ESO Education and Public Relations Dept.
ESO Press Release 15/98
For immediate release: 15 October 1998
A Strange Supernova with a Gamma-Ray Burst
Important Observations with La Silla Telescopes
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
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 . The measurements at ESO will continue during the next
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.
ESO PR Photo 39a/98 ESO PR Photo 39b/98
PR Photo 39a/98 (left) shows a colour composite of three images obtained
with the EMMI multi-mode instrument at the ESO 3.58-m New Technology
Telescope (NTT) at La Silla on May 4, 1998. The short exposures were
obtained through V (green), R (red) and I (near-infrared) filtres. SN
1998bw is the very bright, bluish star at the center (indicated with an
arrow), located on an arm of spiral galaxy ESO 184-G82. There are several
other galaxies in the field. Compare with Photo 39b/98 (right) that was
obtained before the explosion (ESO 1-m Schmidt Telescope; 15 May 1985;
120-min exposure in red light). In both photos, the field of view measures
3.6 x 3.6 arcmin; North is up and East is left. Note that while the
brighter objects are more prominent on the long-exposure Schmidt photo
(39b/98), considerably more details can be seen on that obtained by the
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
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
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.
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.
 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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