# GRB 050820A
**GRB 050820A** was a long, exceptionally energetic gamma-ray burst detected by the Swift Burst Alert Telescope on 20 August 2005 at 06:34:53 UTC (trigger 151207), at a redshift of 2.612 measured from absorption lines with Keck and 2.6147 with VLT.[1](https://swift.gsfc.nasa.gov/archive/grb_table/grb_lookup.php?grb_name=050820A) It is one of the best-sampled broadband data sets of the Swift era, followed simultaneously in gamma-ray, X-ray, and optical bands thanks to a soft precursor pulse about 200 seconds before the main event.[2](https://arxiv.org/abs/astro-ph/0608183) Its prompt gamma-ray energy release and afterglow kinetic energy make it one of the most energetic events for which such values could be determined.[2](https://arxiv.org/abs/astro-ph/0608183)
| Key fact | Detail |
|---|---|
| Detection | Swift BAT trigger 151207, 20 August 2005, 06:34:53 UTC; BAT T90 = 26 s, fluence 34.40 × 10−7 erg cm−2 (15–150 keV)[1](https://swift.gsfc.nasa.gov/archive/grb_table/grb_lookup.php?grb_name=050820A) |
| Distance | Redshift 2.612 (Keck absorption); 2.6147 (VLT absorption)[1](https://swift.gsfc.nasa.gov/archive/grb_table/grb_lookup.php?grb_name=050820A) |
| Precursor | Soft gamma-ray pulse about 200 s before the main event, with no intervening emission, suggesting a mechanism distinct from the GRB itself[2](https://arxiv.org/abs/astro-ph/0608183) |
| Prompt energy | Eγ,iso = (9.7+3.1−1.4) × 1053 erg in a 1–104 MeV rest-frame bandpass, from a Konus/Wind fluence of (6.1+1.9−0.9) × 10−5 erg cm−2 • [3](https://doi.org/10.1088/0004-637X/711/2/641) |
| Collimation | Jet opening angle θ = 6.8–9.3 degrees from an achromatic jet break seen between about 7 and 36 days[2](https://arxiv.org/abs/astro-ph/0608183) |
| Calorimetry | Afterglow kinetic energy EKE = 5.2+7.9−4.1 × 1051 erg; beaming-corrected prompt energy Eγ = 7.5+6.7−2.4 × 1051 erg[4](https://pure.psu.edu/en/publications/multiwavelength-observations-of-grb-050820a-an-exceptionally-ener/) |
| Early optical | Optical and X-ray emission brightened abruptly at t ≈ 220 s in step with the gamma-ray rise; interpreted by the Vestrand team as a reverberation of the prompt energy input[3](https://doi.org/10.1088/0004-637X/711/2/641) • [5](https://www.nature.com/articles/nature04913) |
## Detection and prompt emission
Swift's [Burst Alert Telescope](https://www.edgepedia.org/burst-alert-telescope) triggered on a faint gamma-ray precursor nearly 4 minutes before the bulk of the prompt emission.[3](https://doi.org/10.1088/0004-637X/711/2/641) The gamma-ray light curve is marked by this soft precursor pulse some 200 s before the main event; the lack of any intervening emission suggests it is due to a physical mechanism distinct from the GRB itself.[2](https://arxiv.org/abs/astro-ph/0608183) The large time lag between precursor and main event enabled simultaneous observations in the gamma-ray, X-ray, and optical bandpasses, something achieved for only a handful of events at that time.[2](https://arxiv.org/abs/astro-ph/0608183)
The prompt emission was observed by the Konus/[Wind instrument](https://www.edgepedia.org/wind-instrument) with spectral coverage from 20 keV to 1 MeV.[3](https://doi.org/10.1088/0004-637X/711/2/641) Extrapolating the observed spectrum to a rest-frame bandpass of 1–104 MeV gives a fluence of (6.1+1.9−0.9) × 10−5 erg cm−2, and at z = 2.615 an isotropic prompt energy release of Eγ,iso = (9.7+3.1−1.4) × 1053 erg.[3](https://doi.org/10.1088/0004-637X/711/2/641) Among the bursts in a comparative sample, GRB 050820A had the largest prompt energy.
