A gamma-ray burst, often abbreviated as GRB, is extremely energetic explosion that is observed in distant galaxies and are the brightest electromagnetic events in the universe. The length of a GRB can vary, with some bursts listing ten milliseconds, while the longest last several hours. After an initial flash of gamma rays, a longer-lived “afterglow” is usually emitted at longer wavelengths on the electromagnetic spectrum.
The release of intense radiation that is observed with Gamma-Ray Bursts is generally thought to be the result of a supernova as a high-mass star implodes to form a neutron star or a black hole.
Most Game Ray Burst sources are billions of light years away from Earth, implying that the explosions are both extremely energetic and extremely rare. These bursts are said to be so energetic that a typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime and soo rare that only a few per galaxy per million years will occur. So far to date, scientists have observed GRBs originating from outside the Milky Way galaxy. The only source of GRBs within the Milky Way Galaxy is a related class of phenomena called soft gamma repeater flares, which are associated with magnetars within the Milky Way. It has been hypothesized that a gamma-ray burst in the Milky Way, pointing directly towards the Earth, could cause a mass extinction event.
The Vela satellites in 1967, which were designed to detect covert nuclear weapons tests, were the first to detect Gamma Ray Bursts. However, this discovery was not declassified and released to the world until 1973, which led to scientists postposing hundreds of theoretical models to explain these bursts. These theories kept floating around as there was few information too verify these models until 1996 when scientists detected the first X-ray and optical afterglows and direct measurement of their redshifts using optical spectroscopy, and thus their distances and energy outputs. It was these discoveries and subsequent studies of the galaxies and supernovae associated with GRBs that clarified the distance and luminosity and definitively placing them in distant galaxies.
Deployed in the late 1960’s, the Vela satellites deployed by the US to detect gamma radiation pulses emitted by nuclear weapons tested in space to detect secret Soviet nuclear tests after signing of the Nuclear Test Ban Treaty. However, instead of detecting normal gamma ray patterns, several gamma ray detections were discovered to be unlike anything they expected to see. This led the uncertain scientists to file the data away for later investigation as the matter was seen as not necessarily urgent. as the technology aboard the Vela satellites improved, the scientists at Los Alamos National Laboratory continued to find inexplicable gamma-ray bursts in their data. By analyzing the different arrival times of the bursts as detected by different satellites, the team was able to determine rough estimates for the sky positions of sixteen bursts and definitively rule out a terrestrial or solar origin. The scientists would later declassify and publish the work in 1973.
Early GRB theories thought that they could come from our own Milky Way Galaxy, but based on the plane detected during the bursts, scientists discovered that this could not be the case.
It is worth noting that astronomers think that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817, a gravitational wave event detected in 2017, and associated with the merger of two neutron stars. This is a result of the. similarities between the two events in regards to emissions and the nature of the host galaxies. suggest that the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova.
There are several different types of Gamma Ray Bursts, which depend on the length of time of the burst occurrence.
Short gamma-ray bursts
Accounting for about 30% of all GRBs, short GBRs are classified as events with a duration of less than about two seconds. Since 2005, scientists have been able to detect afterglow from these short bursts and localize these events. These localizations have found that when several of these short GRBs were detected, the area of space are associated with regions of little or no star formation. This has meant that scientific studies have found that these events cannot be related to longer bursts and are not related to massive stars. That is important because these reports have also found that events that cause short gamma-ray bursts are not related to supernovae and are found in large elliptical galaxies and in central regions of large galaxy clusters.
Long gamma-ray bursts
The majority of observed events, 70% at the time of this writing, have a burst duration of greater than two seconds. These bursts of more than 2 seconds is enough to be classified as long gamma-ray bursts and have the brightest afterglows. The bright afterglows and increased length of the bursts means that these types of Gamma Ray Bursts have been observed in much greater detail than the shorter bursts. This has meant that almost every well-studied long gamma-ray burst has been linked to a galaxy with rapid star formation, a core-collapse supernova, and the deaths of massive stars.
Ultra-Long Gamma-Ray Bursts
These ultra long duration GRB last more than 10,000 seconds, which has caused scientists to propose that these bursts form a separate class of GRB. The reasoning is that these ultra long duration bursts are caused by either the collapse of a blue supergiant star, a tidal disruption event or a new-born magnetar. Only a small number have been identified to date, their primary characteristic being their gamma ray emission duration. It is worth noting that the the low detection rate may be a result of low sensitivity of current detectors to long-duration events, rather than a reflection of their true frequency, which could change if current detector sensitivity changes.