Tikalon Blog by Dev Gualtieri

Gamma-Ray Burst GRB 130427A

December 11, 2013

Humans are fragile organisms whose natural environment is all but benign. Even the air we breath is dangerous, since it contains the radioactive gas , radon , a product of the decay of uranium isotopes in minerals . I reviewed environmental radon in a previous article (Radioactive Fountains of the Deep, March 29, 2012), but there are other sources of background radiation .

If we're feeling paranoid , it might be because the universe really is trying to get us. Along with the radon attacking us from below, the universe is bombarding us with cosmic rays . Energetic nuclei and electromagnetic radiation from various cosmic sources impinge on Earth's atmosphere , where they interact with the nuclei of air molecules to create radiation that reaches Earth's surface. The lower atmosphere shields us from some of this, but frequent air travelers get a higher dose .

Aside from these usual radiation sources, there are infrequent radiation events of high intensity, some of which are gamma ray bursts . Although a detailed mechanism for the generation of these gamma ray outbursts is not known, their source is suspected to be rapidly rotating , high mass stars collapsing into neutron stars or black holes . Some of these have been intense enough to disturb Earth's ionosphere .

Gamma radiation incident on Earth's atmosphere will cause additional ionization in the ionosphere, so extraterrestrial radio waves will be blocked, and the strength of long-distance terrestrial radio waves will be enhanced. SGR 1806-20 , a magnetar and gamma ray repeater, is located about 50,000 light years from Earth. There was a gamma ray burst from SGR 1806-20 on December 27, 2004.[1-2]

Even at 50,000 light year's distance, the December 27, 2004, gamma ray burst from SGR 1806-20 ionized the atmosphere down to about 50,000 feet (20 kilometers ), nearly down to the level of commercial aircraft .[1-2] This gamma ray event increased the density of ions at a 60 kilometer altitude by a huge factor, from 0.1 to 10,000 free electrons per cubic foot .[2]

Just this year, on April 27, 2013, Earth was assaulted by an intense gamma ray burst, designated GRB 130427A , with gamma ray intensity reaching 95 billion electron volts (eV). For comparison purposes, thermal energy (k_BT) at room temperature is about 0.025 eV. Data on the GRB 130427A gamma ray burst was reported by a huge international collaboration of 182 authors from 73 institutions in a recent issue of Science .[3-12] Published on the same date were four companion papers in detailed study areas.[4]

Before (left) and after (right) energy maps (>100 MeV) of the GRB 130427A event, as seen by the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope . GRB 130427A was located in the constellation Leo , near its border with Ursa Major , at right ascension 11:32:32.90, and declination +27:41:56.5 (J2000). (Image: NASA/DOE/Fermi LAT Collaboration.)

To quote from the abstract of the Science article,

"GRB 130427A had the largest fluence, highest-energy photon (95 GeV), longest γ-ray duration (20 hours), and one of the largest isotropic energy releases ever observed from a GRB."[3]

Low energy gamma ray bursts are observed about once per day, but GRB 130427A was an energetic exception.[9] The initial surge of gamma rays arrived at Earth at about 3:47 a.m. EDT, April 27, 2013, and it was the second brightest gamma ray burst.[6] Astronomers were prepared for such an event, and optical astronomers were immediately informed of the burster's position. A collection of optical telescopes , called the Rapid Telescopes for Optical Response (RAPTOR) Project detected a magnitude 7 brightening, which is strong enough to be observed with binoculars . The optical flash continued for 80 seconds, and there's a galaxy (SDSS J113232.84+274155.4) nearly coincident with the observed position.[4,6]

A gamma ray intensity of 95 GeV was recorded at the optical peak. This is the highest intensity for a gamma ray associated with a burst. Gamma rays were detected for twenty hours, and it's estimated that the burster was at a distance of about 3.8 billion light years.[4,8] This seems like a long distance, but it's about a third the distance of similar events.[8] Energetic X-rays were observed for more than a day by another NASA space observatory , the Nuclear Spectroscopic Telescope Array (NuSTAR).[4]

Gamma ray bursts are thought to arise from particle jets, ejected at nearly light speed , from a collapsing star.

(Still image from a NASA animation, via YouTube.[11])

The intensity of a gamma ray burst arises from the mechanism for its production. As shown in the above illustration, particles are ejected from a collapsing star in a narrow jet which creates a shockwave with surrounding matter.[4] The star producing GRB 130427A is thought to have had about 20-30 solar masses , so the collapse would have been into a black hole.[5,6] Says Paul Hertz , director of NASA's Astrophysics Division ,

"We expect to see an event like this only once or twice a century, so we're fortunate it happened when we had the appropriate collection of sensitive space telescopes with complementary capabilities available to see it."[4]

Although GRB 130427A was closer to Earth than other gamma ray bursters, its radiation was easily absorbed by Earth's atmosphere. Such a burst within 1,000 light years would be another matter, since it could damage our ozone layer . However, extinction-level gamma ray bursts are expected to occur near Earth every 500 million years.[5] It's thought that the the Ordovician–Silurian extinction event , which caused the extinction of many marine species 450 million years ago, may have been caused by a nearby hypernova .[6] However, a burst comparable to GRB 130427A has a probability at less than 1 in 10 million of occurring in our galaxy .[7]

The Fermi Gamma-ray Space Telescope detected thousands of gamma ray photons during its observations of GRB 130427A, but two of these were at extremely high energies. The first, at nineteen seconds into the burst, had an energy of 73 GeV . The second, at about four minutes, had an energy of 95 GeV. Not only were these energies the highest ever recorded for a gamma ray burst, they were too high in energy to be explained by the prevailing model for gamma ray bursts. Alternative models involving magnetic fields and synchrotron radiation have already been proposed.[9]

It takes a lot of money to maintain such an assemblage of telescopes, and to feed 182 authors from 73 institutions. Funding in the US is from NASA and the U.S. Department of Energy .[4]

Gamma-Ray Burst GRB 130427A

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