It is now accepted that long-duration γ-ray bursts (GRBs) are produced during the collapse of a massive star1,2. The standard `collapsar' model3 predicts that a broad-lined and luminous type Ic core-collapse supernova accompanies every long-duration GRB4. This association has been confirmed in observations of several nearby GRBs5-9. Here we report that GRB060505 (ref. 10) and GRB060614 (ref. 11) were not accompanied by supernova emission down to limits hundreds of times fainter than the archetypal supernova SN1998bw that accompanied GRB980425, and fainter than any type Ic supernova ever observed12. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration and show that the bursts originated in actively star-forming regions. The absence of a supernova to such deep limits is qualitatively different from all previous nearby long-duration GRBs and suggests a new phenomenological type of massive stellar death.
- Pub Date:
- December 2006
- Submitted to Nature on July 7. Posted on Nature website July 12. Revised on August 10 and accepted October 20. Astro-ph version corrected for a few typos and given the final title