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An international team of astronomers led by researchers at the
Max-Planck Institute for Extraterrestrial Physics (MPE) in
Garching (Germany) [2] has discovered powerful infrared flares
from the supermassive black hole at the heart of the Milky Way.

The signals, rapidly flickering on a scale of minutes, must
come from hot gas falling into the black hole, just before it
disappears below the "event horizon" of the monster. The new
observations strongly suggest that the Galactic Centre black
hole rotates rapidly.

Never before have scientists been able to study phenomena in
the immediate neighbourhood of a black hole in such a detail.
The new result is based on observations obtained with the
NACO Adaptive Optics instrument on the 8.2-m VLT KUEYEN
telescope and is published in this week’s edition of the
research journal Nature.

The scene was the usual one in the VLT Control Room at the
Paranal Observatory in the early morning of May 9, 2003.
Groups of astronomers from different nations were sitting
in front of the computer screens, pointing the four giant
telescopes in different directions and recording the sparse
photons from the remotest corners of the Universe. There
were the usual brief exchanges of information, numbers,
wavelengths, strange acronyms, but then suddenly something
happened at the YEPUN desk …

"What is that star doing there?" exclaimed Rainer Schˆdel,
one of the MPE scientists in the team working with the NACO
Adaptive Optics instrument [3] that delivers razor-sharp
images. He and Reinhard Genzel, leader of the team and MPE
Director, were observing the Milky Way Centre, when they
saw the "new" object on the screen in front of them. The
astronomers were puzzled and then became excited — something
unusual must be going on, there at the centre of our galaxy!

And then, a few minutes later, the "star" disappeared from
view. Now the scientists had little doubt — they had just
witnessed, for the first time, a powerful near-infrared flare
from exactly the direction of the supermassive black hole at
the heart of the Milky Way, cf. PR Photo 29a/03 and PR Video
Clip 01/03.

"We had been looking for infrared emission from that black
hole for more than a decade" recalls another team member,
Andreas Eckart of the Cologne University. "We were certain
that the black hole must be accreting matter from time to
time. As this matter falls towards the surface of the black
hole, it gets hotter and hotter and starts emitting infrared

But no such infrared radiation had been seen until that night
at the VLT. This was the wonderful moment of breakthrough.
Never before had anybody witnessed the last "scream" from
matter in the deadly grip of a black hole, about to pass the
point of no return towards an unknown fate.

At the border

ESO PR Photo 29b/03

Captions: PR Photo 29b/03 displays the "light curve" of a
light flare from the galactic centre, as observed in the
K-band (wavelength 2.2 µm) on June 16, 2003. This and a
second flare discovered about 24 hours earlier show
variability on a time scale of a few minutes and appear
to show larger variations (arrows) with a 17-minute
periodicity. The rapid variability implies that the
infrared emission comes from just outside (the event
horizon of) the black hole. If the periodicity is a
fundamental property of the motion of gas orbiting the
black hole, the Galactic Centre black hole must rotate
with about half the maximum spin rate allowed by General
Relativity. The present observations thus probe the
space-time structure in the immediate vicinity of that
event horizon.

A careful analysis of the new observational data, reported in
this week’s issue of the Nature magazine, has revealed that
the infrared emission originates from within a few thousandths
of an arcsecond [4] from the position of the black hole
(corresponding to a distance of a few light-hours) and that
it varies on time scales of minutes (PR Photo 29b/03).

This proves that the infrared signals must come from just
outside the so-called "event horizon" of the black hole, that
is the "surface of no return" from which even light cannot
escape. The rapid variability seen in all data obtained by
the VLT clearly indicates that the region around this horizon
must have chaotic properties — very much like those seen in
thunderstorms or solar flares [5].

"Our data give us unprecedented information about what happens
just outside the event horizon and let us test the predictions
of General Relativity" explains Daniel Rouan, a team member
from Paris-Meudon Observatory. "The most striking result is
an apparent 17-minute periodicity in the light curves of two
of the detected flares. If this periodicity is caused by the
motion of gas orbiting the black hole, the inevitable
conclusion is that the black hole must be rotating rapidly".

Reinhard Genzel is very pleased: "This is a major breakthrough.
We know from theory that a black hole can only have mass, spin
and electrical charge. Last year we were able to unambiguously
prove the existence and determine the mass of the Galactic
Centre black hole (ESO Press Release 17/02). If our assumption
is correct that the periodicity is the fundamental orbital
time of the accreting gas, we now have also measured its spin
for the first time. And that turns out to be about half of
the maximum spin that General Relativity allows".

He adds: "Now the era of observational black hole physics has
truly begun!"

More information

The results described in this ESO press release are presented
in a report published today in the research journal "Nature"
("Near-IR Flares from Accreting Gas around the Supermassive
Black Hole in the Galactic Centre", by Reinhard Genzel and


[1]: This press release is issued in coordination between ESO,
the Max Planck Society (Munich, Germany) and the Centre
National de la Recherche Scientifique (CNRS (Paris, France).
A German and a French versions are also available.

[2]: The team consists of Reinhard Genzel, Rainer Schˆdel,
Thomas Ott and Bernd Aschenbach (Max-Planck-Institut f¸r
extraterrestrische Physik, Garching, Germany), Andreas Eckart
(Physikalisches Institut, Universit‰t zu Kˆln, Cologne,
Germany), Tal Alexander (The Weizmann Institute of Science,
Rehovot, Israel), FranÁois Lacombe and Daniel Rouan (LESIA –
Observatoire de Paris-Meudon, France).

[3]: The NACO facility has two major components, CONICA and
NAOS. The COudÈ Near-Infrared CAmera (CONICA) was developed
by a German Consortium, with an extensive ESO collaboration.
The Consortium consists of Max-Planck-Institut f¸r Astronomie
(MPIA) (Heidelberg) and the Max-Planck-Institut f¸r
Extraterrestrische Physik (MPE) (Garching). The Nasmyth
Adaptive Optics System (NAOS) was developed, with the support
of INSU-CNRS, by a French Consortium in collaboration with
ESO. The French consortium consists of Office National
d’Etudes et de Recherches AÈrospatiales (ONERA), Laboratoire
d’Astrophysique de Grenoble (LAOG) and Observatoire de Paris
(LESIA and GEPI). Adaptive Optics (AO) is a technique that
allows overcoming the image distortions in the optical/
infrared wavelength region caused by the turbulent
terrestrial atmosphere. The wave distortions of the incoming
waves are detected and analyzed in a fast sensor/computer
system, and then "undone" online with a so-called deformable
mirror. Adaptive optics thus allows ~0.040 arcsec resolution
images on the 8.2-m VLT telescopes in the near-infrared, about
10 times sharper than with conventional "seeing limited"
observations and about 4 times sharper than the Hubble Space
Telescope working at this wavelength.

[4]: One thousandth of an arcsecond corresponds to about 2
metres at the distance of the Moon.

[5]: Time variability of the black hole at the centre of the
Milky Way on time scales of hours to days at longer wavelengths
was also independently discovered by a second team at the
University of California, Los Angeles, working with the
William Keck Telescope at Mauna Kea (Hawaii, USA).