George's Random Astronomical Object

Object 34: GRB 111209A

Podcast release date: 16 November 2020

Right ascension: 00:57:22.6

Declination: -46:48:04

Epoch: J2000

Constellation: Phoenix

Corresponding Earth location: About 1450 km south-southwest of Cape Town, South Africa

GRB 111209A is a gamma ray burst. These are objects outside our galaxy that produce very large numbers of gamma rays for relatively short amounts of time. Gamma rays are very rare in space, so any object that produces gamma rays attracts a lot of attention. A variety of objects produce these gamma ray bursts, including merging neutron stars, merging black holes, and supernovae. I discussed another gamma ray burst in episode 18, but that object is quite a bit different from GRB 111209A.

GRB 111209A was discovered on December 9, 2011, by the Swift satellite, a satellite designed specifically for this type of thing [1]. The numbers in the name for this gamma ray burst refer to the year, month, and day that the burst was discovered, while the letter A indicates that it was the first gamma ray burst discovered on that day [2]. Most gamma ray bursts last for anywhere from a fraction of a second to a few minutes, but GRB 111209A lasted for about 25000 seconds, or close to 7 hours [1]. This, by the standards of gamma ray bursts, was long. It's longer than a typical Major League Baseball game [3] or the length of a flight from LAX to Honolulu [4], but it is shorter than season 2 of Mad Men [5]. Most notably, it was the longest gamma ray burst ever observed at the time. Quite honestly, I think this burst is so long that it doesn't seem like the word "burst" can be used to accuractely describe it.

So, long gamma ray bursts normally originate from supernovae, but when GRB 111209A was first discovered, they had not identified any supernova that was associated with the burst. In 2015, though, a science paper was published identifying the gamma ray burst as associated with supernova SN 2011kl [6]. Because of cosmology-related effects, it's a little tricky to describe the distance to this supernova, but it can be thought of as exploding when the universe was 6.3 billion years younger but which we did not see on Earth until nine years ago. SN 2011kl, however, was not a normal supernova; it was a Type Ic supernova [6].

Supernovae are categorized based on their spectra, but the spectra are directly related to the type of object that produces the supernova. When most people who aren't professional astronomers think of supernovae, they think of Type II supernovae, which are created by large stars when they reach the ends of their lifespans. A large star, when it initially forms, fuses hydrogen into helium in its core, but when its core fills up with helium, it fuses that into carbon, with hydrogen still fusing into helium in a shell around the core. When the core fill up with carbon, it fuses that into heavier elements, with the fusion of helium into carbon moving into another shell. This type of accumulation of and fusion of heavier and heavier elements in the core continues until the star's core fills up with iron, which can't be fused to produce energy. When this happens, the star collapses inwards at first, but then a reverse shock causes an explosion, blowing away the outer layers of hydrogen, helium, carbon, and other elements. The hydrogen and helium in these outer layers will absorb specific wavelengths or colors of light, thus leading to a characteristic spectrum that allows astronomers to identify a supernova as specifically a Type II supernova. A Type Ic supernova is thought to be similar to a Type II supernova except that, for some reason, the exploding star does not have an outer layer of hydrogen or helium that gets blown away in the explosion [7]. This causes the spectrum to look different, hence leading to the different classification.

So, the abnormal gamma ray burst GRB 111209A seems to have originated from some sort of abnormal exploding star that was stripped of its outer layers of gas. One of the most likely possibilities is that the star before it exploded was so large and so hot that the fusion processes in the star blew away the star's outer atmosphere. We see stars like this in our galaxy, and they are called Wolf-Rayet stars. I discussed one of these stars, called WR 40, in episode 27. The other possibility is that the exploding star was in a binary star system and the other star gravitationally stripped the outer layers of gas from the really massive star before it exploded.

However, many astronomers think that GRB 111209A is so abnormal compared to other Type Ic supernovae that the abnormal exploding giant star scenario by itself doesn't quite explain where the gamma rays come from. At the moment, the favorite explanation is that the dead core of the exploding star formed a type of neutron star called a magnetar that produced the gamma ray emission [6]. A neutron star is a dense ball of neutrons that has a diameter of about 20 km and a mass of a couple of times the mass of the Sun, and they often forms from the dead cores of massive stars when they explode as supernovae. A magnetar is an abnormally magnetized version of a neutron star. It also rotates like a pulsar, which is another type of neutron star. The thoughts are that, once the magnetar formed at the center of the supernova, the very strong rotating magnetic fields lead to the formation of outflows of gas moving away from the magnetar at speeds that are a fraction of the speed of light. It's this highly energized gas that would have produced the very strong gamma ray emission that we saw in GRB 111209A. However, the magentar would have run out of energy to produce gamma rays in a few hours, which is why the gamma ray emission was not continuous.

The data from GRB 111209A was very important in demonstrating that magnetars are the most likely cause of at least some gamma ray bursts. The data will be used for years and years to further study the exact nature of these types of objects, but all astronomers will agree that GRB 111209A has already shown us that it is possible for a gamma ray burst to last longer than one of the Lords of the Ring movies [8,9,10].

References

[1] Gendre, B. et al., The Ultra-long Gamma-Ray Burst 111209A: The Collapse of a Blue Supergiant?, 2013, Astrophysical Journal, 766, 30

[2] Barthelmy, S. D. et al., A small revision to the GRB-naming convention., 2009, GRB Coordinates Network, 10251, 1

[3] Silverman, Steve, The Average Length of Major League Baseball Games, 2018, SportsRec

[4] Flightsphere, Flight time from Honolulu to Los Angeles, 2020

[5] IMDb, Mad Men (TV Series 2007-2015) - IMDb, 2020

[6] Greiner, Jochen et al., A very luminous magnetar-powered supernova associated with an ultra-long gamma-ray burst, 2015, Nature, 523, 189

[7] Kruesi, Liz, The different types of supernovae, 2012, Astronomy

[8] IMDb, The Lord of the Rings: The Fellowship of the Ring (2001) - IMDb, 2020

[9] IMDb, The Lord of the Rings: The Two Towers (2002) - IMDb, 2020

[10] IMDb, The Lord of the Rings: The Return of the King (2003) - IMDb, 2020

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende, which is distributed by filmmusic.io under a Creative Commons 4.0 Attribution License

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© George Bendo 2020. See the acknowledgments page for additional information.

Last update: 15 November 2020