For a decade, the Fermi Gamma-ray Space Telescope from NASA has skimmed the sky for GRBs (gamma-ray bursts)—the most luminous explosions in the universe.
Now, a novel catalog of the highest-energy blasts offers researchers a new understanding of how they work.
Each burst is in some way unique. It’s only when we can study large samples, as in this catalog, that we begin to understand the common features of GRBs. These in turn give us clues to the physical mechanisms at work.
Magnus Axelsson, Astrophysicist, Stockholm University, Sweden
Published in the June 13th edition of The Astrophysical Journal, the catalog can now be accessed online. The paper included contributions from more than 120 authors, and the study was headed by Axelsson, Giacomo Vianello and Nicola Omodei at Stanford University in California, and Elisabetta Bissaldi at the National Institute of Nuclear Physics and Polytechnic University in Bari, Italy.
Gamma rays—the highest-energy form of light—are emitted by GRBs. A majority of GRBs occur when a few types of gigantic stars produce new black holes after they run out of fuel and collapse together. Other GRBs occur when a couple of neutron stars, extremely dense remnants of stellar explosions, combine.
Both forms of cataclysmic events produce jets of particles that move rapidly, that is, close to the speed of light. The gamma rays are generated during collisions of rapidly moving material within the jets of particles and also when these jets have an interaction with the environment around the star.
The two GRB classes can be distinguished by astronomers by the length of their lower-energy gamma rays. Long bursts from neutron star mergers usually last for a minute or more, while short bursts last for only less than 2 seconds. The latest catalog, which contains 169 long and 17 short bursts, elucidates 186 events observed by Fermi’s Large Area Telescope (LAT) over the last decade.
Fermi uses two instruments to observe these intense bursts. The LAT observes roughly one-fifth of the sky at any time and subsequently records gamma rays whose energies have been recorded to be above 30 million electron volts (MeV), which are millions of times the energy of visible light. GBM, which stands for Gamma-ray Burst Monitor, scans the whole sky that is not blocked by Earth and identifies lower-energy emission. All said and done, the GBM has identified over 2,300 GRBs to date.
The following is a sample of five fascinating and record-setting events from the LAT catalog that provided researchers a deeper understanding of GRBs.
1. GRB 081102B
On November 2nd, 2008, the short burst 081102B took place in the constellation Boötes. It is the shortest LAT-detected GRB and lasted just one-tenth of a second. While this burst appeared in Fermi’s initial year of observations, it was not included in a previous version of the collection published in 2013.
The first LAT catalog only identified 35 GRBs. Thanks to improved data analysis techniques, we were able to confirm some of the marginal observations in that sample, as well as identify five times as many bursts for the new catalog.
Elisabetta Bissaldi, Department of Physics, National Institute of Nuclear Physics and Polytechnic University
2. GRB 160623A
On June 23rd, 2016, a long-lived burst 160623A was observed in the constellation Cygnus and it continued to shine for nearly 10 hours at LAT energies—the longest burst to date in the new catalog. However, it was detected for just 107 seconds at the lower energies recorded by Fermi's GBM instrument. This vivid difference among the instruments demonstrates a trend implied in the first LAT catalog. For short as well as long bursts, the high-energy gamma-ray emission was observed to last longer when compared to the low-energy emission and occurs later.
3. GRB 130427A
Detected by Fermi’s LAT, the highest-energy individual gamma-ray reached 94 billion electron volts (GeV), traveling 3.8 billion light-years from the constellation Leo. 130427A, which also holds the record for emitting the most number of gamma rays—17— with more than 10 GeV energies, emitted this individual gamma ray.
A well-known model suggested that charged particles in the jet, traveling at almost the speed of light, come across a shock wave and immediately change direction, producing gamma rays as a consequence. However, this model cannot contribute to the record-setting light from this burst, which made researchers to rethink their concepts.
The initial findings on GRB 130427A revealed that the LAT instrument was able to trace its emission for twice as long as specified in the new catalog. Owing to the large size of the sample, the researchers implemented the same standardized analysis for all GRBs, leading to somewhat varied numbers than reported in the previous analysis.
4. GRB 080916C
The farthest known GRB, called 080916C, took place 12.2 billion light-years away in the constellation Carina. It was estimated that the explosion contained the power of 9,000 supernovae.
GRBs that are located on these great distances can be observed by telescopes since they are extremely bright; however, it is not easy to pinpoint their precise distance. In the new catalog, distances are known only for 34 of the 186 events.
5. GRB 090510
GRB 090510’s known distance helped in testing Einstein’s theory that the space-time fabric is both continuous and smooth. Fermi identified low-energy as well as high-energy gamma rays at almost the same instant. These gamma rays, after traveling the same distance in the same amount of time, revealed that all light, regardless of its energy, travels at the same speed via the vacuum of space.
The total gamma-ray emission from 090510 lasted less than 3 minutes, yet it allowed us to probe this very fundamental question about the physics of our cosmos. GRBs are really one of the most spectacular astronomical events that we witness.
Nicola Omodei, Senior Research Scientist, Stanford University
GRB 170817A highlighted the first time ripples and light in space-time—known as gravitational waves—were identified from the union of a pair of neutron stars.
The Laser Interferometer Gravitational-Wave Observatory (LIGO), Fermi’s GBM instrument, and the Virgo interferometer successfully captured this event, but the same was not observed by the LAT because it was turned off as the spacecraft traveled through an area of Fermi’s orbit where the activity of particles are known to be high.
Now that LIGO and Virgo have begun another observation period, the astrophysics community will be on the lookout for more joint GRB and gravitational wave events. This catalog was a monumental team effort, and the result helps us learn about the population of these events and prepares us for delving into future groundbreaking finds.
Judy Racusin, Study Co-Author and Fermi Deputy Project Scientist, Goddard Space Flight Center, Greenbelt, NASA, Maryland
This animation shows the most common type of gamma-ray burst, which occurs when the core of a massive star collapses, forms a black hole, and blasts particle jets outward at nearly the speed of light. Viewing into a jet greatly boosts its apparent brightness. A Fermi image of GRB 130427A ends the sequence. (Video credit: NASA’s Goddard Space Flight Center)