GRB 130427A: a chance not to miss

Credit NASA, DOE, Fermi LAT Collaboration (click to enlarge and make the GRB explode)
On 27 April, an incredible opportunity was given to GRB science detectives. As the spring was outbursting here in Stockholm the explosion of a distant star almost blinded the Gamma ray Burst Monitor (GBM) detectors on board the Fermi satellite. GRB130427 is the brightest GRB ever detected in the keV – MeV band and the longest lasting in the GeV energy range: Fermi Large Area Telescope (LAT) could detect it for hours after the trigger.

Gamma ray bursts (GRBs) are cosmological flashes of which the prompt emission, lasting for 0.01-100s, is in the gamma ray band. Their late emission can be detected at lower energy ranges like optical and radio. One or two GRBs per day are typically observed, but their origin and the particle acceleration mechanisms involved remain nowadays unknown. The favourite hypothesis on their origin is the collapse of a supermassive star, while there is not a leading hypothesis for the acceleration mechanisms involved in the outflow responsible for the prompt emission.

This burst was also detected by other experiments such as Swift and Integral which allowed a rapid and precise localization which enabled optical, infrared and radio follow-up observations. The redshift was measured within hours from the original trigger and revealed that the outbursting star was quite close (for this kind of objects): z= 0.34.

With nearly 1000 photons per second and square centimeter in the 10-1000 keV band and 14 photons per second per square meter in the 100 MeV – 10 GeV band (see attached figure), this burst is a unique occasion for the scientific community to probe models for particle acceleration and photon emission in the outflow.

source: arXiv:1303.2908
Soon the Fermi and Swift collaborations will publish their papers and hopefully more news and more papers will follow. We expect to be able to take a step further in understanding the physics of the GRB thanks to this record breaker burst. Keep an eye on it!

The figure (source: arXiv:1303.2908) shows the fluence in two energy band of the Fermi LAT detected burst, the star indicate the position this burst would have in this plot.
The event fluence in the first 20 seconds in the 10-1000 keV band is (1.975 +/- 0.003) E-03 erg/cm^2, while in the fluence in the first 140s in the 100 MeV – 10 GeV band is (1.1 +/- 0.1)E-4 erg/cm^2.

For more information: http://www.nasa.gov/topics/universe/features/shocking-burst.html
http://edition.cnn.com/2013/05/06/opinion/urry-gamma-ray-burst/index.html

– Elena Moretti, (OKC fellow) – moretti@particle.kth.se

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