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Hypernova Reveals Hidden Identity As Gamma-Ray Burst
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Hypernova Reveals Hidden Identity As Gamma-Ray Burst

September 7, 2005
An international research team, led by astronomers from the University of Tokyo, Hiroshima University, and the National Astronomical Observatory of Japan, used the Subaru telescope to obtain the spectrum of SN2003jd, a hypernova unaccompanied by a gamma-ray burst, and found the first evidence that it is a jet-like explosion viewed off-axis. Hypernovae are hyper-energetic Supernovae that are often associated with gamma-ray bursts. This result provides clear and firm evidence that all Hypernovae may be associated with gamma-ray bursts, but that gamma-ray bursts are observable only when jets produced by the hypernova explosion point towards Earth.

An international research team, (*1) led by astronomers from the University of Tokyo, Hiroshima University, and the National Astronomical Observatory of Japan, has obtained a spectrum of Supernova 2003jd (*2) using the Faint Object Camera And Spectrograph (FOCAS) instrument on the Subaru telescope in Hawai'i. The supernova was actually a hypernova, the product of catastrophic core collapse in an extremely massive star. The data, taken on September 12, 2004 (about a year after the initial explosion), exhibited some of the expected emission lines of various elements, including oxygen, nitrogen, and magnesium. However, there was something out of the ordinary: double-peaked oxygen emission lines at 630 and 636 nanometers which suggested that the astronomers were witnessing a donut of oxygen rich debris from the side. The debris was created as two jets of material emerged from the explosion near the speed of light, punching a hole through material that had been a star. This matches predictions of what gamma-ray bursts, some of the most energetic outbursts in the universe, would look like when viewed from the side. This observation marks the first time such a line shape has been observed, overcoming the difficulty of taking spectra of a fading supernova this later after the initial explosion. This discovery gives firm evidence supporting a unified theory that gamma-ray bursts are the products of light-speed jets moving out from asymmetric hypernova explosions.

While many stellar explosions are spherically symmetrical, some are not. In these cases, the deviation from spherical symmetry gives a hint as to what is going on during the explosion. A gamma-ray burst (GRB) is among the most energetic explosions in the universe and is an interesting example of an aspherical explosion.

Astronomers were baffled for decades by gamma-ray bursts. Following the successful observation of a number of gamma-ray bursts and their after-glows, the current understanding is that these events are taking place at cosmological (that is, very large) distances - from as far away as a few billion light-years. At those distances, an explosion would need to be extremely large, bright and energetic to be observable from Earth. If these explosions were spherically symmetric, their energy output would exceed the Sun's total energy output over a lifetime by several times within in a very short period. If the explosion is jet-like rather than spherical, the energy output can be more modest and more realistic. In this case, only gamma-ray bursts whose jet axis happens to point toward Earth would be observable.

Recent research supports the jet hypothesis. A class of long-duration gamma-ray bursts had been linked with hypernova explosions though previous research. For example, astronomers from the University of Tokyo and the National Astronomical Observatory of Japan found that the gamma-ray burst GRB030329 and the hypernova SN 2003dh appeared at the same place and at the same time (*3) in the year 2003. They also discovered possible evidence for high velocity jets associated with the hypernova SN2002ap whose polarized light and unpolarized light have different Doppler shifts. Summarizing these discoveries, the research team proposed a unified model for gamma-ray bursts: a hypernova explosion of a collapsing massive star releasing a pair of high-velocity jets (*4).

This bipolar explosion model predicts that relatively light elements such as oxygen should be ejected in a doughnut-shaped debris ring at the “equator” of an explosion. If the event is viewed from the polar direction, a gamma-ray burst shows up. If it is observed from the side, the gamma-ray burst cannot be seen. Furthermore, oxygen emission lines will show a different appearance that depends on the viewing orientation, because the observed Doppler shift depends on the orientation of the non-spherically symmetric debris. The line should appear as a single-peaked line for a face-on observer in the polar direction. For an edge-on observer placed at the equator, it will appear as a double-peaked line, corresponding to materials moving toward and away from us (*5).

