George's Random Astronomical Object

Object 51: GRB 090926A

Podcast release date: 12 July 2021

Right ascension: 23:33:36.2

Declination: -66:19:26

Epoch: J2000

Constellation: Tucana

Corresponding Earth location: Between 450-500 km from Antarctica in the direction of the Atlantic Ocean

GRB 090926A was a gamma ray burst that was detected in the constellation Tucana on September 26, 2009, by the Fermi Gamma-ray Space Telescope [1]. Gamma ray bursts are objects that produce huge amounts of gamma rays for very short amounts of time. They occur in other galaxies, usually galaxies that are very far from Earth in astronomical terms (which in everyday terms is still very far). They are also relatively rare. The Fermi Gamma-ray Space Telescope only detected 186 gamma ray bursts in its first 10 years of operation [2], which works out to 18 or 19 a year or 1 or 2 every month. Since very few things in space (or on Earth) produce gamma rays, any source of gamma rays, including these gamma ray bursts, will attract a lot of attention.

So, GRB 090926A was a relatively long gamma ray burst by gamma ray burst standards; the gamma rays lasted about 20 seconds [1]. Although this doesn't sound very long to most human beings, including most astronomers, it's a very long amount of time for gamma ray astronomers. (This entire description of gamma ray bursts and gamma ray astronomers who get really excited when they find some gamma rays for a couple of seconds once a month or so makes it sound like they are really desperate people who swipe right for every profile shown in dating apps, but I don't think they're really that desperate in real life. However, they definitely swipe right for any profile that mention gamma rays.)

By the way, short gamma ray bursts are less than 2 seconds long and are thought to originate from when two neutron stars or a neutron star and a black hole merge together [3]. The longer gamma ray bursts like GRB 090926A are generally thought to originate from supernova explosions [3].

While the gamma rays from the supernova that produced GRB 090926A lasted only 20 seconds, astronomers continued to observe this location of the sky at other wavelengths. In the visible part of the electromagnetic spectrum, they were able to observe the afterglow for nearly 23 days after the gamma ray burst [1]. Afterglows from supernovae are rather common, but the length of this afterglow was kind of weird as far as gamma ray bursts go, as they usually fade away much more quickly. GRB 090926A also tended to flare at visible wavelengths [1]. This could indicate that something was going on involving shockwaves at the centre of the remnant left over from the supernova explosion, which could potentially be a neutron star or black hole [1].

What I think was particularly interesting about GRB 090926A was not necessarily the gamma ray burst itself (although, like I said, gamma ray astronomers were really excited to see 20 whole seconds of gamma rays from this object). No, what I think was rather interesting about this gamma ray burst was how astronomers used the afterglow in the visible part of the electromagnetic spectrum to study the interstellar gas within the galaxy that contained the burst and to study intergalactic space between the gamma ray burst and our galaxy.

The way astronomers were able to do this was to look at how atoms in space absorbed the light from the burst. Individual elements and molecules in interstellar gas will absorb light at very specific wavelengths, so when astronomers make spectra of objects in space and see that light is missing at specific wavelengths, they know what elements are present in the interstellar gas. These specific wavelengths all look redshifted compared to what we would observe on Earth because the expansion of the universe makes most other galaxies look like they are moving away from us, which makes this analysis more complicated. On the other hand, the redshifting is actually very useful because the rate at which galaxies are moving away from us is directly related to their distance from Earth, so if we see that something is very redshifted, we know how far away it is.

In the case of the galaxy that contains GRB 090926A, astronomers were able to identify the present of hydrogen, carbon, nitrogen, oxygen, magnesium, aluminum, silicon, sulfur, calcium, manganese, iron, and nickel [4]. Although that long list of elements makes it sound like the galaxy contains a broad variety of atoms, most of the gas is actually hydrogen, and the amounts of most of the other elements relative to hydrogen is rather low [4,5]. Elements heavier than helium are made either in stars that explode as supernovae or stars like the Sun that, when they get so old that they run out of hydrogen to convert into helium in their cores, evolve first into red giants and then into planetary nebulae. It takes a lot of stars and a lot of time to produce the amounts of heavy elements like oxygen and iron that we see in interstellar space in our galaxy, and the lack of these elements in the galaxy that contains GRB 090926A implies that it's a dwarf galaxy that simply has too few stars to produce many of these heavy elements [4,5]. Having said this, the supernova that produced GRB 090926A should inject a lot of new heavy elements into the interstellar medium of the dwarf galaxy.

Also, as I indicated before, the redshifting of light absorbed by the interstellar gas could be used to measure the distance to the galaxy. The light from the galaxy containing GRB 090926A was actually redshifted so that all the wavelengths were 3.1 times larger when they reached Earth as compared to when they were originally emitted [4,5]. This corresponds to light that has been travelling for 10.57 billion years, although because of weird relativity effects, that doesn't quite correspond to a distance of 10.57 billion light years.

As a final note, astronomers also disovered hydrogen gas that was less redshifted than the hydrogen gas in the galaxy that contains the gamma ray burst [4]. This gas is associated with other galaxies between GRB 090926A and Earth, and astronomers have even spent a little bit of time studying the nature of these other galaxies as well.

References

[1] Swenson, C. A. et al., GRB 090926A and Bright Late-time Fermi Large Area Telescope Gamma-ray Burst Afterglows, 2010, Astrophysical Journal Letters, 718, L14

[2] Ajello, M. et al., A Decade of Gamma-Ray Bursts Observed by Fermi-LAT: The Second GRB Catalog, 2019, Astrophysical Journal, 878, 52

[3] Berger, Edo, Short-Duration Gamma-Ray Bursts, 2014, Annual Reviews of Astronomy and Astrophysics, 52, 43

[4] D'Elia, V. et al., VLT/X-shooter spectroscopy of the GRB 090926A afterglow, 2010, Astronomy & Astrophysics, 523, A36

[5] Rau, A. et al., A Very Metal-poor Damped Lyman-alpha System Revealed Through the Most Energetic GRB 090926A, 2010, Astrophysical Journal, 720, 862

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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© George Bendo 2021. See the acknowledgments page for additional information.

Last update: 11 July 2021