Swiftly refining GRBs

Astrophysicists have found current gamma-ray burst models need to be fine-tuned.
By | Published: August 18, 2005
August 17, 2005
Using NASA’s Swift satellite, astrophysicists observed a never-before-seen transition stage between a gamma-ray burst’s (GRB) initial emission and its X-ray afterglow. A newly observed X-ray signal falls off with unexpected speed before the previously known afterglow begins. Neither observation fits current GRB models.

GRBs are grouped into two categories based on their emission duration: short and long. Short-duration GRBs are believed to be associated with the merger of compact objects — such as neutron stars — resulting in a short (a few seconds) energy burst. Long GRBs radiate energy in two stages — a prompt gamma-ray emission followed by a lower-energy afterglow — and are typically associated with the deaths of massive stars.

NASA's Swift Gamma-ray Explorer
NASA’s Swift satellite observes, on average, one GRB every few days. These observations are helping astrophysicists understand how GRBs work.
D. Armbrecht (Spectrum Astro) / NASA
The prompt emission lasts more than 2 seconds and is a result of internal shocks. The afterglow can last months and is a result of the GRB’s ejected material colliding with surrounding material.

A team led by Gianpiero Tagliaferri of the Brera Observatory in Merate, Italy, used Swift to study two long-duration GRBs January 26, 2005, and February 19, 2005. The Burst Alert Telescope (BAT) aboard Swift detected the GRBs and alerted the X-Ray Telescope (XRT).

The XRT slewed to observe the afterglow of each — it took 129 seconds to slew to GRB050126 and 87 seconds to slew to GRB050219a. Scientists using pre-Swift instruments had observed only one afterglow stage.

Now, astrophysicists have detected two stages of afterglow emission. A transition occurs in the first few minutes — the X-ray afterglow following the gamma-ray signal dims sooner than expected — so detecting the signal as quickly as Swift did proved crucial.

The transition-phase observations can help astronomers determine the properties of what initiates GRBs, which is still an astronomical riddle. However, these observations give evidence that the “burst and the early afterglow emission are produced by different mechanisms,” writes the team in a letter to Nature, August 18, 2005.

The team’s data agree with aspects of current GRB models: The gamma-ray emission is an internal effect, while the afterglow is an external effect. But the team is not sure what is causing the steep decline in early X-ray emission.

Future studies will add to astrophysicists’ understanding and help refine current GRB models to account for these new observations.