Author Topic: What are Terrestial Gamma Ray Bursts?  (Read 8745 times)

electrobleme

  • Administrator
  • Plasma Star
  • *****
  • Posts: 1503
  • EUreka?: +1/-0
  • It's time to step out of the Gravity, Well?
    • Electric Universe theory blog
What are Terrestial Gamma Ray Bursts?
« on: December 17, 2014, 02:11:57 »
What are Terrestial Gamma Ray Bursts?

Science suggests that Gamma Ray Bursts (GRBs) source is from immense events and distances in space. A report has suggested finding 1000's of Gamma Ray Bursts being created by Earth thunder and lightning sources.

Quote

Scientists have shed light on a mysterious phenomenon that occurs in thunderstorms.

They have discovered that gamma-ray bursts - the most powerful explosions of energy in the Universe - are far more common on Earth than was thought.

Data from Nasa's Fermi satellite shows that all storms produce the blasts and an estimated 1,100 occur each day.

The findings were presented at the American Geophysical Union's Fall Meeting.

Until recently, it was thought that gamma-ray bursts were only found in deep space.

“These are big, monster bursts of gamma rays, and one would think these must be monster storms producing them - but that's not the case”
Prof Joseph Dwyer University of New Hampshire

The pulses of high-energy light are hurled out when giant stars explode or black holes or neutron stars merge.

But in the 1990s, scientists found that these events also occur in the Earth's atmosphere during storms, although it was thought they were rare.

Now, new research has revealed that almost every type of storm - no matter what its strength - produces these invisible explosions.
Gamma-ray bursts 'common in storms' | bbc.co.uk

electrobleme

  • Administrator
  • Plasma Star
  • *****
  • Posts: 1503
  • EUreka?: +1/-0
  • It's time to step out of the Gravity, Well?
    • Electric Universe theory blog
What are Terrestial Gamma Ray Bursts? Wikipedia explanation
« Reply #1 on: December 17, 2014, 02:17:45 »
What are Terrestial Gamma Ray Bursts? Wikipedia explanation

And for history sake lets keep a record of the explanation for terrestial Gamma ray bursts as of this moment.

Terrestrial gamma-ray flash
Terrestrial gamma-ray flashes (TGFs) are bursts of gamma rays produced in the Earth's atmosphere. TGFs have been recorded to last 0.2 to 3.5 milliseconds, and have energies of up to 20 MeV. It is speculated that TGFs are caused by intense electric fields produced above or inside thunderstorms. Scientists have also detected energetic positrons and electrons produced by terrestrial gamma-ray flashes.[1][2]

Discovery
Terrestrial gamma-ray flashes were first discovered in 1991 by BATSE, or Burst and Transient Source Experiment, on the Compton Gamma-Ray Observatory, a NASA spacecraft.[3] A subsequent study from Stanford University in 1996 linked a TGF to an individual lightning strike occurring within a few ms of the TGF. BATSE detected only a small number of TGF events in nine years (76), due to its having been constructed to study gamma rays from outer space, which last much longer.

The newer RHESSI satellite has observed TGFs with much higher energies than those recorded by BATSE.[4] In addition, the new observations show that approximately 50 TGFs occur each day, more than previously thought but still only representing a very small fraction of the total lightning on Earth (3-4 million lightning events per day on average). However, the number may be much higher than that due to the possibility of flashes in the form of narrow beams that would be difficult to detect, or the possibility that a large number of TGFs may be generated at altitudes too low for the gamma rays to escape the atmosphere.

Mechanism
Though the details of the mechanism are uncertain, there is a consensus forming about the physical requirements. It is presumed that TGF photons are emitted by electrons traveling at speeds very close to the speed of light that collide with the nuclei of atoms in the air and release their energy in the form of gamma rays (bremsstrahlung [5]). Large populations of energetic electrons can form by avalanche growth driven by electric fields, a phenomenon called relativistic runaway electron avalanche (RREA).[6][7] The electric field is likely provided by lightning, as most TGFs have been shown to occur within a few milliseconds of a lightning event (Inan et al. 1996).[8][9][10] Beyond this basic picture the details are uncertain. Recent research has shown that electron-electron (Bremsstrahlung) [11] leads first to an enrichment of high-energy electrons and subsequently enlarges the number of high-energy photons.

Some of standard theoretical frameworks have been borrowed from other lightning-associated discharges like sprites, blue jets, and elves, which were discovered in the years immediately preceding the first TGF observations. For instance, that field may be due to the separation of charges in a thundercloud ("DC" field) often associated with sprites, or due to the electromagnetic pulse (EMP) produced by a lightning discharge, often associated with elves. There is also some evidence that certain TGFs occur in the absence of lightning strikes, though in the vicinity of general lightning activity, which has evoked comparisons to blue jets.

The DC field model requires a very large thundercloud charge to create sufficient fields at high altitudes (e.g. 50–90 km, where sprites form). Unlike the case of sprites, these large charges do not seem to be associated with TGF-generating lightning.[8] Thus the DC field model requires the TGF to occur lower down, at the top of the thundercloud (10–20 km) where a local field can be stronger. This hypothesis is supported by two independent observations. First, the spectrum of the gamma-rays seen by RHESSI matches very well to the prediction of relativistic runaway at 15–20 km.[12] Second, TGFs are strongly concentrated around Earth's equator when compared to lightning.[13] (They may also be concentrated over water compared to lightning in general.) Thundercloud tops are higher near the equator, and thus the gamma-rays from TGFs produced there have a better chance of escaping the atmosphere. The implication would then be that there are many lower-altitude TGFs not seen from space, particularly at higher latitudes.

An alternative hypothesis, the EMP model,[14] relaxes the requirement on thundercloud charge but instead requires a large current pulse moving at very high speed. The required current pulse speed is very restrictive, and there is not yet any direct observational support for this model.

Another hypothetical mechanism is that TGFs are produced within the thundercloud itself, either in the strong electric fields near the lightning channel or in the static fields that exist over large volumes of the cloud. These mechanisms rely on extreme activity of the lightning channel to start the process (Carlson et al. 2010) or on strong feedback to allow even small-scale random events to trigger production.[15]

Conjugate events

It has been suggested that TGFs must also launch beams of highly relativistic electrons and positrons which escape the atmosphere, propagate along Earth's magnetic field and precipitate on the opposite hemisphere .[16][17] A few cases of TGFs on RHESSI, BATSE, and Fermi-GBM have shown unusual patterns that can be explained by such electron/positron beams, but such events are very unusual.

