Gamma Ray Burst Danger and Studies

For 25 years, cosmic gamma ray bursts (GRBs) have been one of the great mysteries of modern astronomy. GRBs have given us several clues as to what they might be. GRBs were seen to occur frequently and appeared to be spread evenly over the sky. Their distribution on the sky indicated that they were the result of events happening either very close or very far (in other galaxies). The events were not, for example, seen to be concentrated in the plane of our Galaxy.

Concentrated study and follow-up observations have shown that GRBs appear to arise in distant galaxies. If they are distant and very bright, then the source of the GRB must be a very energetic event. One possible explanation of GRBs is that they are the result of the merger of two neutron stars, the dense remnant cores of exhausted massive stars.This explanation has accounted for many of the known characteristics of GRBs.

Gamma radiation, also known as gamma rays (denoted as γ), is electromagnetic radiation of high frequency (very short wavelength). They are produced by sub-atomic particle interactions such as electron-positron annihilation, neutral pion decay, radioactive decay (including isomeric transition which involves an inhibited gamma decay), fusion, fission or inverse Compton scattering in astrophysical processes. Gamma rays typically have frequencies above 1019 Hz, and therefore have energies above 100 keV and wavelength less than 10 picometers, often smaller than an atom. Gamma rays from radioactive decay commonly have energies of a few hundred keV, and almost always less than 10 MeV. The upper limit for such energies is about 20 MeV, and there is no lower limit.

Gamma Ray Burst Danger and Studies

Gamma-rays itself are ultra-short and energetic radiations. Fortunately for us, our atmosphere shields us from most far bursts. Gamma-ray astronomy could not really begin until space research became practicable, but during the late 1960s it was found that US satellites, sent up to search for evidence of Soviet nuclear tests, were picking up bursts of gamma-rays. The Russian nuclear tests proved to be as unreal as Iraq's weapons of mass destruction,but the gamma-ray bursts were genuine, and astronomers were intrigued. On5 April 1991 NASA launched the Compton Gamma-Ray Observatory to study them(it was named after the American Nobel Prize winner Arthur Holly Compton, a pioneer in gamma-ray research). The CGRO orbited until 4 June 2000, and carried out a full survey of the sky, discovering almost three hundred sources.

SN2008D CAUGHT IN THE ACT Top: NASA’s Swift satellite acquired these images of galaxy NGC 2770 before SN 2008D exploded. An X-ray image is on the left; a visible-light image is on the right. SN 2007uy is arrowed. Above: On January 9th Swift caught this bright and mysterious X-ray outburst (left). A few days later, SN 2008D emerged in visible light at the same location (right).

Many of these could be identified; for example, the Crab Nebula is a gamma-ray emitter. But the brief, super-energetic flashes, detected by BATSE (the Burst sand Transient Source Experiment) were quite different, and nobody could make out what they were.

For a time it was thought that they might be relatively local, but on 23 January1999 Compton found that one violent burst left an "afterglow" which could be examined spectroscopically, and was obviously a long way away; its distance was given as 4.5 thousand million light-years. Now we have the Swift satellite, which was launched on 20 November 2004 and was put into an almost circular orbit at an altitude of 370 miles (600 km). It was an immediate success, and its BAT (Burst Alert Telescope) is on average detecting one burst per day.

When a burst is found, Swift can slew round and use its X-ray and ultra-violet telescopes to follow the sequence of events. So far, the remotest burst observed lies at a distance of 12.3 thousand million light-years. Gamma-ray bursts (GRBs) are of two main types: short (less than 2 s in duration) and long (several seconds). Their origins are different.

March 21, 2008-Scientists have detected an interstellar explosion so bright that it was briefly visible to the naked eye—from 7.5 billion light-years away.Viewers looking at the right patch of night sky on Wednesday would have seen several afterglows from the massive gamma-ray burst, slightly brighter than the faintest visible stars. NASA's Swift satellite captured the unprecedented spectacle using its X-Ray Telescope (left) and Optical/Ultraviolet Telescope (right). The burst was named GRB 080319B, because it was the second of four bursts detected that day—a first for Swift.GRB 080319B, located more than halfway across the visible universe, crushes the previous record holder for most distant object visible without assistance by three orders of magnitude. That would be the galaxy M33, located just 2.9 million light-years from Earth."This burst was a whopper," Swift principal investigator Neil Gehrels, of NASA Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. "It blows away every gamma-ray burst we've seen so far."

A long burst is believed to be due to the collapse of a hyper giant star, at least 40 times as massive as the Sun, to form a black hole. When the star runs out of fuel and energy production comes to an abrupt stop, its matter swirls downwards towards the core, and the infall results in a pair of jets emerging from the rotational poles of the doomed star; the shock waves break into space, and their immense energy is released in the form of gamma-rays. A short burst is more probably due to a collision between two neutron stars; which hit each other and fuse to form a black hole. 

The whole process takes only a second or two, and there is no afterglow. It is also possible that some flashes are due to flares from magnetars, which are stars with unusually strong magnetic fields, but we know little about magnetars, because only five had been discovered by the end of 2006. Could we be in danger from a gamma-ray burst? If it occurred within a few light years from us, the answer is "yes". 

Even a supernova would make things very uncomfortable from a range of, say, 200 light-years, and comparing a super nova with a GRB is like comparing a match with a searchlight. However, our Galaxy does not seem to be of the type prone to GRBs, and our particular region is reassuringly quiescent. We can survey them from a respectful distance, and see how they behave - the biggest bangs since the original Big Bang almost fourteen thousand million years ago.

