ESA’s X-ray observatory, XMM-Newton, has imaged a spectacular set of
rings which appear to expand, with a speed a thousand times faster
than that of light, around the point in the sky where a powerful
gamma-ray explosion took place in early December. This is the first
time that such a fascinating event, called an `echo’, is seen in X-ray
wavelengths. This echo forms when the powerful radiation of a gamma-ray
burst, coming from far away, crosses a slab of dust in our Galaxy and
is scattered by it, like the beam of a lighthouse in clouds. Using the
expanding rings to precisely pin-point the location of this dust,
astronomers can identify places where new stars and planets are likely
to form.

On 3 December 2003 ESA’s observatory, Integral, detected a burst of
gamma rays, lasting about 30 seconds, from the direction of a distant
galaxy. Within minutes of the detection, thanks to a sophisticated
alert network, many observatories around the world were pointing their
instruments at this mysterious source in the sky, named GRB 031203, in
the attempt to decipher its nature. Also ESA’s X-ray observatory,
XMM-Newton, joined the hunt and observed the source in detail, using
its on-board European Photon Imaging Camera (EPIC).

The fading X-ray emission from GRB 031203 – called the `afterglow’ – is
clearly seen in XMM-Newton’s images. But much more stunning are the two
rings, centred on the afterglow, which appear to expand thousand times
faster than the speed of light. Dr. Simon Vaughan, of the University of
Leicester, United Kingdom, leads an international team of scientists
studying GRB 031203. He explains that these rings are what astronomers
call an `echo’. They form when the X-rays from the distant gamma-ray
burst shine on a layer of dust in our own Galaxy. “The dust scatters
some of the X-rays, causing XMM-Newton to observe these rings, much in
the same way as fog scatters the light from a car’s headlights,” said
Vaughan.

Although the afterglow is the brightest feature seen in XMM-Newton’s
images, the expanding echo is much more spectacular. “It is like a
shout in a cathedral,” Vaughan said. “The shout of the gamma-ray burst
is louder, but the Galactic reverberation, seen as the rings, is much
more beautiful.” The rings seem to expand because the X-rays scattered
by dust farther from the direction of GRB 031203 take longer to reach
us than those hitting the dust closer to the line of sight. However,
nothing can move faster than light. “This is precisely what we expect
because of the finite speed of light,” said Vaughan. “The rate of
expansion that we see is just a visual effect.” He and his colleagues
explain that we see two rings because there are two thin sheets of dust
between the source of the gamma-ray burst and Earth, one closer to us
creating the wider ring and one further away where the smaller ring is
formed.

Since they know precisely at which speed the X-ray light travels in
space, the team in Leicester have determined accurately the distance to
the dust sheets by measuring the size of the expanding rings. The
nearest dust sheet is located 2900 light years away and is probably
part of the Gum nebula, a bubble of hot gas resulting from many
supernova explosions. The other dust layer is about 4500 light years
away. Understanding how dust is distributed in our Galaxy is important
because dust favours the collapse of cool gas clouds, which can then
form stars and planets. Knowing where dust is located helps astronomers
to determine where star and planet formation is likely to occur.

Expanding X-ray dust scattering rings, such as those around GRB 031203,
have never been seen before. Slower-moving rings, caused by a similar
effect, have been seen in visible light around a very few exploding
stars, mostly supernovae.

The expanding rings also provide much needed information on the
gamma-ray burst itself. Gamma-ray bursts are the most powerful
explosive events in the Universe, but astronomers are still trying to
understand the mystery that surrounds their origin. Some occur with the
supernova explosion of a massive star when it has used up all of its
fuel, although only stars which have lost their outer layers and which
collapse to make a black hole seem able to make a gamma-ray burst. The
delayed X-rays from the echo of GRB 031203 are very useful because they
tell astronomers how bright the burst was in the X-ray spectrum when it
went off on 3 December. The only direct data available from that moment
are those obtained by ESA’s Integral observatory in the gamma-ray
range. “XMM-Newton’s measurements are thus crucial to better understand
the nature of the burst,” said Dr. Fred Jansen, XMM-Newton’s project
scientist. “The more details we gather of the burst, the more we can
learn on how black holes are made.”

Today, ESA’s Integral and XMM-Newton observatories provide astronomers
with their most powerful facilities for studying gamma-ray bursts. In
2004 a new gamma-ray satellite, called `Swift’, will be launched as
part of a collaboration between the USA, United Kingdom and Italy.
Swift will add to the flotilla of satellites providing fast and
accurate locations of gamma-ray bursts on the sky, which can then be
followed with XMM-Newton. This will provide even more opportunities for
new discoveries in this cutting-edge field.

Notes to editors

A scientific paper describing this discovery by Dr. Simon Vaughan and
his collaborators has been accepted for publication in “The
Astrophysical Journal” (see http://arxiv.org/abs/astro-ph/0312603).
The other members in Vaughan’s team are R. Willingale, P. O’Brien, J.
Osborne, A. Levan, M. Watson and J. Tedds from the University of
Leicester, United Kingdom; J. Reeves from NASA’s Goddard Space Flight
Center in Greenbelt, USA; D. Watson from the Neils Bohr Institute for
Astronomy in Copenhagen, Denmark; M. Santos-Lleo, P. Rodriguez-Pascual
and N. Schartel from ESA’s XMM-Newton Science Operations Centre in
Villafranca, Spain.

More about XMM-Newton

XMM-Newton can detect more X-ray sources than any previous satellite
and is helping to solve many cosmic mysteries of the violent Universe,
from black holes to the formation of galaxies. It was launched on 10
December 1999, using an Ariane-5 rocket from French Guiana. It is
expected to return data for a decade. XMM-Newton’s high-tech design
uses over 170 wafer-thin cylindrical mirrors spread over three
telescopes. Its orbit takes it almost a third of the way to the Moon,
so that astronomers can enjoy long, uninterrupted views of celestial
objects.

Figure and caption

High resolution image (approximage size 4 MB) available now at:
http://esamultimedia.esa.int/images/spcs/xmmnewton/snr2004-02.tiff

Caption: XMM-Newton’s X-ray EPIC camera shows the expanding rings
caused by a flash of X-rays scattered by dust in our Galaxy. The X-rays
were produced by a powerful gamma-ray burst that took place on 3
December 2003. The slowly fading afterglow of the gamma-ray burst is at
the centre of the expanding rings. Other, unrelated, X-ray sources can
also be seen. The time since the gamma-ray explosion is shown in each
panel in hours. At their largest size, the rings would appear in the
sky about five times smaller than the full moon.

Credit: ESA, S. Vaughan (University of Leicester)

Video and caption
Animation (approximate size 2.1 MB) available now at:

http://esamultimedia.esa.int/images/spcs/xmmnewton/snr2004-02.gif

Caption: XMM-Newton’s X-ray EPIC camera shows the expanding rings
caused by a flash of X-rays scattered by dust in our Galaxy. The X-rays
were produced by a powerful gamma-ray burst that took place on 3
December 2003. The slowly fading afterglow of the gamma-ray burst is at
the centre of the expanding rings. Other, unrelated, X-ray sources can
also be seen. The time since the gamma-ray explosion is shown in each
panel in seconds. At their largest size, the rings would appear in the
sky about five times smaller than the full moon.

Credit: ESA, S. Vaughan (University of Leicester)

Internet publication

On 26 January 2004, at 10.00 CET, the text and figure of this
release will be published on: http://www.esa.int/science/media