About once a day, a gamma-ray burst is detected. When this happens, e-mails get sent around and scientists scramble to detect whatever few photons might have been sent our way. But sometimes things are different…
On April 27th this year, an e-mail alert was sent around signifying the detection of yet another GRB. Yet this event was like no other. Rather than fighting to catch photons, there were suddenly too many to detect! The main emission episode was so bright that the GBM instrument on Fermi became saturated. And not only that – the GeV emission lasted for more than a day!
With all this wealth of information it was clear that this burst, named GRB130427A, would help us test many of the current theories for how gamma-rays are produced. Even if we’ve studied gamma-ray bursts for over 30 years, we still don’t know what makes them shine. Or rather, we know (or think we know) that the energy comes from the birth of a black hole, but we don’t know how the radiation is produced. So which of the theories could explain what we saw in GRB130427A? As it turns out, none of them!
Gamma-ray bursts are the most luminous explosions in the cosmos. At least some of them are the result of a massive star running out of nuclear fuel, collapsing under its own weight to form a black hole. In the process, jets of particles are launched that drill all the way through the collapsing star and into space at nearly the speed of light. If the jet is pointing towards us, we see a gamma-ray burst.
The emission we see is usually divided into two parts, the prompt phase and the afterglow. The prompt phase is usually connected to processes in the jet itself, such as internal shocks or thermal emission from a photosphere. The afterglow is believed to be the result of the jet crashing into the surrounding circumstellar material. Until a few years ago, it was thought that all high-energy emission (MeV and above) was related to the prompt phase, but previous results from the Fermi satellite has showed that at least some GeV emission is related to the afterglow.
Yet GRB130427A gave us more to think about. There are no models which can explain the behavior of the emission during the first pulse of this burst. And we were still counting GeV photons many hours after the onset (the record being a 32 GeV photon detected 9 hours later), which is very difficult to understand using current models of external shocks. But the good news is that to refine our models we need to see the details, and GRB130427A was bright enough to show us plenty of detail.
The results of the Fermi teams are published in two Science papers today:
More exciting gamma-ray burst science is surely to come!
Magnus Axelsson (Researcher at the Oskar Klein Centre) – firstname.lastname@example.org