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For the convenience of our readers, we provide here the press
releases provided by the University
of Amsterdam and the European
Southern Observatory.
Astronomers witness the birth of a black hole
University of Amsterdam, Oct. 15, 1998. An international
team of astronomers have identified the gamma-ray burst of April
25, 1998 with a rare type of supernova. This supernova signalled
the formation of a black hole.
During the 1970's American military satellites detected the
existence of brief outbursts of gamma rays. Gamma rays are very
energetc radiation (similar in nature but more energetic than
light or radio waves), which, for instance, is emitted in nuclear
explosions. The gamma-ray bursts (as they were called) did not
come from bombs but from mysterious sources somewhere in the
Universe. To detect such gamma-ray bursts and accurately locate
them on the sky Dr John Heise of the Dutch Space Research Organisation
SRON built the Wide Field Camera, which is now circling around
the Earth on board of the Italian-Dutch satellite BeppoSAX. With
this camera the position of a gamma-ray burst can be determined
with an accuracy of a few arcminutes across a field of view of
40 by 40 degrees. [note: the diameter of the full moon is 30
arcminutes]. Thanks to these accurate positions a team lead by
astrophysicist Dr Jan van Paradijs (University of Amsterdam and
University of Alabama in Huntsville) was able , in February 1997,
for the first time to identify a gamma-ray burst with a weak
source of optical radiation. Since then optical and radio telescopes
have detected such 'afterglows' for another seven gamma-ray bursts.
It turned out that the sources of these gamma-ray bursts are
located billions of light years away from us, in faint galaxies.
In several cases it was possible to determine their distances
from the redshift of the optical light, which is caused by the
expansion of the Universe as a whole.
On April 25 1998 BeppoSAX registered another gamma-ray burst.
University of Amsterdam graduate students Titus Galama and Paul
Vreeswijk alerted optical observatories in Chile and Australia,
and several hours later the first exposures were made of the
gamma-ray burst location on the sky. From a comparison with images
made during the next several nights Galama and Vreeswijk found
that in that same direction a supernova had occurred, at about
the same time as the gamma-ray burst. This was a big surprise,
since a relation between gamma-ray bursts and supernovae had
never been found before. The explosion had taken place not in
the far recesses of the universe, but in a relatively nearby
galaxy, at a distance of about 120 million light years, cosmologically
speaking just around the corner. A gamma-ray burst from such
a small distance blurred the picture that astrophysicists had
just obtained on the basis of the afterglow observations of gamma-ray
bursts which showed them to be located some hundred times farther
away, and to be intrinsically more than ten thousand times as
powerful than the event of April 25. The probability that the
gamma-ray burst and the supernova would occur in the same direction
within a day of each other just by chance is less than one in
ten thousand according to a conservative estimate. Apparently
nature can produce gamma-ray bursts in two very different ways.
The second surprise is the supernova itself. In the first
place the optical spectrum looks abnormal. It shows no signs
for the presence of hydrogen and helium, by far the most abundant
elements in the Universe. This implies that the supernova was
caused by the collapse of a very massive star that consisted
predominantly of carbon and oxygen. In the second place American
astronomers had found that the supernova is also a strong source
of radio emission.
Japanese astronomers of the University of Tokyo have made
extensive computer simulations of the supernova, on the basis
of the optical observations during the weeks and months following
April 25. They come to the conclusion that in the collapse of
the very massive star which caused the supernova, at least three
times the mass of the Sun was left behind in the firm of a compact
star. According to out present knowledge this is too massive
for a neutron star, hence the conclusion that in this supernova
a black hole was formed.
The research described in this note was done in collaboration
with dr John Heise and dr Jean in 't Zand of the Space Research
Laboratory (Utrecht, the Netherlands), astronomers of the Marshall
Space Flight Center, Huntsville Alabama (Dr Chryssa Kouveliotou
and collaborators) and the European Southern Observatory (Dr
Thomas Augusteijn and collaborators), and the group of Dr Filippo
Frontera of the Bologna Observatory. The research in Amsterdam
is partially funded by the Dutch ASTRON foundation of the Netherlands
Organisation for Research.
A Strange Supernova with a Gamma-Ray Burst:
Important Observations with La Silla Telescopes
European Southern Observatory, Oct. 15, 1998. Several
articles appear today in the scientific journal Nature
about the strange supernova SN 1998bw that exploded earlier this
year in the spiral galaxy ESO184 G-82. These studies indicate
that this event was linked to a Gamma-Ray Burst and may thus
provide new insights into this elusive phenomenon.