Other research
Terrestrial gamma-ray flashes pose a challenge to current theories of lightning, especially with the discovery of the clear signatures of antimatter produced in lightning.[18]

It has been discovered in the past 15 years that among the processes of lightning is some mechanism capable of generating gamma rays, which escape the atmosphere and are observed by orbiting spacecraft. Brought to light by NASA's Gerald Fishman in 1994 in an article in Science,[19] these so-called terrestrial gamma-ray flashes (TGFs) were observed by accident, while he was documenting instances of extraterrestrial gamma ray bursts observed by the Compton Gamma Ray Observatory (CGRO). TGFs are much shorter in duration, however, lasting only about 1 ms.

Professor Umran Inan of Stanford University linked a TGF to an individual lightning stroke occurring within 1.5 ms of the TGF event,[20] proving for the first time that the TGF was of atmospheric origin and associated with lightning strikes.

CGRO recorded only about 77 events in 10 years; however, more recently the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft, as reported by David Smith of UC Santa Cruz, has been observing TGFs at a much higher rate, indicating that these occur about 50 times per day globally (still a very small fraction of the total lightning on the planet). The energy levels recorded exceed 20 MeV.

Scientists from Duke University have also been studying the link between certain lightning events and the mysterious gamma ray emissions that emanate from the Earth's own atmosphere, in light of newer observations of TGFs made by RHESSI. Their study suggests that this gamma radiation fountains upward from starting points at surprisingly low altitudes in thunderclouds.

Steven Cummer, from Duke University's Pratt School of Engineering, said, "These are higher energy gamma rays than those coming from the sun. And yet here they are coming from the kind of terrestrial thunderstorm that we see here all the time."

Early hypotheses of this pointed to lightning generating high electric fields and driving relativistic runaway electron avalanche at altitudes well above the cloud where the thin atmosphere allows gamma rays to easily escape into space, similar to the way sprites are generated. Subsequent evidence however, has suggested instead that TGFs may be produced by driving relativistic electron avalanches within or just above high thunderclouds. Though hindered by atmospheric absorption of the escaping gamma rays, these theories do not require the exceptionally intense lightning that high altitude theories of TGF generation rely on.

The role of TGFs and their relationship to lightning remains a subject of ongoing scientific study.

In 2009, the Fermi Gamma-ray Space Telescope in Earth orbit observed intense burst of gamma rays corresponding to positron annihilations coming out of a storm formation. Scientists wouldn't have been surprised to see a few positrons accompanying any intense gamma ray burst, but the lightning flash detected by Fermi appeared to have produced about 100 trillion positrons. This was reported by news media in January 2011, and had never been previously observed.[21][22]


electrobleme

  • Administrator
  • Plasma Star
  • *****
  • Posts: 1503
  • EUreka?: +1/-0
  • It's time to step out of the Gravity, Well?
    • Electric Universe theory blog
What are Terrestrial Gamma-ray Flashes? Stanford University definition
« Reply #2 on: December 17, 2014, 02:21:45 »
What are Terrestrial Gamma-ray Flashes? Stanford University definition

Quote
Terrestrial gamma-ray flashes (TGFs) are brief bursts of energetic gamma-rays produced in the atmosphere and observed by satellites in low-Earth orbit. First discovered in 1994 (Fishman et al., 1994), these bursts are now known to be associated with lightning (Inan, 2005; Cummer et al., 2005; Stanley et al., 2006; Inan et al., 2006; Cohen et al., 2010; Inan et al., 1996; Cohen et al., 2006), produced in the middle atmosphere (Carlson et al., 2007; Dwyer and Smith, 2005), and consist of photons with individual energies ranging from <10 keV to >40 MeV and total event energy 10 kJ. A sample TGF light curve is shown in Figure 1.

 In spite of much study over the past 10 years, the underlying mechanism by which lightning is associated with TGFs is still a mystery. The energetic photons in a TGF are known to be produced by energetic electrons. Such energetic electrons must be accelerated by electric fields. Though low-energy electrons feel strong frictional forces, relativistic electrons feel much less friction. As a result, in a strong electric field, populations of relativistic electrons can grow like an avalanche, suggesting that a strong electric field and an initial population of relativistic electrons will suffice to produce a TGF. This is not a very simple picture, however, as there are several possible sources of the initial population of relativistic electrons, several ways in which lightning can contribute an electric field, and the population of relativistic electrons must be large and contain sufficiently high-energy electrons to produce a TGF.

Initial populations of energetic electrons may come from cosmic rays, energetic atomic nuclei from outside the solar system that continually bombard the Earth and produce energetic electrons in the atmosphere as a result. These energetic electrons are not common enough, however: if cosmic rays are the only source of energetic electrons, the electric fields necessary to produce an observable TGF are too great to be produced in the atmosphere. Complicating things, however, is ``relativistic feedback,'' where a population of energetic electrons grows so big that it produces enough energetic secondary particles to start a second generation of energetic electrons. When it occurs, this process grows quite large quite rapidly and the large populations of energetic electrons that result are a good candidate for TGF production. One further possible source of the initial population of energetic electrons is lightning itself, where the confined electric fields near the channel can be strong enough to directly accelerate free low-energy electrons to high enough energies.

Whatever the source, these initial seed energetic electrons must be accelerated to very high energies to account for TGF emission. The electric fields involved in this acceleration may appear above the thundercloud as part of a quasi-electrostatic field after a large lightning discharge, but the necessary lightning is unreasonably intense. They may also be produced by an electromagnetic pulse by a fast lightning return stroke, but the necessary speed and intensity of the return stroke renders this possibility unlikely as well. The electric fields normally present in an active thunderstorm are generally too low to directly accelerate electrons to high energies, while the electric fields near lightning channels tend to lack the necessary electric potential. The unknown source of the electric fields involved in TGF is the subject of much debate.

Our research at Stanford focuses on analysis of lightning activity associated with TGFs and on computer models of possible TGF production mechanisms.

Lightning associated with TGFs

Though lightning was suggested as relevant to TGFs even in the first paper, Stanford VLF studies of electrical activity associated with TGFs was the first clear association of electrical activity with TGF emission(Inan et al., 1996). Since then, Stanford has been very active in the study of TGF-associated lightning (see citations below). With the advent of the new high-efficiency global lightning detection network (GLD360) and the high-resolution detection of TGFs by the Fermi spacecraft, the VLF group will continue to improve our observations of TGF-associated lightning.

