Fading embers of exploded star?

These composite-color images from the Hubble Space Telescope show a transient light source that faded from view after gamma-ray photons blasted out from the same point on the sky last November. Each frame is labeled by its age relative to the gamma-ray burst. Ground- and space-based data together suggest that a massive star exploded as a supernova 4 billion years ago in an anonymous galaxy, engendering both the brief burst of ionizing radiation and the fading 'star' shown here.

Caltech-NRAO GRB Collaboration

Evidence is mounting that at least some gamma-ray bursts, or GRBs — the most energetic outbursts of radiation known to science — are generated by supernovae, the relatively familiar if still stupendous explosions of massive stars that have exhausted their nuclear fuel.

In the last few years astronomers have traced GRBs to extremely distant galaxies and determined that they pack enough punch to blow entire stars apart. But experts remain in the dark about the events' true nature. The latest chapters in the decades-long GRB saga are being debated this week in Cambridge, MA, at a conference on the universe’s biggest explosions.

One intriguing development is the discovery of a handful of GRBs that seem to have originated from (or given rise to) supernovae. Arguably the best case comes courtesy GRB 011121, a GRB that flared up in the far-southern constellation Chamaeleon last November 21st. Initially detected by the since-retired BeppoSAX spacecraft, the burst led Lukasz Wyrzykowski (Warsaw University Observatory, Poland), Krzysztof Stanek (Harvard-Smithsonian Center for Astrophysics), and Peter Garnavich (Notre Dame University) to a rapidly fading star, which they found with a 1.3-meter telescope at Chile's Las Campanas Observatory. Briefly shining at 18th magnitude (a few percent of Pluto’s apparent brightness), that "star" was presumably the burst's visible-light aftermath: a cooling, expanding fireball generated by some kind of cataclysmic stellar explosion.

Soon after, a panoply of ground-based instruments and the Hubble Space Telescope were aimed at the rapidly fading "star." Spectra from the recently commissioned 6.5-meter Walter Baade Telescope immediately revealed that the explosion took place 4 billion years ago in a star-forming galaxy. And brightness measurements made from the ground and from Hubble over the following weeks collectively revealed that the object didn't fade at the steadily accelerating rate typical of other so-called GRB afterglows. Rather, it leveled off around 23rd magnitude nearly 2 weeks after BeppoSAX detected the GRB. This "bump" was just what Joshua Bloom and Shrinivas Kulkarni (Caltech) and their collaborators were hoping to see when they applied for their Hubble observing time: the characteristic light curve of a core-collapse supernova.

As if to ice the cake, the Warsaw-Harvard-Notre Dame group obtained with the Baade telescope a spectrum of the suspected supernova itself. (The object was too faint for Hubble, with its much smaller primary mirror, to examine spectroscopically.) Unfortunately, the spectrum is too coarse to show a supernova's "fingerprint": wide, deep absorption lines from chemicals forged as the massive stellar precursor burned hotter and hotter in its desperate attempt to stave off gravitational collapse. As a result, says Harvard-Smithsonian theorist Abraham Loeb, "there is no proof still of a definitive link between gamma-ray bursts and supernovae."


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