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.
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 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.