GRB130427A – a challenge to our models

This image illustrates the ingredients of the most common type of gamma-ray burst. The core of a massive star (left) has collapsed, forming a black hole that sends a jet moving through the collapsing star and out into space at near the speed of light. Radiation across the spectrum arises from hot ionized gas (plasma) in the vicinity of the newborn black hole, collisions among shells of fast-moving gas within the jet (internal shock waves), and from the leading edge of the jet as it sweeps up and interacts with its surroundings (external shock). Credit: NASA's Goddard Space Flight Center
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!

These maps, both centered on the north galactic pole, show how the sky looks at gamma-ray energies above 100 million electron volts (MeV). Left: The sky during a three-hour interval prior to the detection of GRB 130427A. Right: A three-hour interval starting 2.5 hours before the burst and ending 30 minutes into the event, illustrating its brightness relative to the rest of the gamma-ray sky. GRB 130427A was located in the constellation Leo near its border with Ursa Major, whose brightest stars form the familiar Big Dipper. Credit: NASA/DOE/Fermi LAT Collaboration

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:

Fermi-LAT Observations of the Gamma-Ray Burst GRB 130427A

The First Pulse of the Extremely Bright GRB 130427A: A Test Lab for Synchrotron Shocks

More exciting gamma-ray burst science is surely to come!

Magnus Axelsson (Researcher at the Oskar Klein Centre) –

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