANTU telescope
304k] and FORS1 on May 18 UT during a 20
min exposure in 0.9 arcsec seeing
conditions. North is up and east is
left.
Further photometric and spectroscopic observations with the ESO VLT,
performed by Klaus Beuermann, Frederic Hessman and Klaus Reinsch of the
Gvettingen group of the FORS instrument team (Germany), have revealed the
character of some of the objects that are seen close to the image of the
afterglow of GRB 990510 (also referred to as the "Optical Transient" - OT).
Two objects to the North are cool foreground stars of spectral types dM0 and
about dM3, respectively; they are located in our Milky Way Galaxy. The
object just to the South of the OT is probably also a star.
A V(isual)-band image (PR Photo 22g/99) taken during the night between May
17 and 18 with the VLT/ANTU telescope and FORS1 now shows the OT at
magnitude V = 24.5, with still no evidence for the host galaxy that is
expected to appear when the afterglow has faded sufficiently.
Outlook
The great distances (high redshifts) of Gamma-Ray Bursts, plus the fact that
a 9th magnitude optical flash was seen when another GRB exploded on January
23 this year, has attracted the attention of astronomers outside the GRB
field. In fact, GRBs may soon become a very powerful tool to probe the early
universe by guiding us to regions of very early star formation and the
(proto)-galaxies and (proto)-clusters of which they are part. They will also
allow the study of the chemical composition of absorbing clouds at very
large distances.
At the end of this year, the NASA satellite HETE-II will be launched, which
is expected to provide about 50 GRB alerts per year and, most importantly,
accurate localisations in the sky that will allow very fast follow-up
observations, while the optical counterparts are still quite bright. It will
then be possible to obtain more spectra, also of extremely distant bursts,
and many new distance determinations can be made, revealing the distribution
of intrinsic brightness of GRB's (the "luminosity function"). Other types of
observations (e.g. polarimetry, as above) will also profit, leading to a
progressive refinement of the available data.
Thus there is good hope that astronomers will soon come closer to
identifying the progenitors of these enormous explosions and to understand
what is really going on. In this process, the huge light-collecting power of
the VLT and the many other facilities at the ESO observatories will
undoubtedly play an important role.
Notes
[1] Gamma-Ray Bursts are brief flashes of high-energy radiation. Satellites
in orbit around the Earth and spacecraft in interplanetary orbits have
detected several thousand such events since they were first discovered in
the late 1960s.
Earlier investigations established that they were so evenly distributed in
the sky that they must be very distant (and hence very powerful) outbursts
of some kind. Only in 1997 it became possible to observe the fading
"afterglow" of one of these explosions in visible light, thanks to accurate
positions available from the BeppoSAX satellite. Soon thereafter, another
optical afterglow was detected; it was located in a faint galaxy whose
distance could be measured. In 1998, a gamma-ray burst was detected in a
galaxy over 8,300 million light-years away.
Even the most exotic ideas proposed for these explosions, e.g. supergiant
stars collapsing to black holes, black holes merging with neutron stars or
other black holes, and other weird and wonderful notions have trouble
accounting for explosions with the power of 10,000 million million suns.
[2] The various reports issued by astronomers working on this and other
gamma-ray burst events are available as GCN Circulars on the GRB Coordinates
Network web-noteboard.
[3] See also the Press Release, issued by SAAO on this occasion.
[4] In astronomy, the redshift (z) denotes the fraction by which the lines
in the spectrum of an object are shifted towards longer wavelengths. The
observed redshift of a distant galaxy or intergalactic cloud gives a direct
estimate of the universal expansion (i.e. the "recession velocity"). The
detailed relation between redshift and distance depends on such quantities
as the Hubble Constant, the average density of the universe, and the
'Cosmological' Constant. For a standard cosmological model, redshift z = 1.6
corresponds to a distance of about 7,000 million light-years.
[5] Assuming a Hubble Constant H0 = 70 km/s/Mpc, mean density Omega0 = 0.3
and a Cosmological Constant Lambda = 0.
------------------------------------------------------------------------