X-ray and Observational Astronomy

News February 2004: Discovery of an Expanding Galactic X-ray halo due to GRB 031203

XMM-Newton's EPIC X-ray cameras show 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 GRB is at the centre of the rings. Other, unrelated, X-ray sources can also be seen. The time since the burst is shown. At its largest, the inner ring is about one fifth the size of the full moon. Credit: ESA, S. Vaughan (University of Leicester)

A simulation of the expanding rings associated with the gamma-ray burst. The size and intensity of the rings are derived from a careful analysis of the XMM-Newton images. This simulation starts with the very bright X-ray flash of the gamma-ray burst, and continues to the end of the XMM-Newton observation, indicating when XMM-Newton was observing. The two movies do not run at the same speed. Credit: R. Willingale (University of Leicester)

Dr Simon Vaughan from the University of Leicester (with a team from Leicester, NASA, the University of Copenhagen, and ESA) has discovered a unique phenomenon: a beautiful set of expanding X-ray halos surrounding a gamma-ray burst.

Gamma-ray bursts (GRB) are the most powerful explosive events in the Universe and can be used to probe any material between us and the burst. In this case the GRB lies behind the plane of our Galaxy, its light has to travel through the gas and dust in the Galactic disc to reach us.

ESA's gamma-ray observatory satellite Integral detected the 30 second long GRB 031203 on 2003 December 3rd and the halos were discovered in a follow-up observation with ESA's XMM-Newton X-ray satellite that started 6 hours after the burst. Dr Vaughan's web page describes the result in detail and provides further images and movies.

The fading X-ray emission from the GRB - the afterglow - is clearly seen in the image from the X-ray cameras on XMM-Newton. Uniquely, two rings centred on the afterglow were also seen. Dr Vaughan said "These rings are due to dust in our own Galaxy which is illuminated by the X-rays from the gamma-ray burst. The dust scatters some of the X-rays causing the rings in the same way as fog scatters the light from a car's headlights." Although the afterglow is the brightest feature seen in XMM-Newton's images, the expanding echo is much more spectacular. "Its 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." This illustrated by a diagram showing the later arrival of X-rays away from the GRB direction. Dr Vaughan and his colleagues explain that because we see two rings there must be 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.

The team determined accurately the distance to the dust sheets by measuring the size of the expanding rings. The distances have an uncertainty of just 2%, a remarkable level of accuracy for an object in our Galaxy. 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 to determine where star and planet formation is likely to occur.

Expanding X-ray dust scattering rings have never been seen before. Slower moving rings seen in visible light around a very few supernovae are caused by a similar effect. The famous SN1987A has rings which can still be seen almost 5 years after the explosion. Hubble has also produced a cartoon showing the principle of dust scattering (also know as a light echo).

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 GRB. The delayed X-rays from the echo of GRB 031203 are very useful because they tell us 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 Integral and XMM-Newton provide astronomers with their most powerful facilities for studying gamma-ray bursts, and XMM-Newton has been responding quickly to the Integral alerts, but 2004 will see the launch of a new NASA/UK/Italian satellite, Swift, which will be dedicated to GRBs. 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 discoveries in this cutting-edge field.

A scientific paper describing this discovery has been accepted for publication in the Astrophysical Journal. The authors are: Dr Simon Vaughan (University of Leicester), Dr Dick Willingale (Leicester), Dr Paul O'Brien (Leicester), Dr Julian Osborne (Leicester), Dr James Reeves (GSFC, NASA), Andrew Levan (Leicester), Dr Mike Watson (Leicester), Dr Jonathan Tedds (Leicester), Dr Darach Watson (Neils Bohr Institute for Astronomy, Copenhagen), Dr Maria Santos-Lleo (Vilspa, ESA), Dr Pedro Rodriguez-Pascual (Vilspa, ESA) and Dr Norbert Schartel (Vilspa, ESA).

A second scientific paper by the same team has also been submitted for publication in the Astrophysical Journal. This presents an analysis of the X-ray afterglow of GRB 031203, which is itself very unusual. The GRB is shown to actually be a so-called 'X-ray Flash', that is a GRB with more X-rays than gamma-rays. Because the object is so close (the apparent host is a small galaxy at a redshift of 0.1, ie about a billion light-years away, making it the second closest GRB known), the burst and afterglow are much less luminous than typical GRBs, and appear to provide a link between normal GRBs and the nearby and very under-luminous GRB 980425 associated with supernova 1998bw.

The EPIC X-ray cameras on XMM-Newton used in this observation were provided by an international team led by Dr Martin Turner of the University of Leicester. Dr Martin Turner was awarded a CBE in the UK 2004 New Years Honours list for services to X-ray astronomy.

Astrophysics research at the University of Leicester is funded by the UK Particle Physics and Astronomy Research Council. XMM-Newton and Integral are ESA science missions funded by European Space Agency member states and the USA (NASA).

This news item is closely related to an ESA press release issued on 26 Jan. Press releases were also issued by PPARC and the University of Leicester. Other media version of this story can be found at space.com and at SpaceRef. The XMM-Newton movie is the 30 Jan 2004 Astronomy Picture of the Day.

Other news items are available here.

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Last updated: 2004 February 2 by Julian Osborne