Astronomers Agog Over Stellar Explosion

Gamma-Ray Burst
This artist's conception depicts the violent pair of jets emitted by a typical gamma-ray burst (GRB). Astronomers now think that some supernovae channel some of their energy into jets of material traveling at near light speed. The GRB arises from shock waves within the jets. Click on the image for a larger view.
Courtesy David Aguilar, Harvard-Smithsonian Center for Astrophysics.
The astronomical community is abuzz with activity following the closest long-duration gamma-ray burst (GRB) observed since 1998. Major telescopes on the ground and in space are being trained on the source. Research astronomers have high hopes that further study of this event will provide crucial details about the connection of long GRBs (extraordinarily powerful bursts of very-high-energy radiation that last several seconds to several minutes) to supernovae (the explosions of massive stars). Advanced amateur astronomers will monitor the explosion's expanding fireball, which should brighten to about 16th magnitude over the next one to two weeks.

NASA's Swift satellite detected the GRB at 3:43:30 Universal Time on February 18th. In less than 3 minutes Swift's Ultraviolet/Optical Telescope slewed to the correct coordinates, in the constellation Aries, and found the afterglow of the burst. Astronomers all over the world were alerted, and an armada of telescopes began to observe the fading afterglow.

At first the burst appeared rather strange because it lasted for about 30 minutes, which is 100 times longer than a typical long-duration GRB. Many astronomers wondered if it was instead a transient object within our Milky Way Galaxy. But follow-up observations from numerous ground-based telescopes quickly associated the afterglow with a small, 20th-magnitude star-forming galaxy about 470 million light-years from Earth. Only a GRB could be so energetic to be seen by Swift at this distance. Even so, that's much closer than nearly all other GRBs. "We've been waiting several years for a nearby GRB," says Dale Frail (National Radio Astronomy Observatory), who is observing the event at radio wavelengths with the Very Large Array in New Mexico.

Before and after Feb. 18 GRB
The left image from the Sloan Digital Sky Survey shows a field in Aries before the February 18th GRB. The right image, from Swift's Ultraviolet/Optical Telescope, shows how the point of light associated with this stellar explosion outshines its entire host galaxy. Each image is 5 arcminutes across. Click on the image for a larger view.
Courtesy SDSS (left); NASA / Swift / UVOT (right).

On February 21st Alicia Soderberg (Caltech) and her colleagues used the 8.1-meter Gemini South telescope in Chile to find the emerging light of a supernova (named SN 2006aj) at the exact coordinates of the GRB, and the supernova's light now outshines the fading GRB afterglow. The contemporaneous supernova confirms the prevailing theory for long-duration GRBs: that they occur when massive stars explode as supernovae and channel some of the energy into ejecta moving at nearly the speed of light.

Most GRBs detected by Swift occur at distances of billions of light-years. The only GRB with a closer confirmed distance was observed by the Italian-Dutch BeppoSAX satellite on April 25, 1998, at a distance of about 120 million light-years. That event was also associated with a supernova (SN 1998bw), and was one of the crucial events that bolstered the idea that long GRBs are linked to supernovae.

Both the April 1998 and February 2006 events were 10 to 100 times less energetic that most observed GRBs (despite the February 2006 event's long duration). In fact, both would have been invisible if they had occurred billions of light-years away. "This new discovery confirms that there is an underlying population of sub-energetic bursts," says Soderberg. Satellites such as Swift can only detect these low-luminosity GRBs if they occur relatively nearby. "If this were a normal GRB at that distance," adds Frail, "it would have blown out every detector in space."

GRB jet simulation
This frame from a computer simulation of the collapsar model shows a GRB jet breaking out from a dying star. Click on the image for a larger view.
Weiqun Zhang and Stan Woosley (University of California, Santa Cruz)

In the prevailing collapsar model for long GRBs, developed by Stan Woosley (University of California, Santa Cruz) and Andrew MacFadyen (now at the Institute for Advanced Study) in the early 1990s, GRBs are triggered when the core of a massive star collapses to form either a black hole or a neutron star. Infalling stellar gas swirls into an accretion disk around the collapsed core. Magnetic fields channel some of the disk material into two counterflowing jets traveling at very near light speed. Shock waves within the jets generate the actual gamma rays, and the star itself blows apart as a supernova of either Type Ib or Ic (meaning the supernova's spectrum lacks hydrogen, presumably because the outer hydrogen-rich layers were blown off in the star's wind prior to the explosion). Both SN 1998bw and SN 2006aj are Type Ic, in accordance with the collapsar model.

But the collapsar model leaves a critical question unanswered: Why are some Type Ib or Ic supernovae accompanied by GRBs, and others are not? Radio observations by Soderberg and her colleagues demonstrate that only about 1 percent of Type Ib and Ic supernovae are accompanied by GRBs, regardless of where the jets may be aimed. Woosley thinks the key ingredient is rotation. In a supernova with a GRB, the progenitor was spinning rapidly prior to its collapse, and the explosion somehow taps some of that rotational energy to produce the GRB. In low-luminosity GRBs such as the April 1998 and February 2006 events, most of the explosion's energy goes into making the supernova.

"These nearby events will enable us to test these ideas," says Frail, who notes that most GRBs are so far away that astronomers have no chance of seeing the accompanying supernova. "We'll learn much more from a few of these nearby events than from a large sample of far examples. We'll learn a lot about the fundamental question of what types of supernovae give rise to GRBs."

"These observations will be crucial in helping us understand the gap between the ordinary supernovae that don't produce GRBs and the very energetic supernovae that do manage to give rise to gamma-ray emission," adds Soderberg.

Frail points out another bonus of this nearby GRB: Swift's quick alert allowed various telescopes to view the early rise of the supernova. "Swift captures the instant of core collapse and slews instruments right there within seconds of it happening," says Frail. "This is a huge leap forward in terms
of what we do today."

Besides working with Frail on radio observations, Soderberg will scrutinize the supernova in the coming months with both the Hubble Space Telescope and the Chandra X-ray Observatory. These observations will help nail down the explosion's total energy and the properties of the host galaxy — vital clues to unraveling the GRB's fundamental nature.

Half-degree field. Click for larger view.
This half-degree-wide finder field in eastern Aries serves for narrowing in on the GRB's location. The box at upper right shows the area covered by the deeper close-up image below. The GRB site is at right ascension 3h 21m 39.7s, declination +16° 52' 02' (2000 coordinates). A 9.6-magnitude star lies 2' north of it. North is up and east is left. Click on the image for an enlarged view.
Courtesy Digitized Sky Survey.

Astronomers predict that the supernova will brighten to perhaps 16th magnitude around March 5th. This brightness is easily within the grasp of high-quality amateur telescopes equipped with CCD cameras. The American Association of Variable Star Observers (AAVSO) is organizing a worldwide observing campaign to monitor the supernova's light output as it brightens to a peak and then fades from view.

4-arcminute-wide field
This close-up from the Sloan Digital Sky Survey shows the burst's host galaxy, which is 20th magnitude in red light and 470 million light-years away. The frame is 4 arcminutes wide, with north up and east left.
Courtesy Sloan Digital Sky Survey.

"The emerging supernova is well positioned in the early evening sky for professional and amateur monitoring," says Aaron Price of the AAVSO. "Determining the timing of the supernova's peak is very important for scientists. Amateurs can do it effectively since they are spread all over the globe and thus get good time resolution between their observations. Daylight and poor weather are no match for an amateur campaign like this."