The observation of SN 2003jd (Figure 1) provided astronomers a golden opportunity to link models of such explosions with observed evidence of bipolar structure and its effect on the expected observations of a gamma-ray burst. Earlier observations of the supernova during its peak in brightness had already revealed that it was a hypernova, an explosion caused by the catastrophic core collapse of an extremely massive star. Since there was no known gamma-ray burst accompanying the hypernova, SN2003jd was an excellent observational candidate to test the unified theory of gamma-rays and Hypernovae. Up to the present, the spectra of Hypernovae had shown only single-peaked oxygen emissions lines. This is because astronomers preferentially observed hypernovas associated with gamma-ray bursts, and because such observations are difficult to do and thus few in number. The difficulty arises because hypernovas fade very rapidly. The light gathering and resolving power the Subaru telescope's large 8.2 meter diameter aperture gave the research team a chance to observe a double-peaked emission line in SN2003jd.

Indeed, when the team observed SN 2003jd on September 12, 2004, about one year after the initial explosion, there were prominent oxygen lines in the red part of the spectrum at 630 and 636 nm matching the double-peaked profile predicted by the theory. This discovery provides strong evidence that the unified model proposed by the research group is correct: a Gamma-ray burst is produced by light-speed jets from a very asymmetric hypernova explosion.

This work was published in the May 27, 2005, edition of the journal Science.

*1: The research team includes astronomers from the University of Tokyo, Hiroshima University, the National Astronomical Observatory, Max-Planck Institute (Germany), Trieste Observatory (Italy), the University of California (US), Princeton Institute for Advanced Study (US), Padova Observatory (Italy), the National Astronomical Observatory of China, Japan Aerospace Exploration Agency, Graduate University for Advanced Studies, the National Optical Astronomy Observatory (US), Lawrence Berkeley Laboratory (US), and the California Institute of Technology (US).

*2: SN 2003jd was discovered on 25 October 2003 (UT) by Lick Observatory Supernova Search using the Katzman Automatic Imaging Telescope. It is located in a spiral galaxy MCG-01-59-21 (Figure 1) at the distance about three hundred million light-years away from the Earth. A spectrum around its maximum brightness showed it was a hyper-energetic supernova (hypernova) from a core-collapse massive star, similar to Gamma-Ray Burst related supernovae SNe 1998bw and 2003dh.

*3: “Gamma-Ray Bursts = Hypernovae?!” Subaru Telescope Scientific Results on 13 June 2003. http://www.naoj.org/Pressrelease/2003/06/index.html

*4: It is generally agreed that a massive star, whose mass exceeds eight times that of the Sun, ends its life when its central iron core collapses, and rebounds to create a gigantic explosion. This is called a core-collapse supernova. A hypernova is believed to be the outcome of a supernova explosion from a star at least 20 times heavier than the sun (*3).

*5: According to the bipolar explosion model for an asymmetric supernova and calculations of nucleosynthsis, iron is ejected toward the polar direction while oxygen is along the equatorial direction in a doughnut-like shaped debris (Figure 2). A profile of an emission line of oxygen is different depending on the inclination between the polar and the observer's directions. The oxygen emission line (a blend of lines at 630, 636nm) showed a double-peaked profile (Figure 2, right-bottom panel), in contrast to the single peak seen in another hypernova SN 1998bw (Figure 2, top panel). The double-peaked profile is in good accordance with a theoretical expectation for a jet-like aspherical hypernova viewed from the side. The analysis of the emission line profile, together with the non-detection of a Gamma-Ray Bursts associated with SN 2003jd, added a new and firm evidence for the theoretical model, that is, a Gamma-Ray bursts emerges as a consequence of a bipolar, aspherical hypernova explosion.