First detection of a gamma-ray burst afterglow in very-high-energy gamma-ray light

Posted by Paul O’Brien at Nov 20, 2019 12:00 AM |
After a decade-long search Leicester physicists are involved in the first detection of a gamma-ray burst in very-high-energy gamma-ray light.
First detection of a gamma-ray burst afterglow in very-high-energy gamma-ray light

The large central H.E.S.S. telescope with 614 m² mirror area that was used for the first detection of a GRB in VHE gamma-ray light (Credit: MPIK / Christian Föhr) Right: GRB 180720B in very-high-energy gamma light, 10 to 12 hours after the burst.

Extremely energetic cosmic explosions generate gamma-ray bursts (GRB), typically lasting for only a few tens of seconds. They are the most luminous explosions in the universe. The burst is followed by a longer lasting afterglow, seen mostly in the optical and X-ray spectral regions whose intensity decreases rapidly. The prompt high energy gamma-ray emission is mostly composed of photons several thousands to millions of times more energetic than visible light, that can only be observed by satellite-based instruments. Whilst these space-borne observatories have detected a few photons with even higher energies, the question if very-high-energy (VHE) gamma radiation (at least 100 billion times more energetic than visible light and only detectable with ground-based telescopes) is emitted, has remained unanswered until now.


On 20 July 2018, the Fermi Gamma-Ray Burst Monitor and a few seconds later the Swift Burst Alert Telescope notified the world of a gamma-ray burst, GRB 180720B. Immediately after the alert, several observatories turned to look at this position in the sky. For H.E.S.S. (the ‘High Energy Stereoscopic System’), this location became visible only 10 hours later and reveal very high energy emission many hours after the initial event, deep in the afterglow phase. The discovery of the first GRB to be detected at such very-high-photon energies is reported in a publication by the H.E.S.S. collaboration et al., “A new very-high-energy component deep in the Gamma-ray Burst afterglow”, in the journal 'Nature' on Nov. 20, 2019.


Before this H.E.S.S. observation, it had been assumed that such bursts likely are observable only within the first seconds and minutes at these extreme energies, and not many hours after the explosion. Professor Paul O’Brien, the Leicester lead on H.E.S.S., noted that “the shape of the H.E.S.S. spectrum, and the energy range of the emission at such late times presents a major challenge to emission models”. GRBs are a prime target for the new Cherenkov Telescope Array, currently under construction and which includes cameras built at Leicester. Professor O’Brien noted “We can expect a future with a great number of GRBs detections at very-high energies and with this, a deeper understanding of these exciting phenomena".


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