Record cosmic explosion brightens student's first day

Adam Goldstein's first day on the job tending the Gamma-ray Burst Monitor (GBM) instrument on NASA's Fermi Gamma-ray Space Telescope was a doozy. A graduate physics student at The University of Alabama in Huntsville, Goldstein was still learning the ropes the evening of 16 September 2008, nearing the end of his 12-hour on-call shift when the GBM called his cell phone to signal that a burst had been detected.

That in itself wasn't remarkable: GBM detects about one burst a day and it keeps Goldstein's cell phone number handy, along with those of the other GBM team members.

This burst, however, lasted 23 minutes - almost 700 times as long as the two-second average for high-energy gamma-ray bursts. And that was just for starters.

'I was in class the next morning when Alexander (van der Horst, a NASA post-doctoral fellow) called me up and told me the LAT (Fermi's Large Area Telescope) had found photons from that same burst,' Goldstein recalls. 'At the time, when you get a burst you oooh and aaah but it's not until you can sit down and do the spectral analysis that you know what you've found. And if another instrument looked at it, then you've got the chance to do some real science.'

The first significant gamma-ray burst detected by the LAT (Fermi was lifted into orbit in June), this burst bursts with superlatives. When the analysis of spectral data collected by a telescope on the ground was finished in November, the burst's 'red shift' put its point of origin about 12 billion light years from Earth. (Seen from Earth it came from just below the star Chi Carina in the southern sky.)

When that distance is factored with the burst's brightness at the Fermi sensors, it becomes the most powerful gamma-ray event ever detected - four times as powerful at the source as the second strongest burst ever detected, said Dr Valerie Connaughton, a scientist in UAHuntsville's Centre for Space Plasma and Aeronomic Research (CSPAR) and a member of the GBM team.

'This is the most spectacular burst ever seen at high energy,' she said. 'If the event that caused this blew out in every direction instead of being a focused beam, it would be equivalent to 4.9 times the mass of the sun being converted to gamma rays in a matter of minutes.'

This theory-bruising burst is the subject of research published today in Science Express, the on-line scientific journal of the American Association for the Advancement of Science. A collaborative effort by more than 250 scientists around the world, it is the first gamma-ray burst findings to be reported from the Fermi telescope.

The day after the burst, when Goldstein learned that his first burst was noteworthy, he called his parents in Pineville, Missouri, to share the news that his dreams were coming to fruition.

'The next day I talked to them when I found out what a big deal it was,' said Goldstein, who is completing a catalogue of gamma-ray burst data from an earlier orbiting detector as part of his thesis research. 'I have always wanted to work with NASA, so for me this is an ideal place to be.'

Goldstein's enthusiasm has spread to his family. One of the 'honours' accorded a scientist when a burst is seen on his or her shift is the responsibility of writing a circular describing the burst's coordinates and characteristics for the Gamma-ray burst Coordinates Network (GCN). Since posting his description of the Sept. 16 burst, Goldstein said, his father Scott has taken to routinely checking the GCN to see if his son has posted anything new.

The Sept. 16 burst is a theory bender because theories developed to explain gamma-ray bursts - believed to be the most powerful explosions since the Big Bang - don't 'allow' some of the behaviours seen by the Fermi instruments.

This includes the 23-minute duration. Roaring through space for 12 billion years tends to s-t-r-e-t-c-h waves of electromagnetic energy. Accounting for that stretching means the burst was a solid four minutes in duration when it was created.

'It is difficult to imagine keeping a central gamma-ray 'engine' active for that period of time,' said Dr Michael Briggs, a CSPAR scientist and GBM team member.

Another problem is in the energy itself. Most gamma-ray bursts start hot with high-energy gamma rays, then fade to progressively weaker rays. The Sept. 16 burst started 'cool,' with the high-energy gamma rays showing up almost five seconds later. That wasn't expected.

And the burst had both high and low energy photons at the same time for about 200 seconds (also not expected), said Briggs. 'It means everything that created both sets of rays happened in the same space at the same time, which is very difficult to explain.'

After not quite three and a half minutes the cooler gamma rays became too weak to detect, but the high-energy rays continued for at least 20 more minutes. (It was still going when the burst moved out of the LAT's field of view.) If the cataclysmic cosmic event that caused the burst was fading away, why would the weaker gamma rays disappear while the strong ones stick around?

Gamma rays are at the highest end of the energy spectrum, with as much as one million times as much energy per photon as X-rays. Gamma-ray bursts are believed to come from dying stars that explode or collapse, potentially releasing as much energy in a few seconds (or minutes) as our sun will generate in billions of years.

Goldstein was the first (and is still the only) UAHuntsville graduate student to join the GBM team but several post-doctoral students have joined since the success of his first night, swelling the team to about ten. While the GBM instrument notifies team members and other scientists around the world when it detects a burst, someone has to be on-duty tending the instrument at all times. This responsibility is rotated in 12-hour shifts between the team in UAHuntsville's Cramer Hall and scientists at the Max Planck Institute in Germany.