Gamma-Ray Bursts = Hypernovae ?!
June 13, 2003
Observations by astronomers from NAOJ, the University of Tokyo and the University of Texas using the Subaru telescope confirm that the optical counter part to gamma-ray burst GRB 030329 is a hypernova. The FOCAS spectra of the optical after glow of the burst show all the characteristics of a hypernova spectrum. This is the first gamma-ray burst with spectra that show such a clear resemblance to hypernovae.
Gamma-ray bursts (Note 1) are extremely energetic but short-lived phenomena that were discovered more than 30 years ago. Their origin has been a puzzle during the past few decades because it is very difficult to pin point their location and to do follow up studies before they fade away. However, concerted efforts to monitor the sky continuously for gamma-ray bursts, and to attempt immediate follow up observations at other wavelengths is leading to a dramatic improvement in our understanding.
Hypernovae are extreme versions of stellar explosions called Type-Ib/Ic or Type-II supernovae. These are the explosive deaths of stars that are more than eight times heavier than our Sun. Hyper-novae are thought to be the explosion of stars more than twenty times heavier than our Sun. These explosions can outshine an entire galaxy containing hundreds of millions of stars.
GRB 030329 was discovered on March 29, 2003 by HETE-2 (http://space.mit.edu/HETE/), a satellite designed to detect gamma-ray bursts. Within two hours of the initial detection, several groups of astronomers in Japan and Australia were able to detect the after glow of the burst in visible light.(Note 2) The after glow was given a new name, SN 2003dh, after spectral observations by US astronomers in early April showed similarities to a hypernova. The new Subaru observations were carried out on May 7 and 8, a little over a month after the burst using FOCAS, the Faint Object Camera and Spectrograph.
The FOCAS spectra of GRB 030329/SN 2003dh shows the characteristic features of the spectra of hypernovae. Figure 1 compares the spectra of GRB 030329/SN 2003dh (black) with two other hyper-novae, SN 1997ef (green) and SN 1998bw (red). The FOCAS data shows greater resemblance to SN 1997ef, the spectrum of a 42 day old hypernova. By contrast, spectra taken earlier show a closer resemblance to SN 1998bw, a 36 day old spectra.
Changes in the spectra over time probably reflect differences in the inner and outer structure of the hypernovae. There have been previous observations hinting at a connection between hypernova and gamma-ray bursts, but this is the first gamma-ray burst who's spectra show such a clear resemblance to hypernovae. These observations provide precious insight into the nature of both gamma-ray bursts and hypernovae.
The new results from Subaru were reported in IAU Circular No. 8133 on May 17, 2003.
Note 1: Gamma-rays are electromagnetic waves (light) with the shortest wavelengths ever observed. They are more energetic than X-rays. Astronomical objects that suddenly shine bright (typically for a few seconds to a couple minutes) in gamma-rays are called gamma-ray bursts.
Note 2: Images of the after glow from Tokyo Institute of Technology
Figure 1: The spectrum of GRB 030329/SN 2003dh (black) compared to the hypernovae SN1997ef (green) and SN1998bw (red). The horizontal axis shows wavelength and the vertical axis shows the light's strength. The similarities, particilaurly between GRB 030329/SN 2003dh and SN1997ef, become apparent by comparing features at 590-660 nm (silicon), 740-800 nm (oxygen), and 800-890 nm (calcium) where the spectra of hypernovae have a characteristic shape (dips at shorter wavelengths, bumps at longer wavelengths).