Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked
This image is from a computer simulation of the beginning of a gamma-ray burst. Here we see the jet 9 seconds after its creation at the center of a Wolf Rayet star by the newly formed, accreting black hole within. The jet is now just erupting through the surface of the Wolf Rayet star, which has a radius comparable to that of the sun. Blue represents regions of low mass concentration, red is denser, and yellow denser still. Note the blue and red striations behind the head of the jet. These are bounded by internal shocks.
The jet passed through the outer shell of the star and, in conjunction with vigorous winds of newly forged radioactive nickel-56 blowing off the disk inside, shattered the star. This shattering represents the supernova event. Meanwhile, collisions among pieces of the jet moving at different velocities, all very close to light speed, created the gamma-ray burst. This "collapsar" model, introduced by Woosley in 1993, best explains the observation of GRB 030329, as opposed to the "supranova" and "merging neutron star" models.
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Washington - Jun 22, 2003
A very bright burst of gamma-rays was observed on March 29, 2003 by NASA's High Energy Transient Explorer (HETE-II), in a sky region within the constellation Leo. Within 90 min, a new, very bright light source (the "optical afterglow") was detected in the same direction by means of a 40-inch telescope at the Siding Spring Observatory (Australia) and also in Japan. The gamma-ray burst was designated GRB 030329, according to the date.
And within 24 hours, a first, very detailed spectrum of this new object was obtained by the UVES high-dispersion spectrograph on the 8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory (Chile). It allowed to determine the distance as about 2,650 million light-years (redshift 0.1685).
Continued observations with the FORS1 and FORS2 multi-mode instruments on the VLT during the following month allowed an international team of astronomers [1] to document in unprecedented detail the changes in the spectrum of the optical afterglow of this gamma-ray burst. Their detailed report appears in the June 19 issue of the research journal "Nature".
The spectra show the gradual and clear emergence of a supernova spectrum of the most energetic class known, a "hypernova". This is caused by the explosion of a very heavy star -- presumably over 25 times heavier than the Sun. The measured expansion velocity (in excess of 30,000 km/sec) and the total energy released were exceptionally high, even within the elect hypernova class.
From a comparison with more nearby hypernovae, the astronomers are able to fix with good accuracy the moment of the stellar explosion. It turns out to be within an interval of plus/minus two days of the gamma-ray burst. This unique conclusion provides compelling evidence that the two events are directly connected.
These observations therefore indicate a common physical process behind the hypernova explosion and the associated emission of strong gamma-ray radiation. The team concludes that it is likely to be due to the nearly instantaneous, non-symmetrical collapse of the inner region of a highly developed star (known as the "collapsar" model).
The March 29 gamma-ray burst will pass into the annals of astrophysics as a rare "type-defining event", providing conclusive evidence of a direct link between cosmological gamma-ray bursts and explosions of very massive stars.
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