Fermi mission unveils gamma ray bursts at highest energies

For 10 years, NASA's Fermi gamma-ray space telescope swept the sky in search of gamma ray bursts (GRBs), the brightest explosions in the universe. A new catalog of high-energy explosions provides scientists with new information on how they work.

"Every burst is somehow unique," said Magnus Axelsson, an astrophysicist at Stockholm University in Sweden. "It is only when we can study large samples, as in this catalog, that we begin to understand the common features of the GRBs, which in turn give us clues about the physical mechanisms at the time. ;artwork."

The catalog was published in the June 13 edition of The astrophysical journal and is now available online. More than 120 authors contributed to this article, led by Axelsson, Elisabetta Bissaldi of the National Institute of Nuclear Physics and the Polytechnic University of Bari, Italy, and Nicola Omodei and Giacomo Vianello of the University of Bari. Stanford University in California.

GRBs emit gamma rays, the form of light at the highest energy. Most GRBs occur when certain types of massive stars run out of fuel and collapse to create new black holes. Others arise when two neutron stars, super remains of stellar explosions, merge. Both types of cataclysmic events create jets of particles that move at a speed close to that of light. Gamma rays are produced during collisions of fast moving materials inside jets and when jets interact with the environment around the star.

Astronomers can distinguish the two classes of GRB by the duration of their low energy gamma rays. Neutron star fusions last less than 2 seconds, whereas they usually last a minute or more. The new catalog, which includes 17 short sequences and 169 long sequences, describes 186 events observed by Fermi's Large Area Telescope (LAT) over the last 10 years.

Fermi observes these powerful bursts with the help of two instruments. The LAT sees about a fifth of the sky at all times and registers gamma rays with energies greater than 30 million electron volts (MeV), millions of times the energy of visible light. The gamma ray burst monitor (GBM) detects the entire sky that is not blocked by the Earth and detects low energy emissions. All in all, the GBM has detected more than 2,300 GRB to date.

Below you will find a sample of five events in the LAT catalog that helped record and intrigue, and helped scientists learn more about GRBs.

1. GRB 081102B

The short burst 081102B, which appeared in the Boötes constellation on November 2, 2008, is the shortest GRB detected by the LAT, lasting only a tenth of a second. Although this gust appeared during Fermi's first year of observations, it did not feature in an earlier version of the collection published in 2013.

"The first LAT catalog only identified 35 GRB," Bissaldi said. "Through improved data analysis techniques, we were able to confirm some of the marginal observations from this sample, as well as five times more bursts for the new catalog."

2. GRB 160623A

Long-lasting explosion 160623A, noticed on June 23, 2016 in the constellation Cygnus, shone for nearly 10 hours at LAT energies, the longest explosion of the catalog. But at the lowest energies recorded by the Fermi GBM instrument, it was only detected for 107 seconds. This clear difference between the instruments confirms the trend evoked in the first LAT catalog. The high energy gamma ray emission lasts longer than the low energy emission and occurs later, whether for long or short bursts.

3. GRB 130427A

The individual gamma ray of the highest energy detected by the Fermi LAT has reached 94 billion electron volts (GeV) and has traveled 3.8 billion light years since the constellation Leo. It was issued by 130427A, which also holds the record for the highest number of gamma rays – 17 – of energies above 10 GeV.

A popular model proposes that particles charged into the jet, moving at almost the speed of light, encounter a shock wave and suddenly change direction, emitting gamma rays. But this model can not explain the record light of this burst, forcing scientists to rethink their theories.

Initial findings on 130427A show that the LAT instrument tracked its emission twice as long as the one indicated in the catalog. Due to the large sample size, the team adopted the same standardized analysis for all GRBs, which gave slightly different figures than those reported in the previous study.

4. GRB 080916C

The best-known GRB is produced at 12.2 billion light-years in the constellation of Carina. Called 080916C, the researchers calculate that the explosion contains the power of 9,000 supernovae.

Telescopes can observe GRBs over these long distances because they are very bright, but it is difficult to accurately determine their distance. Distances are only known for 34 of the 186 events in the new catalog.

5. GRB 090510

The known distance up to 090510 allowed to test Einstein's theory that the tissue of space-time is smooth and continuous. Fermi detected a high energy gamma ray and a low energy gamma ray almost at the same time. Having traveled the same distance at the same time, they showed that all the light, whatever its energy, moves at the same speed in the void of space.

"The total gamma ray emission of 090510 lasted less than 3 minutes, but it allowed us to probe this very fundamental question about the physics of our cosmos," said Omodei. "The GRBs are really one of the most spectacular astronomical events we are witnessing."

What is missing?

GRB 170817A was the first time that light and space-time ripples, called gravitational waves, were detected from the fusion of two neutron stars. The event was captured by the laser interferometer gravitational wave observatory (LIGO), the Virgo interferometer and the Fermi GBM instrument, but was not observed by the LAT because the instrument been disabled because the probe has passed through a region of Fermi orbit where the particles activity is high.

"Now that LIGO and Virgo have begun a new period of observation, the astrophysics community will be on the lookout for other GRB and gravitational joint events," said Judy Racusin, co author and researcher at NASA's Goddard Space Flight Center project in Greenbelt, Maryland. "This catalog was a monumental team effort, and the result helps us to know the population of these events and prepares us to deepen our future revolutionary discoveries."

The Fermi Gamma Ray Space Telescope is an astrophysics and particle physics partnership run by NASA's Goddard Space Flight Center in the Greenbelt of Maryland. Fermi was developed in collaboration with the US Department of Energy, with significant contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.