X-ray shout echoing through spaceJan 27, 2004( taken from SpaceFlightNow ) |
The European Space Agency's X-ray observatory, XMM-Newton, has imaged a spectacular set of
rings which appear to expand, with a speed a thousand times fasterthan that of light, around the point in the sky where a powerfulgamma-ray explosion took place in early December. This is the firsttime that such a fascinating event, called an 'echo', is seen in X-raywavelengths. This echo forms when the powerful radiation of a gamma-rayburst, coming from far away, crosses a slab of dust in our Galaxy andis scattered by it, like the beam of a lighthouse in clouds. Using theexpanding rings to precisely pin-point the location of this dust,astronomers can identify places where new stars and planets are likelyto form.
 XMM-Newton's X-ray EPIC camera shows the expanding rings caused by a flash of X-rays scattered by dust in our Galaxy. Credits: ESA, S. Vaughan (University of Leicester) |
The fading X-ray emission from GRB 031203 - called the 'afterglow' - isclearly seen in XMM-Newton's images. But much more stunning are the tworings, centred on the afterglow, which appear to expand thousand timesfaster than the speed of light. Dr. Simon Vaughan, of the University ofLeicester, United Kingdom, leads an international team of scientistsstudying GRB 031203. He explains that these rings are what astronomerscall an 'echo'. They form when the X-rays from the distant gamma-rayburst shine on a layer of dust in our own Galaxy. "The dust scatterssome of the X-rays, causing XMM-Newton to observe these rings, much inthe same way as fog scatters the light from a car's headlights," saidVaughan.
Although the afterglow is the brightest feature seen in XMM-Newton'simages, the expanding echo is much more spectacular. "It is like ashout in a cathedral," Vaughan said. "The shout of the gamma-ray burstis louder, but the Galactic reverberation, seen as the rings, is muchmore beautiful." The rings seem to expand because the X-rays scatteredby dust farther from the direction of GRB 031203 take longer to reachus than those hitting the dust closer to the line of sight. However,nothing can move faster than light. "This is precisely what we expectbecause of the finite speed of light," said Vaughan. "The rate ofexpansion that we see is just a visual effect." He and his colleaguesexplain that we see two rings because there are two thin sheets of dustbetween the source of the gamma-ray burst and Earth, one closer to uscreating the wider ring and one further away where the smaller ring isformed.
Since they know precisely at which speed the X-ray light travels inspace, the team in Leicester have determined accurately the distance tothe dust sheets by measuring the size of the expanding rings. Thenearest dust sheet is located 2900 light years away and is probablypart of the Gum nebula, a bubble of hot gas resulting from manysupernova explosions. The other dust layer is about 4500 light yearsaway. Understanding how dust is distributed in our Galaxy is importantbecause dust favours the collapse of cool gas clouds, which can thenform stars and planets. Knowing where dust is located helps astronomersto 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 similareffect, have been seen in visible light around a very few explodingstars, mostly supernovae.
The expanding rings also provide much needed information on thegamma-ray burst itself. Gamma-ray bursts are the most powerfulexplosive events in the Universe, but astronomers are still trying tounderstand the mystery that surrounds their origin. Some occur with thesupernova explosion of a massive star when it has used up all of itsfuel, although only stars which have lost their outer layers and whichcollapse to make a black hole seem able to make a gamma-ray burst. Thedelayed X-rays from the echo of GRB 031203 are very useful because theytell astronomers how bright the burst was in the X-ray spectrum when itwent off on 3 December. The only direct data available from that momentare those obtained by ESA's Integral observatory in the gamma-rayrange. "XMM-Newton's measurements are thus crucial to better understandthe nature of the burst," said Dr. Fred Jansen, XMM-Newton's projectscientist. "The more details we gather of the burst, the more we canlearn on how black holes are made."
Today, ESA's Integral and XMM-Newton observatories provide astronomerswith their most powerful facilities for studying gamma-ray bursts. In2004 a new gamma-ray satellite, called 'Swift', will be launched aspart of a collaboration between the USA, United Kingdom and Italy.Swift will add to the flotilla of satellites providing fast andaccurate locations of gamma-ray bursts on the sky, which can then befollowed with XMM-Newton. This will provide even more opportunities fornew discoveries in this cutting-edge field.
XMM-Newton can detect more X-ray sources than any previous satelliteand is helping to solve many cosmic mysteries of the violent Universe,from black holes to the formation of galaxies. It was launched on 10December 1999, using an Ariane-5 rocket from French Guiana. It isexpected to return data for a decade. XMM-Newton's high-tech designuses over 170 wafer-thin cylindrical mirrors spread over threetelescopes. Its orbit takes it almost a third of the way to the Moon,so that astronomers can enjoy long, uninterrupted views of celestialobjects.
