An Italian team of astronomers has observed the afterglow ofa Gamma-Ray Burst that is the farthest known ever. With a measuredredshift of 6.3, the light from this very remote astronomical sourcehas taken 12,700 million years to reach us. It is thus seen when theUniverse was less than 900 million years old, or less than 7 percentits present age.
"This also means that it is among theintrinsically brightest Gamma-Ray Burst ever observed", said GuidoChincarini from INAF-Osservatorio Astronomico di Brera and Universityof Milano-Bicocca (Italy) and leader of a team that studied the objectwith ESO's Very Large Telescope. "Its luminosity is such that within afew minutes it must have released 300 times more energy than the Sunwill release during its entire life of 10,000 million years."
Gamma-raybursts (GRBs) are short flashes of energetic gamma-rays lasting fromless than a second to several minutes. They release a tremendousquantity of energy in this short time making them the most powerfulevents since the Big Bang. It is now widely accepted that the majorityof the gamma-ray bursts signal the explosion of very massive, highlyevolved stars that collapse into black holes.
This discovery notonly sets a new astronomical record, it is also fundamental to theunderstanding of the very young Universe. Being such powerful emitters,these Gamma Ray Bursts serve as useful beacons, enabling the study ofthe physical conditions that prevailed in the early Universe. Indeed,since GRBs are so luminous, they have the potential to outshine themost distant known galaxies and may thus probe the Universe at higherredshifts than currently known. And because Gamma-ray Burst are thoughtto be associated with the catastrophic death of very massive stars thatcollapse into black holes, the existence of such objects so early inthe life of the Universe provide astronomers with important informationto better understand its evolution.
The Gamma-Ray Burst GRB050904was first detected on September 4, 2005, by the NASA/ASI/PPARC Swiftsatellite, which is dedicated to the discovery of these powerfulexplosions.
Immediately after this detection, astronomers inobservatories worldwide tried to identify the source by searching forthe afterglow in the visible and/or near-infrared, and study it.
Firstobservations by American astronomers with the Palomar Robotic 60-inchTelescope failed to find the source. This sets a very stringent limit:in the visible, the afterglow should thus be at least a million timesfainter than the faintest object that can be seen with the unaided eye(magnitude 21). But observations by another team of Americanastronomers detected the source in the near-infrared J-band with amagnitude 17.5, i.e. at least 25 times brighter than in the visible.
Thiswas indicative of the fact that the object must either be very far awayor hidden beyond a large quantity of obscuring dust. Furtherobservations indicated that the latter explanation did not hold andthat the Gamma-Ray Burst must lie at a distance larger than 12,500million light-years. It would thus be the farthest Gamma-Ray Burst everdetected.
Italian astronomers forming the MISTICI collaboration then used Antu, one of four 8.2-m telescopes that comprise ESO'sVery Large Telescope (VLT) to observe the object in the near-infraredwith ISAAC and in the visible with FORS2. Observations were donebetween 24.7 and 26 hours after the burst.
Indeed, the afterglowwas 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 thebrightness of the source in the various bands, the astronomers coulddeduce its redshift and, hence, its distance. "The value we derived hassince then been confirmed by spectroscopic observations made by anotherteam using the Subaru telescope", said Angelo Antonelli (RomaObservatory), another member of the team.
: TheMISTICI collaboration consists of astronomers from OsservatorioAstronomico di Roma (INAF), Osservatorio Astronomico di Brera (INAF),Osservatorio Astronomico di Arcetri (INAF), Universita` degli Studi diMilano - Bicocca, International School for Advanced Studies (SISSA) andObservatori Astronomic of Universitat de Valencia (Spain). Inparticular, Angelo Antonelli, Daniele Malesani, Vincenzo Testa, PaoloD'Avanzo, Stefano Covino, Alberto Fernandez-Soto, GianpieroTagliaferri, Guido Chincarini, Sergio Campana, Massimo Della Valle,Felix Mirabel, and Luigi Stella were notably active with the dataanalysis and observations. Prof. Guido Chincarini is the ItalianPrincipal Investigator of the Italian research on GRBs related to theSwift satellite, which is funded by the Italian Space Agency (ASI).
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