TGF theory

On the theoretical side, VLF group theorists are working to try to understand TGF production mechanism. The quasi-electrostatic fields produced after lightning discharges (Figure 2, left, Lehtinen et al. (1996)), the electromagnetic pulses radiated by rapid lightning return strokes (Figure 2, middle, Inan and Lehtinen (2005)), and the direct production of radiation by the lightning channel itself (Figure 2, right, Carlson et al. (2009)) are all under consideration.

None of these mechanisms provides a completely convincing picture of TGF production, so further work is underway to refine the models and work out the remaining details.
Terrestrial Gamma-ray Flashes | stanford.edu



electrobleme

  • Administrator
  • Plasma Star
  • *****
  • Posts: 1503
  • EUreka?: +1/-0
  • It's time to step out of the Gravity, Well?
    • Electric Universe theory blog
What are terrestrial gamma-ray flashes? livescience.com details
« Reply #3 on: December 17, 2014, 02:26:22 »
What are terrestrial gamma-ray flashes? livescience.com details

Thunderstorms are powerful enough to generate flashes of gamma-rays, the highest-energy form of light, in Earth's atmosphere, and now scientists are uncovering how they do so.

These gamma-ray flashes typically last for less than a millisecond. They can actually create antimatter, heeding Einstein's famous equation E=mc2, which revealed that energy can be converted into mass and vice versa. (Gamma rays are also emitted by powerful explosions in the distant universe, though these are separate phenomena.)

These bursts of energy, known as terrestrial gamma-ray flashes, come from lightning. The powerful electric fields of lightning bolts hurl avalanches of electrons near the speed of light, which give off gamma-rays after they slam into air molecules. Satellites revealed these flashes originate from "intra-cloud lightning," which arcs within thunderclouds. This is the most common form of lightning .

Uncovering more about the link between terrestrial gamma-ray flashes and lightning has proven difficult, as the chances of a satellite flying over a lightning storm that scientists are monitoring at exactly the time such gamma-ray flashes occur is very, very small. Nevertheless, researchers have now managed to analyze bursts of radio waves that gamma ray-emitting bolts give off, shedding light on what parameters of lightning might cause this radiation.

Lightning happens in stages. First, a streamer of electricity travels from one charged area to another, say, from a cloud to the ground, or from one layer within a cloud to another. This prompts a return stroke with the reverse charge to go in the opposite direction. The initial streamer electrified the air it moved through, creating a path of least resistance that allows the return stroke to carry a much greater current.

Scientists analyzed 56 terrestrial gamma-ray flashes with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite from 2004 to 2009 and compared them with bursts of radio waves that happened at the same time.

The large majority of terrestrial gamma-ray flashes are apparently caused by so-called "positive lightning," where the initial streamers are positively charged. Although most cloud-to-ground lightning is negative, most intracloud lightning is positive.

"Terrestrial gamma-ray flashes are produced by what seems in many ways like very ordinary and common lightning," said researcher Steven Cummer, an electrical engineer at Duke University. "But that they also seem rare according to gamma-ray detection is part of the puzzle of terrestrial gamma-ray flashes."

These gamma-ray flashes accompany 2-to-6-millisecond-long pulses of ultra-low-frequency radio waves, which are signs they are linked with unusually strong movements of electrical charge.

"Now that we've identified an important terrestrial gamma-ray flash-associated process, we can begin to use our radio measurements of lightning to find out how common it is," Cummer told OurAmazingPlanet. "Now that we have what appears to be a fairly reliable signature of a terrestrial gamma-ray flash that can be found from ground-based lightning measurements alone, we can dramatically expand the number and detail of events we can look at."
Quote
text
How Lightning Sparks High-Energy Bursts | livescience.com

electrobleme

  • Administrator
  • Plasma Star
  • *****
  • Posts: 1503
  • EUreka?: +1/-0
  • It's time to step out of the Gravity, Well?
    • Electric Universe theory blog
What are Earths gamma ray flashes? Florida Institute of Technology explanation


Quote
Electron avalanches could be generating some of the highest energy radiation bursts ever discovered on Earth.

The study in Geophysical Research Letters, suggests bursts known as terrestrial gamma ray flashes (TGF), are possibly produced by dark lightning generated by an avalanche of electrons.

TGFs were first detected by chance by NASA's Earth-orbiting Compton gamma ray telescope in 1994.

Compton was searching for gamma ray bursts from exploding stars in deep space, when it unexpectedly began detecting very strong bursts of high energy x-rays and gamma rays, coming from Earth.

"These bursts last about a thousandth of a second, so they're very short and they're very bright," says the study's lead author Professor Joseph Dwyer of the Florida Institute of Technology.

"In fact they're so bright, that they temporarily blind spacecraft."

The flashes were originally thought to be coming from the top of Earth's atmosphere, but spacecraft measurements and energy modelling show they're coming from altitudes below 20 kilometres.

"People now know they're coming from deeper down, from thunderstorms at about the same altitudes where aircraft fly," says Dwyer.

"We've been struggling to figure out how thunderstorms could generate these flashes."

Dwyer and colleagues have been using computer models and simulations to try and solve the problem.

The authors say, there are two possible ways in which thunderstorms are producing these powerful events.

Scientists already know high energy x-rays can be generated by normal lightning, close to the ground. So it's possible normal lightning is also producing x-rays and gamma rays at higher altitudes inside thunderstorms.

"The second idea, is that there's this sort of exotic type of discharge, which we've coined dark lightning," says Dwyer.

"It produces a lot of high energy electrons and their anti-matter counterparts called positrons. This generates lots of gamma rays, but not much visible light, which is why we call it dark lightning.

"We're pretty sure that in order to make gamma rays inside thunderstorms we need high energy electrons, accelerated by strong electric fields to almost the speed of light," says Dwyer.

"When these electrons hit air atoms, they make gamma-rays, so we're pretty sure that's what's going on. But for that to work, we need lots of high energy electrons to be made very quickly, and figuring out how that happens is challenging."
Electron avalanche

Dwyer thinks, part of the process involves an electron avalanche.

"You start with just a couple of high energy particles from other sources like cosmic rays," says Dwyer.

The electron avalanche builds up quickly, generating more and more high energy particles, in an ever increasing feedback loop, eventually generating a terrestrial gamma ray flash.