## Early optical emission and the reverberation interpretation
The event settled a question about when the early afterglow begins. Optical emission in the first minutes after a burst had been sorted into two classes: prompt optical emission correlated with the prompt gamma rays, and early optical afterglow emission uncorrelated with them.[5](https://www.nature.com/articles/nature04913) Observations of GRB 050820A, principally with the RAPTOR telescope at [Los Alamos National Laboratory](https://www.edgepedia.org/los-alamos-national-laboratory), showed for the first time the temporal relationship and relative strength of the two optical components.[5](https://www.nature.com/articles/nature04913)
Both the X-ray and the optical emission brightened abruptly in concert with the onset of the main gamma-ray episode at t ≈ 220 s, suggesting a common origin.[3](https://doi.org/10.1088/0004-637X/711/2/641) The contemporaneous X-rays are the low-energy tail of the prompt emission, but the optical does not directly track the gamma-ray flux; the early optical data instead appear consistent with the forward-shock synchrotron peak passing through the optical band, and are therefore likely the beginning of the afterglow.[2](https://arxiv.org/abs/astro-ph/0608183) The Vestrand team interpreted the early afterglow as a reverberation of the energy input measured by the prompt emission, meaning the early afterglow does not begin at the trigger time as normally assumed.[5](https://www.nature.com/articles/nature04913) Because such reverberations probe the structure of the environment around the burst, and because many long GRBs come from massive stars that died when the [Universe](https://www.edgepedia.org/universe) was less than ten per cent of its current age, they offer clues about the environments around some of the first stars.[5](https://www.nature.com/articles/nature04913)
## Afterglow behavior and energy injection
On hour timescales the X-ray and optical light curves are inconsistent with a simple adiabatic expansion of the shock into the surrounding medium and instead indicate a period of energy injection.[2](https://arxiv.org/abs/astro-ph/0608183) The radio light curve shows a bright flare at t ≈ 1 day that is probably reverse-shock emission.[3](https://doi.org/10.1088/0004-637X/711/2/641) A later decomposition of Swift-era light curves found that the X-ray emission of GRB 050820A is dominated by a "late prompt" component up to 200 ks in the rest frame (720 ks in the observer frame), with the standard afterglow taking over after that; in the optical, the standard afterglow prevails throughout except around 5 ks rest frame (18 ks observer frame), where the two contributions become comparable.[6](https://academic.oup.com/mnras/article/403/3/1131/1046479)
The afterglow was bright enough that its decay could be traced to late times, including [Hubble Space Telescope](https://www.edgepedia.org/hubble-space-telescope) observations at t ≈ 36 days.[3](https://doi.org/10.1088/0004-637X/711/2/641) The optical light curve shows a distinct break between the last ground-based detection at t ≈ 7 days and those HST observations, and late-time X-ray data establish the same break in the X-ray band, identifying it as a jet break.[3](https://doi.org/10.1088/0004-637X/711/2/641)
## Energetics and jet collimation
The jet break constrains the collimation angle of the relativistic outflow to θ = 6.8–9.3 degrees.[2](https://arxiv.org/abs/astro-ph/0608183) Correcting the isotropic energy for beaming gives a prompt energy release of Eγ = 7.5+6.7−2.4 × 1051 erg, alongside an afterglow kinetic energy of EKE = 5.2+7.9−4.1 × 1051 erg.[4](https://pure.psu.edu/en/publications/multiwavelength-observations-of-grb-050820a-an-exceptionally-ener/) Together these make GRB 050820A one of the most energetic events for which such values could be determined.[2](https://arxiv.org/abs/astro-ph/0608183)
Modeling of the afterglow found that no high-quality fits were possible assuming a wind-like circumburst medium.[3](https://doi.org/10.1088/0004-637X/711/2/641) In a 2022 broadband reanalysis using the afterglowpy forward-shock code, the jet was found to be nearly a top-hat structure, with a half opening angle of only about 0.015 rad and a circumburst medium density as low as 10−7 cm−3; the authors interpret this as evidence for a metal-poor progenitor star that lost mass at a low rate.[7](https://www.raa-journal.org/issues/all/2022/v22n8/202208/P020220805559622102403.pdf)
## Significance
GRB 050820A served as a test case for afterglow theory in several ways at once. Its delayed main event produced one of the few simultaneous gamma-ray, X-ray, and optical records of a burst's first minutes, showing that the early afterglow can begin as a response to prompt energy release rather than at trigger time.[2](https://arxiv.org/abs/astro-ph/0608183) • [5](https://www.nature.com/articles/nature04913) Its clear achromatic jet break contributed to the evidence that bright Swift bursts are collimated explosions, while the large opening angles found for such events argued against a strictly canonical energy release of about 1051 erg for all GRBs.[3](https://doi.org/10.1088/0004-637X/711/2/641) Its high redshift and abundant multi-wavelength afterglow data have kept it a reference event for jet-structure modeling.[7](https://www.raa-journal.org/issues/all/2022/v22n8/202208/P020220805559622102403.pdf)
## References
1. [GRB 050820A, Swift GRB table, NASA Goddard Space Flight Center](https://swift.gsfc.nasa.gov/archive/grb_table/grb_lookup.php?grb_name=050820A)
2. [Cenko, S. B. et al. (2006). Multi-Wavelength Observations of GRB 050820A: An Exceptionally Energetic Event Followed from Start to Finish. arXiv:astro-ph/0608183](https://arxiv.org/abs/astro-ph/0608183)
3. [Cenko, S. B. et al. (2010). The Collimation and Energetics of the Brightest Swift Gamma-Ray Bursts. The Astrophysical Journal 711, 641](https://doi.org/10.1088/0004-637X/711/2/641)
4. [Multiwavelength observations of GRB 050820A, Penn State research portal](https://pure.psu.edu/en/publications/multiwavelength-observations-of-grb-050820a-an-exceptionally-ener/)
5. [Vestrand, W. T. et al. (2006). Energy input and response from prompt and early optical afterglow emission in γ-ray bursts. Nature 442, 172–175](https://www.nature.com/articles/nature04913)
6. [Testing a new view of gamma-ray burst afterglows. Monthly Notices of the Royal Astronomical Society 403, 1131](https://academic.oup.com/mnras/article/403/3/1131/1046479)
7. [Li, X.-Y., He, H.-N., & Wei, D.-M. (2022). On the Jet Structures of GRB 050820A and GRB 070125. Research in Astronomy and Astrophysics 22, 085021](https://www.raa-journal.org/issues/all/2022/v22n8/202208/P020220805559622102403.pdf)
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