GRBs, first noted in the 1960s thanks to Cold War H-bomb detection measures, divide into two categories, long or short, depending on whether the duration is more or less than a few seconds. About two-thirds are long, and these alone are later accompanied by "afterglows," emissions at x-ray, optical, and radio wavelengths that may endure for several months. The spectrum of an afterglow facilitates finding a distance from its redshift, as well as identifying the host as a star-formation region in a galaxy. Prior to the GRB reported by #5 there were strong hints that three GRBs (of nearly 3,000 such events) matched supernova counterparts, but in each case uncertainties clouded the conclusion. Paper #5 confidently blasts away these doubts.

In 1998 observers made a groundbreaking discovery when they co-located one GRB to a subsequent supernova. However, far from exciting observers, this solitary match led to dismay. The distance to the GRB, a mere 120 million light years, suggested that observers would be seriously challenged in relating GRBs billions of light years away to supernovae. At such cosmological distances optical telescopes simply cannot see the supernovae; they are too faint.

The very intense GRB of 29 March 2003, bagged by NASA's High Energy Transient Explorer HETE-2, punched a gaping hole through the impasse. This 25-second burst sparked a global alert to ground-based observers. First on the case was Rie Sato, a graduate student at the Tokyo Institute of Technology, Japan. He bounded to the rooftop observatory where he slewed into action a modest 300-mm Meade telescope, a model favored by amateur astronomers. Just 67 minutes after the GRB he discovered the afterglow, the earliest ever of such reports. Over the following two nights, further observations suggested that excited emission from the normal interstellar medium surrounding the burster caused the afterglow.

Paper #5 highlights the research of the collaboration led by Jens Hjorth (University of Copenhagen, Denmark), which used the Very Large Telescope (VLT) at the European Southern Observatory, Chile. Their redshift z=0.1685 for the afterglow situates the GRB at ~800 Mpc (2.5 billion light years), the second closest ever studied, but nevertheless a classical cosmological GRB rather than a local event.

The brightness of the afterglow faded quickly, which offered the possibility that an underlying supernova might be detectable, so Hjorth's team launched their quest for the smoking gun. Their VLT spectra quickly showed the emission lines characteristic of a supernova, whose light is produced from the beta decay of 56Ni made in the explosion to 56Fe. The spectra speak of Type I, the endpoint of a massive star whose core has imploded after burning all the nuclear fuel. In this cataclysm there was a marked departure from the norm: the dying star ejected its outermost layers at 36,000 km s-1, 0.12c. Awesome. By this measure alone this GRB supernova is upgraded to the elite status of hypernova, the term applied to exploding stars with an expansion velocity exceeding 30,000 km s-1.

The driver behind the strong citation interest in #5 is the support it gives for the link between GRBs and the core collapse of massive stars. Theorists will probably be satisfied that all long-duration GRBs are caused by the prompt emission of photons at the moment of core collapse, with the sighting of the supernova being the delayed response.