* Astronomy

Title: Swift observations of GRB050904: the most distant cosmic explosion ever observed
Authors: G. Cusumano, V. Mangano, G. Chincarini, A. Panaitescu, D.N. Burrows, V. La Parola, T. Sakamoto, S. Campana, T. Mineo, G. Tagliaferri, L. Angelini, S.D. Barthelemy, A.P. Beardmore, P.T. Boyd, L. Cominsky, C. Gronwall, E.E. Fenimore, N. Gehrels, P. Giommi, M. Goad, K. Hurley, S. Immler, J.A. Kennea, K.O. Mason, F. Marshall, P. Meszaros, J.A. Nousek, J.P. Osborne, D.M. Palmer, P.W.A. Roming, A. Wells, N.E. White, B. Zhang
(revised v2)

Swift discovered the high redshift (z=6.29) GRB050904 with the Burst Alert Telescope (BAT) and began observing with its narrow field instruments 161 s after the burst onset. This gamma-ray burst is the most distant cosmic explosion ever observed.

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Universe Re-Ionised 900 Million Years after Birth

Observations by the Subaru telescope of the most distant known gamma-ray bust show that most hydrogen between the galaxies had become ionised 900 million years after the Big Bang. This study by researchers from Kyoto University, Tokyo Institute of Technology, and the National Astronomical Observatory of Japan dates the formation of the first generations of stars and galaxies to an ever precise time than previous studies, and demonstrates the power of gamma-ray bursts to illuminate the early universe.

Fourteen billion years ago the universe came into existence as a fiery furnace of energy and matter. As it expanded, it cooled. After 300 thousand years it reached a temperature of about 3000 degrees, allowing atomic nuclei and electrons to combine and form electrically neutral atoms. After that, gravity brought matter together to form the stars and galaxies we know today.
However, we also know from previous studies of quasars, energetic beacons from the early universe, that most of the sparse matter in between galaxies is now ionised - split back into nuclei and elections. How and when this happened is one of the central questions of modern astronomy because the re-ionisation of the universe and the early genealogy of stars and galaxies are closely linked. Ultra- violet radiation from the first generations of stars is one of the strongest candidates for cause of the re-ionisation.
In recent years, astronomers have learned that gamma-ray bursts can be even better beacons than quasars for illuminating the process of the re-ionisation of the universe. Quasars shine from the release of gravitational energy as matter falls into a black hole. When observing quasars, it is difficult to disentangle the quasar, the quasar's effect on its immediate environment, and the environment in the vastness between the quasar and Earth. Gamma-ray bursts are now known to originate from extremely energetic versions of a supernova, the death throws of a massive star. These events do not disturb their immediate environment, so extracting information about the gas between the galaxies is much easier.

IMAGE (62kb, 521 x 561)

On September 4, 2005, the Swift Gamma-Ray Burst Mission detected a gamma-ray burst, GRB 050904, that had occurred 900 million years after the birth of the universe. Japanese researchers using the Subaru telescope were able to catch a high-resolution spectrum of the visible light after-glow of the burst. A detailed analysis of this spectrum has revealed that at the time of the burst, more than 80% of the hydrogen between galaxies was already ionised.

"Our technology to observe gamma-ray bursts is really maturing - allowing us to understand earlier times of the universe with ever greater detail. This event demonstrates the power and potential of studying the early universe using gamma-ray bursts. It opens a new frontier of observational cosmology for astronomers worldwide" - Dr. Tomonori Totani.

Now the team awaits the detection of an even more distant gamma-ray burst.

These results will be published in the June 25, 2006 edition of the Publications of the Astronomical Society of Japan.

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Title: Detection of a huge explosion in the early Universe
Authors: G. Cusumano, V. Mangano, G. Chincarini, A. Panaitescu, D.N. Burows, V. La Parola, T. Sakamoto, S. Campana, T. Mineo, G. Tagliaferri, L. Angelini, S.D. Barthelemy, A.P. Beardmore, P.T. Boyd, L. Cominsky, C. Gronwall, E.E. Fenimore, N. Gehrels, P. Giommi, M. Goad, K. Hurley, J.A. Kennea, K.O. Mason, F. Marshall, P. Meszaros, J.A. Nousek, J.P. Osborne, D.M. Palmer, P.W.A. Roming, A. Wells, N.E. White, B. Zhang

Gamma-ray Bursts (GRBs) are bright flashes of high energy photons that can last from about 10 milliseconds to 10 minutes. Their origin and nature have puzzled the scientific community for about 25 years until 1997, when the first X-ray afterglows of long (greater than 2 seconds duration) bursts were detected and the first optical and radio counterparts were found.
These measurements established that long GRBs are typically at high redshift (z 1.6) and are in sub-luminous star-forming host galaxies. They are likely produced in core-collapse explosions of a class of massive stars that give rise to highly relativistic jets (collapsar model).
Internal inhomogeneities in the velocity field of the relativistic expanding flow lead to collisions between fast moving and slow moving fluid shells and to the formation of internal shock waves. These shocks are believed to produce the observed prompt emission in the form of irregularly shaped and spaced pulses of gamma-rays, each pulse corresponding to a distinct internal collision.

The expansion of the jet outward into the circumstellar medium is believed to give rise to "external" shocks, responsible for producing the smoothly fading afterglow emission seen in the X-ray, optical and radio bands.
Here researchers report on the gamma-ray and x-ray observation of the most distant gamma-ray burst ever observed: its redshift of 6.29 translates to a distance of 13 billion light-years from Earth, corresponding to a time when the Universe was just 700 million to 750 million years old.
The discovery of a gamma-ray burst at such a large redshift implies the presence of massive stars only 700 million years after the Big Bang. The very high redshift bursts represent a good way to study the re-ionization era soon after the Universe came out of the Dark Ages.

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"This is uncharted territory. This burst smashes the old distance record by 500 million light-years. We are finally starting to see the remnants of some of the oldest objects in the universe" - Daniel Reichart, University of North Carolina.

The previous most distant gamma-ray burst had a redshift of 4.5.

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This burst was also very long, lasting more than 200 seconds, whereas most bursts last only about 10 seconds. The detection of this burst confirms that massive stars mingled with the oldest quasars.



Only one quasar has been discovered at a greater distance. Quasars are super-massive black holes containing the mass of billions of stars. This burst comes from a lone star. Scientists say it is puzzling how a single star could have generated so much energy as to be seen across the entire Universe. The science team has not yet determined the nature of the exploded star. A detailed analysis is forthcoming.



The most distant explosion ever detected occurred deep in the constellation Pisces.