On average each thunderstorm produces at least one TGF.

"Passengers flying through a thunderstorm when a TGF occurs would receive a significant dose of radiation, comparable to a full body CT scan," says Dwyer.

He says it amounts to a fair fraction of your lifetime radiation budget.

"There's probably a half-dozen ways a thunderstorm can hurt you if you're in a plane inside one, and now we've found another way," says Dwyer.

"The good news is this would be a very rare event. Pilots already know to stay away from thunderstorms, they're dangerous places even without gamma rays."
Illuminating Earth's mysterious gamma ray flashes | abc.net.au

electrobleme

  • Administrator
  • Plasma Star
  • *****
  • Posts: 1503
  • EUreka?: +1/-0
  • It's time to step out of the Gravity, Well?
    • Electric Universe theory blog
What are Gamma Ray Bursts? Wikipedia explanation
« Reply #5 on: December 17, 2014, 02:38:37 »
Quote

Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe.[1] Bursts can last from ten milliseconds to several minutes. The initial burst is usually followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).[2]

Most observed GRBs are believed to consist of a narrow beam of intense radiation released during a supernova or hypernova as a rapidly rotating, high-mass star collapses to form a neutron star, quark star, or black hole. A subclass of GRBs (the "short" bursts) appear to originate from a different process – this may be due to the merger of binary neutron stars. The cause of the precursor burst observed in some of these short events may be due to the development of a resonance between the crust and core of such stars as a result of the massive tidal forces experienced in the seconds leading up to their collision, causing the entire crust of the star to shatter.[3]

The sources of most GRBs are billions of light years away from Earth, implying that the explosions are both extremely energetic (a typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime) and extremely rare (a few per galaxy per million years[4]). All observed GRBs have originated from outside the Milky Way galaxy, although a related class of phenomena, soft gamma repeater flares, 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.[5]

GRBs were first detected in 1967 by the Vela satellites, a series of satellites designed to detect covert nuclear weapons tests. Hundreds of theoretical models were proposed to explain these bursts in the years following their discovery, such as collisions between comets and neutron stars.[6] Little information was available to verify these models until the 1997 detection of the first X-ray and optical afterglows and direct measurement of their redshifts using optical spectroscopy, and thus their distances and energy outputs. These discoveries, and subsequent studies of the galaxies and supernovae associated with the bursts, clarified the distance and luminosity of GRBs. These facts definitively placed them in distant galaxies and also connected long GRBs with the explosion of massive stars, the only possible source for the energy outputs observed.

On November 21, 2013, NASA released detailed data about one of the strongest gamma-ray bursts, designated GRB 130427A, that was observed on April 27, 2013.[7][8]

History
Gamma-ray bursts were first observed in the late 1960s by the U.S. Vela satellites, which were built to detect gamma radiation pulses emitted by nuclear weapons tested in space. The United States suspected that the USSR might attempt to conduct secret nuclear tests after signing the Nuclear Test Ban Treaty in 1963. On July 2, 1967, at 14:19 UTC, the Vela 4 and Vela 3 satellites detected a flash of gamma radiation unlike any known nuclear weapons signature.[9] Uncertain what had happened but not considering the matter particularly urgent, the team at the Los Alamos Scientific Laboratory, led by Ray Klebesadel, filed the data away for investigation. As additional Vela satellites were launched with better instruments, the Los Alamos team 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[9] and definitively rule out a terrestrial or solar origin. The discovery was declassified and published in 1973 as an Astrophysical Journal article entitled "Observations of Gamma-Ray Bursts of Cosmic Origin".[10]

Many theories were advanced to explain these bursts, most of which posited nearby sources within the Milky Way Galaxy. Little progress was made until the 1991 launch of the Compton Gamma Ray Observatory and its Burst and Transient Source Explorer (BATSE) instrument, an extremely sensitive gamma-ray detector. This instrument provided crucial data that showed the distribution of GRBs is isotropic—not biased towards any particular direction in space, such as toward the galactic plane or the galactic center.[11] Because of the flattened shape of the Milky Way Galaxy, if the sources were from within our own galaxy they would be strongly concentrated in or near the galactic plane. The absence of any such pattern in the case of GRBs provided strong evidence that gamma-ray bursts must come from beyond the Milky Way.[12][13][14][15] However, some Milky Way models are still consistent with an isotropic distribution.[12][16]
Counterpart objects as candidate sources

For decades after the discovery of GRBs, astronomers searched for a counterpart at other wavelengths: i.e., any astronomical object in positional coincidence with a recently observed burst. Astronomers considered many distinct classes of objects, including white dwarfs, pulsars, supernovae, globular clusters, quasars, Seyfert galaxies, and BL Lac objects.[17] All such searches were unsuccessful,[nb 1] and in a few cases particularly well-localized bursts (those whose positions were determined with what was then a high degree of accuracy) could be clearly shown to have no bright objects of any nature consistent with the position derived from the detecting satellites. This suggested an origin of either very faint stars or extremely distant galaxies.[18][19] Even the most accurate positions contained numerous faint stars and galaxies, and it was widely agreed that final resolution of the origins of cosmic gamma-ray bursts would require both new satellites and faster communication.[20]

Afterglow
Several models for the origin of gamma-ray bursts postulated[21] that the initial burst of gamma rays should be followed by slowly fading emission at longer wavelengths created by collisions between the burst ejecta and interstellar gas. This fading emission would be called the "afterglow." Early searches for this afterglow were unsuccessful, largely due to the difficulties in observing a burst's position at longer wavelengths immediately after the initial burst. The breakthrough came in February 1997 when the satellite BeppoSAX detected a gamma-ray burst (GRB 970228[nb 2]) and when the X-ray camera was pointed towards the direction from which the burst had originated, it detected fading X-ray emission. The William Herschel Telescope identified a fading optical counterpart 20 hours after the burst.[22] Once the GRB faded, deep imaging was able to identify a faint, distant host galaxy at the location of the GRB as pinpointed by the optical afterglow.[23][24]

