Observations Reveal Aspherical Supernova Explosion as Possible Source of Gamma-Ray Burst

May 27, 2005

An international collaboration of researchers with participation of the Max Planck Institute for Astrophysics has confirmed their prediction that massive stars can explode highly aspherically. As reported in Science May 27, their observations with some of the world's biggest telescopes also confirm such explosions as possible sources of the enigmatic cosmic gamma-ray bursts.

Aspherical explosion of a massive star and direction dependent shape of the oxygen line. In case of Supernova 1998bw, from which a gamma-ray burst was observed, the jet and the gamma burst were directed towards the Earth. The oxygen line in the supernova spectrum had one sharp maximum (red: model, black: observation). In case of Supernova 2003jd the observer was positioned at a large angle relative to the jet axis and the oxygen line had two maxima.

Stars shine because nuclear reactions take place in their cores, converting lighter elements into heavier ones, liberating energy in the process. When a massive star exhausts its nuclear fuel, gravitational forces cause its iron core to collapse, triggering the star to explode. Most of the remaining material that formed the star is ejected at very high velocities. Nuclear burning and heating by an outgoing shock wave at the time of the explosion produce a very bright light display, which for a few days makes the exploding star as bright as an entire galaxy. These events are known as supernovae.

A particular group of supernovae, called Type Ic events, have been linked to gamma-ray bursts (GRBs). GRBs are extremely bright but short - only a few seconds - flashes of gamma- and X-ray radiation, whose location and origin baffled astronomers for decades. Thanks to orbiting X- and gamma-ray telescopes however, we now know that GRBs come from distant galaxies.

To explain the very large energies released by gamma-ray bursts, scientists believe that they must be strongly aspherical, jet-like explosions, whereas supernovae have been thought to be mostly spherical explosions.

However, the supernovae linked to GRBs may themselves be strongly aspherical. Scientists of the Max Planck Institute for Astrophysics (MPA), the Italian National Institute for Astrophysics (INAF) and the universities of Tokyo and Berkeley have built models for such aspherical explosions. Based on these models, in 2002 they predicted that the spectra of these supernovae should appear differently, depending on the angle at which the explosions are viewed. In particular, spectra obtained several months after the explosion were predicted to be significantly dependent on the direction from which the supernova is observed. Observations at such late epochs can probe the entire ejected material because the expansion makes the material transparent. For example, the strongest line in the spectrum, a line of neutral oxygen, is expected to show a single narrow peak if the explosion is viewed near the direction of the jet, the case in which the GRB is observed. In contrast, the line should consist of two separate peaks if the line of sight is almost perpendicular to the jet direction. In the latter case, however, a gamma-ray burst is not expected to be observed, and the supernova is consequently more difficult to detect due to the lack of a tell-tale, very bright gamma-ray signal.

To verify this prediction a programme of observation was started at some of the world's largest telescopes to obtain spectra of a number of Type Ic Supernovae.

In the current issue of Science magazine, Paolo Mazzali, Koji Kawabata, Keiichi Maeda, Kenichi Nomoto, Alexei Filippenko and coworkers report recent observations of SN 2003jd with the Japanese 8.2 meter Subaru telescope at Mauna Kea on Hawaii in which spectral lines of supernova material show an unusual double peak structure indicative of an aspherical explosion. These results were confirmed by the 8 meter Keck telescope, also operated on Hawaii by a consortium of US universities.

The results support the notion that Type Ic supernovae can be highly aspherical explosions. Figure 1 shows how the observed oxygen line profile depends on the orientation of the explosion relative to the observer: Supernova 1998bw, associated with a gamma-ray burst (GRB 980425) had a sharp, single peak, unlike SN 2003jd as expected from the theoretical models. Since Supernova 2003jd shares many characteristic properties with the Supernova 1998bw, for example its exceptional brightness, these results also reinforce the hypothesis that highly aspherical supernovae like 2003jd can produce gamma-ray bursts. These bursts, however, remain unseen on Earth if they point in the wrong direction.

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