A New Type of Cosmic Explosion

I want to thank everyone who has responded to my postings. I’m new at this game, and I’m very pleased to see people writing to the blog with such interesting and insightful comments. Writing magazine articles is essentially a one-way street, with the flow of information and interpretation going from the author to the reader. I really enjoy this blog format, since I can express myself more freely and less formally, but more important, I am receiving quick and valuable feedback from readers.

My original plan for today’s blog installment was to discuss your Pluto comments, and point out that the committee’s proposed definition of a “planet” will be released to the public tomorrow morning. I have seen the press release announcing the definition, but it is embargoed by the International Astronomical Union until 2:00am EDT Wednesday, so I’m not supposed to say anything about it. I can tell you, however, that Pluto aficionados will probably be happy. But check our Web site tomorrow morning for more details.

My plans for today’s posting changed when I saw the new papers posted on Astro-ph last night. Astro-ph is an Internet site where astronomers post their research papers before they actually appear in printed journals. This Web site is a great way for astronomers to disseminate their work quickly to the entire professional community. Last night, I saw a paper that totally blew me away, and it’s so exciting that I want to tell you about it. It suggests that there is a new type of mega-explosion process operating in our universe. Scientifically, this is much, much, much more important than the Pluto/planet debate.

The paper, written by a large group of distinguished astronomers with Avishay Gal-Yam of Caltech as lead author, reports the detection and subsequent observation of a relatively nearby gamma-ray burst (GRB) on June 14, 2006. GRBs are extraordinarily violent and bizarre events. For those of you who have been following the GRB story, including my feature in the August 2006 S&T, you probably recall that GRBs come in two classes: long-duration bursts (lasting several seconds to several minutes) that come from exploding massive stars, and short bursts (lasting a fraction of a second to perhaps 2 or 3 seconds) that originate from a variety of processes, including merging neutron stars. As my August 2006 article explains, astronomers have accumulated compelling evidence that long bursts are powered by massive stars collapsing to form black holes or neutron stars, and in the process they generate high-speed jets that punch through the dying star. Colliding blobs of material within these jets probably trigger the actual burst of gamma rays.

One of the key pieces of evidence in favor of this scenario is the fact that whenever astronomers have pinpointed the location of a long GRB relatively close to Earth, astronomers have always seen a supernova. This association unambiguously links long GRBs to the explosion of massive stars, in line with theoretical predictions. But in this new paper, Gal-Yam and his colleagues report that the relatively nearby long GRB detected by NASA’s Swift satellite on June14th is not associated with a supernova. In other words, the GRB was close enough to Earth (about 1.7 billion light-years) that a supernova should have been visible. Yet when astronomers looked for a supernova with the Hubble Space Telescope and other instruments, they only saw evidence for an afterglow — the interaction of the jets with the surrounding interstellar gas. Either there was no supernova, or it was too faint to be detected, which means it was the lowest-luminosity supernova on record.

Another group, led by Johan Fynbo (University of Copenhagen, Denmark), is just about to come out with a paper reporting a second long GRB without an underlying supernova. This burst was detected by Swift on May 5, 2006. Fynbo's group also failed to find any evidence of a supernova accompanying the June 14th burst.

Theorists are just coming to grips with these discoveries, and it might take months or years for a consensus to emerge. Right now, my speculation as an armchair theorist is that astronomers might have seen two events in which the core of a dying star collapsed to form a black hole, and a supernova either failed to occur or was smothered by inrushing stellar material. Many theorists have predicted that such “failed supernovae” should exist.

But it’s also possible that there is a range of supernova luminosities, and these two events just happen to fall at the low end. In this scenario, the collapsing stellar core initially formed a neutron star, but it later accreted enough infalling stellar gas to form a black hole. Computer models suggest that the subsequent explosion will fail to eject large quantities of nickel-56, the radioactive isotope that is responsible for most of a supernova's early luminosity.

A third possibility is that these events are an extreme version of short bursts, which are not associated with supernovae. But that raises the question of why these bursts lasted so long — about 2 minutes for the June 14th GRB — which is much longer than other short GRBs. Perhaps we’re seeing a powerful and previously unknown explosion process.

Right now, astronomers have more questions than answers. Just when scientists thought they were beginning to understand GRBs, Mother Nature found a way to stay one step ahead of the curve. No doubt about it, the planet controversy will dominate the media spotlight over the next few days. But ultimately, these GRB observations will tell us a lot more about how the universe works than what a committee of astronomers decides to call Pluto.