Long-lasting but dim brethren of cosmic flashes

Astronomers, using ESO’s Very Large Telescope, have for the first time made the link between an X-ray flash and a supernova. Such flashes are the little siblings of gamma-ray bursts (GRB) and this discovery suggests the existence of a population of events less luminous than ‘classical’ GRBs, but possibly much more numerous.

“This extends the GRB–supernova connection to X-ray flashes and fainter supernovae, implying a common origin”, said Elena Pian (INAF, Italy), lead-author of one of the four papers related to this event appearing in the 31 August issue of Nature.

The event began on 18 February 2006: the NASA/PPARC/ASI Swift satellite detected an unusual gamma-ray burst, about 25 times closer and 100 times longer than the typical gamma-ray burst. GRBs release in a few seconds more energy than that of the Sun during its entire lifetime of more than 10,000 million years. The GRBs are thus the most powerful events since the Big Bang known in the Universe.

The explosion, called GRB 060218 after the date it was discovered, originated in a star-forming galaxy about 440 million light-years away toward the constellation Aries. This is the second-closest gamma-ray burst ever detected. Moreover, the burst of gamma rays lasted for nearly 2,000 seconds; most bursts last a few milliseconds to tens of seconds. The explosion was surprisingly dim, however.

A team of astronomers has found hints of a budding supernova. Using, among others, ESO's Very Large Telescope (VLT) in Chile, the scientists have watched the afterglow of this burst grow brighter in optical light. This brightening, along with other telltale spectral characteristics in the light, strongly suggests that a supernova was unfolding. Within days, the supernova became apparent.

The observations with the VLT started on 21 February 2006, just three days after the discovery. Spectroscopy was then performed nearly daily for seventeen days, providing the astronomers with a large data set to document this new class of events.

The group led by Elena Pian indeed confirmed that the event was tied to a supernova called SN 2006aj a few days later. Remarkable details about the chemical composition of the star debris continue to be analysed.

The newly discovered supernova is dimmer than hypernovae associated with normal long gamma-ray bursts by about a factor of two, but it is still a factor of 2–3 more luminous than regular core-collapse supernovae.

All together, these facts point to a substantial diversity between supernovae associated with GRBs and supernovae associated with X-ray flashes. This diversity may be related to the masses of the exploding stars.

Whereas gamma-ray bursts probably mark the birth of a black hole, X-ray flashes appear to signal the type of star explosion that leaves behind a neutron star. Based on the VLT data, a team led by Paolo Mazzali of the Max Planck Institute for Astrophysics in Garching, Germany, postulate that the 18 February event might have led to a highly magnetic type of neutron star called a magnetar.

Mazzali and his team find indeed that the star that exploded had an initial mass of ‘only’ 20 times the mass of the Sun. This is smaller, by about a factor two at least, than those estimated for the typical GRB–supernovae.

“The properties of GRB 060218 suggest the existence of a population of events less luminous than ‘classical’ GRBs, but possibly much more numerous”, said Mazzali. “Indeed, these events may be the most abundant form of X- or gamma-ray bursts in the Universe, but instrumental limits allow us to detect them only locally”.

The astronomers find that the number of such events could be about 100 times more numerous than typical gamma-ray bursts.