Because of the very faint luminosity of this galaxy, its exact distance was not measured for several years. Well before then, another major breakthrough occurred with the next event registered by BeppoSAX, GRB 970508. This event was localized within four hours of its discovery, allowing research teams to begin making observations much sooner than any previous burst. The spectrum of the object revealed a redshift of z = 0.835, placing the burst at a distance of roughly 6 billion light years from Earth.[25] This was the first accurate determination of the distance to a GRB, and together with the discovery of the host galaxy of 970228 proved that GRBs occur in extremely distant galaxies.[23][26] Within a few months, the controversy about the distance scale ended: GRBs were extragalactic events originating within faint galaxies at enormous distances. The following year, GRB 980425 was followed within a day by a coincident bright supernova (SN 1998bw), indicating a clear connection between GRBs and the deaths of very massive stars. This burst provided the first strong clue about the nature of the systems that produce GRBs.[27]

BeppoSAX functioned until 2002 and CGRO (with BATSE) was deorbited in 2000. However, the revolution in the study of gamma-ray bursts motivated the development of a number of additional instruments designed specifically to explore the nature of GRBs, especially in the earliest moments following the explosion. The first such mission, HETE-2,[28] launched in 2000 and functioned until 2006, providing most of the major discoveries during this period. One of the most successful space missions to date, Swift, was launched in 2004 and as of 2014 is still operational.[29][30] Swift is equipped with a very sensitive gamma ray detector as well as on-board X-ray and optical telescopes, which can be rapidly and automatically slewed to observe afterglow emission following a burst. More recently, the Fermi mission was launched carrying the Gamma-Ray Burst Monitor, which detects bursts at a rate of several hundred per year, some of which are bright enough to be observed at extremely high energies with Fermi's Large Area Telescope. Meanwhile, on the ground, numerous optical telescopes have been built or modified to incorporate robotic control software that responds immediately to signals sent through the Gamma-ray Burst Coordinates Network. This allows the telescopes to rapidly repoint towards a GRB, often within seconds of receiving the signal and while the gamma-ray emission itself is still ongoing.[31][32]

New developments over the past few years include the recognition of short gamma-ray bursts as a separate class (likely due to merging neutron stars and not associated with supernovae), the discovery of extended, erratic flaring activity at X-ray wavelengths lasting for many minutes after most GRBs, and the discovery of the most luminous (GRB 080319B) and the former most distant (GRB 090423) objects in the universe.[33][34] The most distant known GRB, GRB 090429B, is now the most distant known object in the universe.

Classification
While most astronomical transient sources have simple and consistent time structures (typically a rapid brightening followed by gradual fading, as in a nova or supernova), the light curves of gamma-ray bursts are extremely diverse and complex.[35] No two gamma-ray burst light curves are identical,[36] with large variation observed in almost every property: the duration of observable emission can vary from milliseconds to tens of minutes, there can be a single peak or several individual subpulses, and individual peaks can be symmetric or with fast brightening and very slow fading. Some bursts are preceded by a "precursor" event, a weak burst that is then followed (after seconds to minutes of no emission at all) by the much more intense "true" bursting episode.[37] The light curves of some events have extremely chaotic and complicated profiles with almost no discernible patterns.[20]

Although some light curves can be roughly reproduced using certain simplified models,[38] little progress has been made in understanding the full diversity observed. Many classification schemes have been proposed, but these are often based solely on differences in the appearance of light curves and may not always reflect a true physical difference in the progenitors of the explosions. However, plots of the distribution of the observed duration[nb 3] for a large number of gamma-ray bursts show a clear bimodality, suggesting the existence of two separate populations: a "short" population with an average duration of about 0.3 seconds and a "long" population with an average duration of about 30 seconds.[39] Both distributions are very broad with a significant overlap region in which the identity of a given event is not clear from duration alone. Additional classes beyond this two-tiered system have been proposed on both observational and theoretical grounds.[40][41][42][43]

Short gamma-ray bursts
Events with a duration of less than about two seconds are classified as short gamma-ray bursts. These account for about 30% of gamma-ray bursts, but until 2005, no afterglow had been successfully detected from any short event and little was known about their origins.[45] Since then, several dozen short gamma-ray burst afterglows have been detected and localized, several of which are associated with regions of little or no star formation, such as large elliptical galaxies and the central regions of large galaxy clusters.[46][47][48][49] This rules out a link to massive stars, confirming that short events are physically distinct from long events. In addition, there has been no association with supernovae.[50]

The true nature of these objects (or even whether the current classification scheme is accurate) remains unknown, although the leading hypothesis is that they originate from the mergers of binary neutron stars[51] or a neutron star with a black hole. The mean duration of these events of 0.2 seconds suggests a source of very small physical diameter in stellar terms: less than 0.2 light-seconds (about 37,000 miles—four times the Earth's diameter) This alone suggests a very compact object as the source. The observation of minutes to hours of X-ray flashes after a short gamma-ray burst is consistent with small particles of a primary object like a neutron star initially swallowed by a black hole in less than two seconds, followed by some hours of lesser energy events, as remaining fragments of tidally disrupted neutron star material (no longer neutronium) remain in orbit to spiral into the black hole, over a longer period of time.[45] A small fraction of short gamma-ray bursts are probably produced by giant flares from soft gamma repeaters in nearby galaxies.[52][53]

Long gamma-ray bursts
Most observed events (70%) have a duration of greater than two seconds and are classified as long gamma-ray bursts. Because these events constitute the majority of the population and because they tend to have the brightest afterglows, they have been studied in much greater detail than their short counterparts. Almost every well-studied long gamma-ray burst has been linked to a galaxy with rapid star formation, and in many cases to a core-collapse supernova as well, unambiguously associating long GRBs with the deaths of massive stars.[54] Long GRB afterglow observations, at high redshift, are also consistent with the GRB having originated in star-forming regions.[55]

Ultra-long gamma-ray bursts
These events are at the tail end of the long GRB duration distribution, lasting more than 10,000 seconds. They have been proposed to form a separate class, possibly the result of the collapse of a blue supergiant star.[56] Only a small number have been identified to date, their primary characteristic being their gamma ray emission duration. So far, the known and well established ultra long GRBs are GRB 091024A, GRB 101225A, and GRB 111209A.[57][58] A recent study,[59] on the other hand, shows that the existing evidence for a separate ultra-long GRB population with a new type of progenitor is inconclusive, and further multi-wavelength observations are needed to draw a firmer conclusion.

Tidal disruption events
This new class of GRB-like events was first discovered through the detection of GRB 110328A by the Swift Gamma-Ray Burst Mission on 28 March 2011. This event had a gamma-ray duration of about 2 days, much longer than even ultra-long GRBs, and was detected in X-rays for many months. It occurred at the center of a small elliptical galaxy at redshift z = 0.3534. There is an ongoing debate as to whether the explosion was the result of stellar collapse or a tidal disruption event accompanied by a relativistic jet, although the latter explanation has become widely favoured.