Gamma-ray Astronomy

Energetic Gamma Ray Experiment Telescope (EGRET) was one of four instruments outfitted on NASA’s Compton Gamma Ray Observatory satellite. Since lower energy gamma rays cannot be accurately detected on Earth’s surface, EGRET was built to detect gamma rays while in space. EGRET was created for the purpose of detecting and collecting data on gamma rays ranging in energy level from 30 MeV to 30 GeV.

To accomplish its task, EGRET was equipped with a spark chamber, calorimeter, and plastic scintillator anti-coincidence dome. The spark chamber was used to induce a process called electron-positron pair production as a gamma ray entered the telescope. The calorimeter on the telescope was then used record the data from the electron or positron. To reject other energy rays that would skew the data, scientists covered the telescope with a plastic scintillator anti-coincidence dome. The dome acted as a shield for the telescope and blocked out any unwanted energy rays.

The telescope was calibrated to only record gamma rays entering the telescope at certain angles. As these gamma rays entered the telescope, the rays went through the telescopes spark chamber and started the production of an electron and positron. The calorimeter then detected the electron or positron and recorded its data, such as energy level.

Compton Gamma Ray Observatory (CGRO) was the second of the NASA "Great Observatories" to be launched to space, following the Hubble Space Telescope. CGRO was named after Dr. Arthur Holly Compton (Washington University in St. Louis), Nobel prize winner, for work involved with gamma ray physics.

CGRO was built by TRW (now Northrop Grumman Aerospace Systems) in Redondo Beach, CA. Following 14 years of effort, the observatory was launched on the Space Shuttle Atlantis, mission STS-37, on 5 April 1991 and operated until its deorbit on 4 June 2000.

It was deployed in low earth orbit at 450 km (280 miles) to avoid the Van Allen radiation belt. It was the heaviest astrophysical payload ever flown at that time at 17,000 kilograms (37,000 lb).

The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra Observatory is the third of NASA's four Great Observatories. The first was Hubble Space Telescope; second the Compton Gamma Ray Observatory, launched in 1991; and last is the Spitzer Space Telescope.

Prior to successful launch, the Chandra Observatory was known as AXAF, the Advanced X-ray Astrophysics Facility. AXAF was assembled and tested by TRW (now Northrop Grumman Space Technology) in Redondo Beach, California. Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, due primarily to the high angular resolution of the Chandra mirrors.

Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes, requiring a space-based telescope to make these observations.

NASA scientists have discovered two enormous "gamma-ray-emitting bubbles" which appear to emanate from the center of the galaxy, each extending 25,000 light years "north and south of the galactic center." The Milky Way has a diameter of about 100,000 light years in total, and the structure covers more than half of the visible sky. According to USA Today, "each one emits the energy of about 100,000 exploding supernova stars."

Swift Gamma-Ray Burst Telescope

Swift is a multi-wavelength space-based telescope used to the study of gamma-ray bursts. It has three instruments that work together to observe the bursts and their afterglows in the gamma-ray, X-ray, ultraviolet, and optical wavebands. Based on continuous scans of the area of the sky which one of the instruments monitors, Swift uses momentum wheels to autonomously slew into the direction of possible GRBs. 

The name "Swift" is not a mission-related acronym, it refers to its rapid movement capability and the bird of the same name. All of Swift's discoveries are transmitted to the ground and those data are available to other observatories which join Swift in observing the GRBs. In the time between GRB events, Swift is available for other scientific investigations, and scientists from universities and other organisations meanwhile can submit proposals for observations. 

The Swift Mission Operation Center (MOC), where commanding of the satellite is performed, is located in State College, Pennsylvania and operated by the Pennsylvania State University and industry subcontractors. The Swift main ground station is located at the Broglio Space Centre near Malindi on the coast of Eastern Kenya, and is operated by the Italian Space Agency. The Swift Science Data Center (SDC) and archive are located at the Goddard Space Flight Center outside Washington D.C.. The UK Swift Science Data Centre is located at the University of Leicester. The Swift spacecraft bus was built by Spectrum Astro, which was later acquired by General Dynamics Advanced Information Systems.

March 3, 2009 The Fermi Gamma-ray Space telescope, which was launched on June 11 last year, has borne witness to the most violent gamma-ray burst ever observed – a monster that exceeded the power of 8000 supernovae. The GRB 080916C burst appeared in the Carina constellation, 12.2 billion light years from Earth, and was analyzed by five French teams, which published their results in the February 19 issue of Science Express. 

The GRB 080916C burst emitted matter at over 99.9999% of the speed of light, with radiation containing more than 30 billion times the energy of visible light.GRB 080916C was first detected by Fermi’s Gamma-ray Burst Monitor, which provides advanced notice of gamma ray incidents by studying the low energy field of space. The Large Area Telescope was able to study the burst at high energies, and the ground-based Gamma-Ray Burst Optical/Near-Infrared Detector documented the afterglow phase.. Kyle Sherer

X-RAY OUTBURST of supernova SN 2008D [upper right] captured by NASA's Swift satellite during a study of an earlier supernova, SN 2007uy [lower left]. Image: NASA / Swift Science Team/ Stefan Immler

NASA’s Swift captured this image of 73P/Schwassmann-Wachmann 3 as it bypassed the Ring Nebula. Image credit: NASA.

Magi from Swift’s UV/Optical (white, purple) and x-ray telescopes (yellow and red) were combined in this view of GRB 110328A. The blast was detected only in x-rays, which were collected over a 3-4-hour period on March 28. Credit: NASA/Swift/Stefan Immler

This is the view of GRB 090429B from Swift's X-Ray Telescope, which imaged the burst less than 107 seconds after the gamma-ray trigger. Credit: NASA/Swift/Stefan Immler

Sources: Wikipedia, Sky & Telescope Magazine, Patrick Moore : The Sky at Night, ScienceDaily, NASA and Researches.

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