ESO Press Release 22/05

12 September 2005

Under Embargo until September 12, 2005, 00:00 CET

Star Death Beacon at the Edge of the Universe

Astronomers Find Farthest Known Gamma-Ray Burst with ESO VLT

An Italian team of astronomers has observed the afterglow of a Gamma-Ray Burst that is the farthest known ever. With a measured redshift of 6.3, the light from this very remote astronomical source has taken 12,700 million years to reach us. It is thus seen when the Universe was less than 900 million years old, or less than 7 percent its present age.

"This also means that it is among the intrinsically brightest Gamma-Ray Burst ever observed", said Guido Chincarini from INAF-Osservatorio Astronomico di Brera and University of Milano-Bicocca (Italy) and leader of a team that studied the object with ESO's Very Large Telescope. "Its luminosity is such that within a few minutes it must have released 300 times more energy than the Sun will release during its entire life of 10,000 million years."

ESO PR Photo 27a/05 The Distant Gamma-Ray Burst GRB 050904 [Preview - JPEG: 400 x 468 pix - 54k] [Normal - JPEG: 800 x 935 pix - 332k] [Full Res - JPEG: 1537 x 1796 pix - 2.4M]

ESO PR Photo 27b/05 GRB 050904 Observed in Various Bands [Preview - JPEG: 600 x 182 pix - 18k] [Full Res - JPEG: 1247 x 379 pix - 213k]

Caption: ESO PR Photo 27a/05 shows the position of the distant Gamma-Ray Burst GRB 050904. The image is a colour-composite based on images obtained with ISAAC on the VLT in three different bands in the near-infrared (J, H and K). ESO PR Photo 27b/05 presents observations of the Gamma-Ray Burst GRB 050904 in different filters in the visible (z- and I-band) and near-infrared (J-, H-, and K-band). The observations were done with FORS2 and ISAAC on the 8.2m Antu (UT1) telescope, which is part of ESO's VLT at Paranal. The burst is seen in the lower middle. It is obvious that the GRB is much fainter in the I-band and becomes more and more brighter as the wavelength increases. This indicates the object is at a high redshift, hence, is very far away.

Gamma-ray bursts (GRBs) are short flashes of energetic gamma-rays lasting from less than a second to several minutes. They release a tremendous quantity of energy in this short time making them the most powerful events since the Big Bang. It is now widely accepted that the majority of the gamma-ray bursts signal the explosion of very massive, highly evolved stars that collapse into black holes.

This discovery not only sets a new astronomical record, it is also fundamental to the understanding of the very young Universe. Being such powerful emitters, these Gamma Ray Bursts serve as useful beacons, enabling the study of the physical conditions that prevailed in the early Universe. Indeed, since GRBs are so luminous, they have the potential to outshine the most distant known galaxies and may thus probe the Universe at higher redshifts than currently known. And because Gamma-ray Burst are thought to be associated with the catastrophic death of very massive stars that collapse into black holes, the existence of such objects so early in the life of the Universe provide astronomers with important information to better understand its evolution.

The Gamma-Ray Burst GRB050904 was first detected on September 4, 2005, by the NASA/ASI/PPARC Swift satellite, which is dedicated to the discovery of these powerful explosions.

Immediately after this detection, astronomers in observatories worldwide tried to identify the source by searching for the afterglow in the visible and/or near-infrared, and study it.

First observations by American astronomers with the Palomar Robotic 60-inch Telescope failed to find the source. This sets a very stringent limit: in the visible, the afterglow should thus be at least a million times fainter than the faintest object that can be seen with the unaided eye (magnitude 21). But observations by another team of American astronomers detected the source in the near-infrared J-band with a magnitude 17.5, i.e. at least 25 times brighter than in the visible.

This was indicative of the fact that the object must either be very far away or hidden beyond a large quantity of obscuring dust. Further observations indicated that the latter explanation did not hold and that the Gamma-Ray Burst must lie at a distance larger than 12,500 million light-years. It would thus be the farthest Gamma-Ray Burst ever detected.

ESO PR Photo 27c/05 GRB 050904 at Two Epochs [Preview - JPEG: 400 x 658 pix - 74k] [Normal - JPEG: 800 x 1316 pix - 280k] [Full Res - JPEG: 1328 x 2185 pix - 1.5M]

ESO PR Photo 27d/05 Spectral Energy Distribution of GRB 050904 [Preview - JPEG: 400 x 530 pix - 37k] [Normal - JPEG: 800 x 1060 pix - 137k] [Full Res - JPEG: 1581 x 2095 pix - 487k]

Caption: ESO PR Photo 27c/05 shows the observation of the Gamma-Ray Burst GRB 050904 at two different epochs, i.e. about one day after its discovery with the Swift satellite (left), and four days after (right), in the near-infrared J-band. The burst becomes quickly fainter as can be seen by comparing the two images. ESO PR Photo 27d/05 shows the magnitude of the Gamma-Ray Burst GRB 050904 as observed with FORS2 and ISAAC in the various filters. The bandpasses of the ESO filters are overplotted as well as the best-fitting template which allowed the astronomers to measure the photometric redshift. The clear drop of the flux of the object in the I-band compared to the others is the telltale signature of a high-redshift object.

Italian astronomers forming the MISTICI collaboration [1] then used Antu, one of four 8.2-m telescopes that comprise ESO's Very Large Telescope (VLT) to observe the object in the near-infrared with ISAAC and in the visible with FORS2. Observations were done between 24.7 and 26 hours after the burst.

Indeed, the afterglow was detected in all five bands in which they observed (the visible I- and z-bands, and the near-infrared J, H, and K-bands). By comparing the brightness of the source in the various bands, the astronomers could deduce its redshift and, hence, its distance. "The value we derived has since then been confirmed by spectroscopic observations made by another team using the Subaru telescope", said Angelo Antonelli (Roma Observatory), another member of the team.

More information

Preliminary communication about these results have been done via the "Gamma Ray Bursts Coordinates Network".

Note

[1]: The MISTICI collaboration consists of astronomers from Osservatorio Astronomico di Roma (INAF), Osservatorio Astronomico di Brera (INAF), Osservatorio Astronomico di Arcetri (INAF), Universita` degli Studi di Milano - Bicocca, International School for Advanced Studies (SISSA) and Observatori Astronomic of Universitat de Valencia (Spain). In particular, Angelo Antonelli, Daniele Malesani, Vincenzo Testa, Paolo D'Avanzo, Stefano Covino, Alberto Fernandez-Soto, Gianpiero Tagliaferri, Guido Chincarini, Sergio Campana, Massimo Della Valle, Felix Mirabel, and Luigi Stella were notably active with the data analysis and observations. Prof. Guido Chincarini is the Italian Principal Investigator of the Italian research on GRBs related to the Swift satellite, which is funded by the Italian Space Agency (ASI).

Contact

Guido Chincarini
Osservatorio Astronomico di Brera (INAF), Italy
Mobile: +39 340 280 3612
Phone: +39 39 999 1157
Email: guido@merate.mi.astro.it

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