A tidal disruption event of this sort is when a star interacts with a supermassive black hole shredding the star, and in some cases creating a relativistic jet which produces bright emission of gamma ray radiation. The event GRB 110328A (also denoted Swift J1644+57) was initially argued to be produced by the disruption of main sequence star by a black hole of several million times the mass of the Sun,[60][61][62] although it has subsequently been argued that the disruption of a white dwarf by a black hole of mass about 10 thousand times the Sun may be more likely.[63]

Energetics and beaming
Gamma-ray bursts are very bright as observed from Earth despite their typically immense distances. An average long GRB has a bolometric flux comparable to a bright star of our galaxy despite a distance of billions of light years (compared to a few tens of light years for most visible stars). Most of this energy is released in gamma rays, although some GRBs have extremely luminous optical counterparts as well. GRB 080319B, for example, was accompanied by an optical counterpart that peaked at a visible magnitude of 5.8,[64] comparable to that of the dimmest naked-eye stars despite the burst's distance of 7.5 billion light years. This combination of brightness and distance implies an extremely energetic source. Assuming the gamma-ray explosion to be spherical, the energy output of GRB 080319B would be within a factor of two of the rest-mass energy of the Sun (the energy which would be released were the Sun to be converted entirely into radiation).[33]

No known process in the Universe can produce this much energy in such a short time. Rather, gamma-ray bursts are thought to be highly focused explosions, with most of the explosion energy collimated into a narrow jet traveling at speeds exceeding 99.995% of the speed of light.[65][66] The approximate angular width of the jet (that is, the degree of spread of the beam) can be estimated directly by observing the achromatic "jet breaks" in afterglow light curves: a time after which the slowly decaying afterglow begins to fade rapidly as the jet slows and can no longer beam its radiation as effectively.[67][68] Observations suggest significant variation in the jet angle from between 2 and 20 degrees.[69]

Because their energy is strongly focused, the gamma rays emitted by most bursts are expected to miss the Earth and never be detected. When a gamma-ray burst is pointed towards Earth, the focusing of its energy along a relatively narrow beam causes the burst to appear much brighter than it would have been were its energy emitted spherically. When this effect is taken into account, typical gamma-ray bursts are observed to have a true energy release of about 1044 J, or about 1/2000 of a Solar mass (M?) energy equivalent[69]—which is still many times the mass-energy equivalent of the Earth (about 5.5 × 1041 J). This is comparable to the energy released in a bright type Ib/c supernova and within the range of theoretical models. Very bright supernovae have been observed to accompany several of the nearest GRBs.[27] Additional support for focusing of the output of GRBs has come from observations of strong asymmetries in the spectra of nearby type Ic supernova[70] and from radio observations taken long after bursts when their jets are no longer relativistic.[71]

Short (time duration) GRBs appear to come from a lower-redshift (i.e. less distant) population and are less luminous than long GRBs.[72] The degree of beaming in short bursts has not been accurately measured, but as a population they are likely less collimated than long GRBs[73] or possibly not collimated at all in some cases.[74]

Progenitors
Because of the immense distances of most gamma-ray burst sources from Earth, identification of the progenitors, the systems that produce these explosions, is particularly challenging. The association of some long GRBs with supernovae and the fact that their host galaxies are rapidly star-forming offer very strong evidence that long gamma-ray bursts are associated with massive stars. The most widely accepted mechanism for the origin of long-duration GRBs is the collapsar model,[75] in which the core of an extremely massive, low-metallicity, rapidly rotating star collapses into a black hole in the final stages of its evolution. Matter near the star's core rains down towards the center and swirls into a high-density accretion disk. The infall of this material into a black hole drives a pair of relativistic jets out along the rotational axis, which pummel through the stellar envelope and eventually break through the stellar surface and radiate as gamma rays. Some alternative models replace the black hole with a newly formed magnetar,[76] although most other aspects of the model (the collapse of the core of a massive star and the formation of relativistic jets) are the same.

The closest analogs within the Milky Way galaxy of the stars producing long gamma-ray bursts are likely the Wolf–Rayet stars, extremely hot and massive stars which have shed most or all of their hydrogen due to radiation pressure. Eta Carinae and WR 104 have been cited as possible future gamma-ray burst progenitors.[77] It is unclear if any star in the Milky Way has the appropriate characteristics to produce a gamma-ray burst.[78]

The massive-star model probably does not explain all types of gamma-ray burst. There is strong evidence that some short-duration gamma-ray bursts occur in systems with no star formation and where no massive stars are present, such as elliptical galaxies and galaxy halos.[72] The favored theory for the origin of most short gamma-ray bursts is the merger of a binary system consisting of two neutron stars. According to this model, the two stars in a binary slowly spiral towards each other due to the release of energy via gravitational radiation[79][80] until the neutron stars suddenly rip each other apart due to tidal forces and collapse into a single black hole. The infall of matter into the new black hole produces an accretion disk and releases a burst of energy, analogous to the collapsar model. Numerous other models have also been proposed to explain short gamma-ray bursts, including the merger of a neutron star and a black hole, the accretion-induced collapse of a neutron star, or the evaporation of primordial black holes.[81][82][83][84]

An alternative explanation proposed by Friedwardt Winterberg is that in the course of a gravitational collapse and in reaching the event horizon of a black hole, all matter disintegrates into a burst of gamma radiation.[85]

Emission mechanisms
The means by which gamma-ray bursts convert energy into radiation remains poorly understood, and as of 2010 there was still no generally accepted model for how this process occurs.[86] Any successful model of GRB emission must explain the physical process for generating gamma-ray emission that matches the observed diversity of light curves, spectra, and other characteristics.[87] Particularly challenging is the need to explain the very high efficiencies that are inferred from some explosions: some gamma-ray bursts may convert as much as half (or more) of the explosion energy into gamma-rays.[88] Recent observations of the bright optical counterpart of GRB 080319B, whose light curve was correlated with the gamma-ray light curve,[64] has suggested that inverse Compton may be the dominant process in some events. In this model, pre-existing low-energy photons are scattered by relativistic electrons within the explosion, augmenting their energy by a large factor and transforming them into gamma-rays.[89]