Important observations of SN 1998bw have been made with several
astronomical telescopes at the ESO La Silla Observatory by some
of the co-authors of the Nature articles [1]. The measurements
at ESO will continue during the next years.
The early observations
On April 25, the BeppoSAX satellite detected a Gamma-Ray Burst
from the direction of the constellation Telescopium, deep in
the southern sky. Although there is now general consensus that
they originate in very distant galaxies, the underlying physical
causes of these events that release great amounts of energy within
seconds are still puzzling astronomers.
Immediately after reports about the April 25 Burst had been
received, astronomers at La Silla took some images of the sky
region where the gamma-rays were observed as a "Target of
Opportunity" (ToO) programme. The aim was to check if the
visual light of one of the objects in the field had perhaps brightened
when compared to exposures made earlier. This would then provide
a strong indication of the location of the Gamma-Ray Burst.
The digital exposures were transferred to the Italian/Dutch
group around BeppoSax that had requested these ToO observations.
Astronomers of this group quickly noticed a new, comparatively
bright star, right on the arm of a small spiral galaxy. This
galaxy was first catalogued in the 1970's during the ESO/Uppsala
Survey of the Southern Sky and received the designation ESO184-G82.
It is located at a distance of about 140 million light-years.
The ESO astronomers at La Silla decided to continue observations
of the new star-like object and set up a comprehensive programme
with several telescopes at that observatory. During the subsequent
weeks and months, they obtained images through various filtres
to determine the brightness in different colours, as well as
detailed spectra. These observations soon showed the object to
be a supernova. This is a heavy star that explodes during a late
and fatal evolutionary stage. The new supernova now received
the official designation SN 1998bw.
From a careful study based on these observations, it has been
concluded that SN 1998bw underwent an exceptionally powerful
explosion, more violent than most other supernovae observed so
far. It was also unusual in the sense that very strong radio
emission was observed within a few days after the explosion -
normally this only happens after several weeks. In fact, at radio
wavelengths, SN 1998bw was the brightest supernova ever observed.
The origin of the Gamma-Ray Burst
SN 1998bw is obviously an unusual supernova. It is therefore
of particular significance that a Gamma-Ray Burst was observed
from the same sky region just before it was discovered in optical
light. It is very unlikely that these two very rare events would
happen in the same region of the sky without being somehow related.
Most astronomers therefore tend to believe that the gamma-rays
do indeed originate in the supernova explosion.
But can a single supernova be sufficiently energetic to produce
a powerful Gamma-Ray Burst? New theoretical calculations, also
published today in Nature, indicate that this may be so. Moreover,
if the Gamma-Ray Burst observed on April 25 did originate in
this supernova that is located in a relatively nearby galaxy,
it was intrinsically much fainter than some of the other Gamma-Ray
Bursts that are known to have taken place in extremely distant
galaxies.
The main idea is that while the centres of most other supernovae
collapse into neutron stars at the moment of explosion, a black
hole was created in a very massive star consisting mostly of
carbon and oxygen. If so, a very strong shockwave may be produced
that is capable of generating the observed gamma rays.
A comparison of synthetic spectra from such a supernova model,
based on a new spectrum-modelling technique developed by Leon
Lucy at the ESA/ESO Space Telescope/European Coordinating Facility
(ST/ECF), with the spectra of SN 1998bw observed at La Silla,
show good agreement, thus lending credibility to the new models.
Future work
Much data has already been collected at ESO on the strange
supernova SN 1998bw. More observations will be obtained by the
astronomers at the ESO observatories in the future during a long-term
monitoring programme of SN 1998bw. There is a good chance that
this effort will ultimately provide fundamental information on
the explosion mechanism and the nature of the progenitor star
of this exceptional object.
This supernova's connection with a Gamma-Ray Burst will significantly
enhance our understanding of the nature of these powerful and
enigmatic events. In view of the range in emitted energy, it
now seems likely that there may be more than one class of Gamma-Ray
Burst.
According to some models for Gamma-Ray Bursts that include
beaming (emission of the radiation in one prefered direction),
it is possible that these events are only detected if they have
a favourable angle with respect to the line of sight. In the
case of SN 1998bw this is probably not the case, however, and
it was only detected in gamma-rays, because it is so relatively
nearby. The question of differences in intrinsic brightness and
possible different classes of objects is far from settled yet.
Note:
[1] The ESO astronomers involved in this work are Thomas Augusteijn,
Hermann Boehnhardt, James Brewer, Vanessa Doublier, Jean-Francois
Gonzalez, Olivier Hainaut, Bruno Leibundgut, Christopher Lidman
and Fernando Patat.
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