The nature of the longer-wavelength afterglow emission (ranging from X-ray through radio) that follows gamma-ray bursts is better understood. Any energy released by the explosion not radiated away in the burst itself takes the form of matter or energy moving outward at nearly the speed of light. As this matter collides with the surrounding interstellar gas, it creates a relativistic shock wave that then propagates forward into interstellar space. A second shock wave, the reverse shock, may propagate back into the ejected matter. Extremely energetic electrons within the shock wave are accelerated by strong local magnetic fields and radiate as synchrotron emission across most of the electromagnetic spectrum.[90][91] This model has generally been successful in modeling the behavior of many observed afterglows at late times (generally, hours to days after the explosion), although there are difficulties explaining all features of the afterglow very shortly after the gamma-ray burst has occurred.[92]

Rate of occurrence and potential effects on life on Earth
All GRBs observed to date have occurred well outside the Milky Way galaxy and have been harmless to Earth. However, if a GRB were to occur within the Milky Way, and its emission were beamed straight towards Earth, the effects could be devastating for the planet. Currently, orbiting satellites detect on average approximately one GRB per day. The closest observed GRB as of March 2014 was GRB 980425, located 40Mpc[93] (130 million light years) away in a (z=0.0085) SBc-type dwarf galaxy.[94] GRB 980425 was far less energetic than the average GRB and was associated with the Type Ib supernova SN 1998bw.[95]

Estimating the exact rate at which GRBs occur is difficult, but for a galaxy of approximately the same size as the Milky Way, the expected rate (for long-duration GRBs) is about one burst every 100,000 to 1,000,000 years.[96] Only a small percentage of these would be beamed towards Earth. Estimates of rate of occurrence of short-duration GRBs are even more uncertain because of the unknown degree of collimation, but are probably comparable.[97]

Since GRBs are thought to involve beamed emission along two jets in opposing directions, only planets in the path of these jets would be subjected to the high energy gamma radiation.[98]

Depending on its distance from Earth, a GRB and its ultraviolet radiation could damage even the most radiation resistant organism known, the bacterium Deinococcus radiodurans. These bacteria can endure 2,000 times more radiation than humans. Life surviving an initial onslaught, including those located on the side of the earth facing away from the burst, would have to contend with the potentially lethal after-effect of the depletion of the atmosphere's protective ozone layer by the burst.[99]

Hypothetical effects of gamma-ray bursts in the past
GRBs close enough to affect life in some way might occur once every five million years or so – around a thousand times since life on Earth began.[100]

The major Ordovician–Silurian extinction event of 450 million years ago may have been caused by a GRB. The late Ordovician species of trilobite that spent some of its life in the plankton layer near the ocean surface was much harder hit than deep-water dwellers, which tended to stay put within quite restricted areas. Usually it is the more widely spread species that fare better in extinction, and hence this unusual pattern could be explained by a GRB, which would probably devastate creatures living on land and near the ocean surface, but leave deep-sea creatures relatively unharmed.[5]

A case has been made that the cause of the carbon 14 (and Be 10) spike in 774 or 775 was the result of a short GRB.

Hypothetical effects of gamma-ray bursts in the future
The greatest danger is believed to come from Wolf–Rayet stars, regarded by astronomers as likely GRB candidates. When such stars transition to supernovae, they may emit intense beams of gamma rays, and if Earth were to lie in the beam zone, devastating effects may occur. Gamma rays would not penetrate Earth's atmosphere to impact the surface directly, but they would chemically damage the stratosphere.[5]

For example, if WR 104, at a distance of 8,000 light-years, were to hit Earth with a burst of 10 seconds duration, its gamma rays could deplete about 25 percent of the world's ozone layer. This would result in mass extinction, food chain depletion, and starvation. The side of Earth facing the GRB would receive potentially lethal radiation exposure, which can cause radiation sickness in the short term, and, in the long term, results in serious impacts to life due to ozone layer depletion.[5]

Effects after exposure to the gamma-ray burst on Earth's atmosphere
Longer-term, gamma ray energy may cause chemical reactions involving oxygen and nitrogen molecules which may create nitrogen oxide then nitrogen dioxide gas, causing photochemical smog. The GRB may produce enough of the gas to cover the sky and darken it. Gas would prevent sunlight from reaching Earth's surface, producing a "cosmic winter" effect - a similar situation to an impact winter, but not caused by an impact. GRB-produced gas could also even further deplete the ozone layer.
Gamma-ray burst | wikipedia.org

electrobleme

  • Administrator
  • Plasma Star
  • *****
  • Posts: 1503
  • EUreka?: +1/-0
  • It's time to step out of the Gravity, Well?
    • Electric Universe theory blog
What are Gamma-Ray Burst Physics? Penn State University suggestions
« Reply #6 on: December 17, 2014, 02:44:26 »
What are Gamma-Ray Burst Physics? Penn State University suggestions

Quote
Gamma-ray bursts (GRB) are sudden, intense flashes of gamma-rays which, for a few blinding seconds, light up in an otherwise fairly dark gamma-ray sky. They are detected at the rate of about once a day, and while they are on, they outshine every other gamma-ray source in the sky, including the sun. Major advances have been made in the last three or four years, including the discovery of slowly fading x-ray, optical and radio afterglows of GRBs, the identification of host galaxies at cosmological distances, and finding evidence for many of them being associated with star forming regions and possibly supernovae. Progress has been made in understanding how the GRB and afterglow radiation arises in terms of a relativistic fireball shock model. This is described in a recent non-specialist GRB review or a more detailed review on GRB and afterglows. A summary of some of the specific research activities on GRB at Penn State is given in the previous link. The rest of this page gives a general overview of GRB.

Until a few years ago, GRB were thought to be just that, bursts of gamma-rays which were largely devoid of any observable traces at any other wavelengths. GRBs were first reported in 1973, based on 1969-71 observations by the Vela military satellites monitoring for nuclear explosions in verification of the Nuclear Test Ban Treaty. When these mysterious gamma-ray flashes were first detected, which did not come from Earth's direction, the first suspicion (quickly abandoned) was that they might be the product of an advanced extraterrestrial civilization. Soon, however, it was realized that this was a new and extremely puzzling cosmic phenomenon. A major advance occurred in 1991 with the launch of the Compton Gamma-Ray Observatory (CGRO), whose results have been summarized Fishman & Meegan 1995. The all-sky survey from the Burst and Transient Experiment (BATSE) onboard CGRO, which measured about 3000 bursts, showed that they were isotropically distributed, suggesting a cosmological distribution, with no dipole and quadrupole components. Some of the related work at Penn State on the cosmological GRB distribution is in the previous link. This isotropic distribution and the brightness distribution (log N- log P) provided strong support for a cosmological origin, and the detailed gamma-ray spectra and time histories imposed significant constraints on viable models, which led to the development of the fireball shock model.

A dramatic development in the last several years has been the measurement and localization of fading x-ray signals a number of GRBs by the Beppo-SAX satellite . These afterglows, lasting typically for weeks, made possible the optical and radio detection of afterglows, which, as fading beacons, mark the location of the fiery and brief GRB event. These afterglows in turn enabled the measurement of redshift distances, the identification of host galaxies, and the confirmation that GRB were, as suspected, at cosmological distances of the order of billions of light-years, similar to those of the most distant galaxies and quasars. Even at those distances they appear so bright that their energy output during its brief peak period has to be larger than that of any other type of source, of the order of a solar rest-mass if isotropic, or some percent of that if collimated. This energy output rate is comparable to burning up the entire mass-energy of the sun in a few tens of seconds, or to emit over that same period of time as much energy as our entire Milky Way does in a hundred years.

The energy density in a GRB event is so large that an optically thick pair/photon fireball is expected to form, which will expand carrying with itself some fraction of baryons. The main challenge in the early 90's was not so much the ultimate energy source, but how to turn this energy into predominantly gamma rays with the right nonthermal broken power law spectrum with the right temporal behavior. To explain the observations, the relativistic fireball shock model was proposed by Rees and Meszaros (1992, 1994), following pioneering earlier earlier work by Cavallo & Rees, Paczynski, Goodman and Shemi & Piran. This model has been quite succesful in explaining the various features of GRB, and a general discussion of it is given, e.g. here.

Much of the recent work has concentrated on GRB afterglows, a highlight of which was the successful prediction (Meszaros & Rees, ApJ 476, 232, Feb 10, 1997) of the general X-ray and optical behavior of GRB 970228. Since then more than 40 afterglows have been studied in detail, and a number of interestinf developments have occured. A prompt optcial flash (also predicted by theory) was found in one burst; many afterglows were found to be collimated, easing the energy constraints; X-ray lines believed to be from Iron and other metals have been reported from a number of bursts; and a new variety of softer bursts dubbed "X-ray flashes" has been identified, which are very similar to classical GRB but have a softer spectrum. Other work has concentrated on identifying the progenitors of GRB. Many of the afterglows identified by Beppo-SAX (all belonging to the class of "long" bursts, >10 s duration) have been shown to be associated with massive young stars, and in some cases a peculiar supernova "(hypernova") may be associated, as suggested by Woosley and Paczynski. This has led to work by Meszaros, Rees, Lazzati and others using X-ray lines as a diagnostic for distinguishing a massive progenitor. Other work has concentrated on modeling the central engine resposible for the energy release. The main ideas invoke the formation of a several solar mass black hole with a disrupted debris torus which is rapidly accrreted, which feeds an MHD or electron-positron-baryon jet. This can result from either the merger of a compact binary, such as a double neutron star (which is expected to lead to short bursts (< 10 s), observed in gamma-rays but so far without identified long-wavelenght afterglows) or by the collapse of the fast-totating core of a massive star, in some cases dubbed a collapsar, which leads to long bursts (>10 s) and could be associated with a suupernova-like phenomenon. More details and references are given in my recent review on GRB and afterglows.
Gamma-Ray Burst Physics | psu.edu

electrobleme

  • Administrator
  • Plasma Star
  • *****
  • Posts: 1503
  • EUreka?: +1/-0
  • It's time to step out of the Gravity, Well?
    • Electric Universe theory blog
What are Galatic Gamma-Ray Bursts? University of California, Berkeley explains

Quote
Gamma-Ray Bursts

Enigmatic explosions from the distant universe


About once a day, something remarkable happens: the sky is lit up by a brilliant flash of energy. For a fleeting few seconds, this mysterious burst - coming from a seemingly random direction, different every time - ranks among the brightest objects in the sky.

Yet no one has ever witnessed such a flash directly: the energy comes almost entirely in the form of gamma rays, which human eyes cannot detect. Even if our eyes were sensitive to this extremely energetic form of radiation, gamma rays cannot penetrate the atmosphere. Only via orbiting satellites do we know of the presence of these mysterious blasts.

These events are known as gamma-ray bursts, or GRBs. They represent the most powerful explosions of energy in the cosmos since the Big Bang itself, corresponding to the equivalent of a thousand Earths vaporized into pure energy in a matter of seconds. One of the most enduring mysteries of the universe since their discovery in the 1960s, only recently have they begun to reveal their secrets.

What is a gamma-ray burst?
We define a gamma-ray burst based on its observational properties: an intense flash of gamma rays, lasting anywhere from a fraction of a second to up to a few minutes.

Gamma-ray bursts have a few other common features. We believe them to be beamed - the energy does not escape from the explosion everywhere equally, but is focused into a narrow jet (or more likely, two oppositely-directed jets.) The burst itself is also normally followed by a much longer-lived (but also much fainter) signal, visible at optical and other wavelengths. This so-called "afterglow", discovered only in the 1990s, allows us to pinpoint the origin of the GRB - something not possible from the short-lived gamma-ray signal alone.

Where do gamma-ray bursts come from?
For a long time, it was believed that GRBs must come from within our own Galaxy. It seemed impossible that they could be much more distant: for a gamma-ray burst to have come from a distant galaxy, it would have to be incredibly powerful to explain its observed brightness.

And yet we now know that, except perhaps for a few rare exceptions, most GRBs do indeed come from other galaxies - often from among the most distant galaxies in the known universe! The closest GRB known to date is still over a hundred million light-years away, and most of them come from billions of light years. To outshine our own Galaxy's closest stars in our sky from distances that are literally billions of times further away, stupendous amounts of energy are required.

What makes a gamma-ray burst?
No one knows for sure! Our best theory to date is based upon several observed facts. First, the only way to generate huge quantities is via gravitational collapse, and black holes can be very efficient at turning this energy into explosive power. Second, some of the closest GRBs appear to occur simultaneously with supernovae: explosions of stars at the end of their lives. Finally, almost all GRBs happen in galaxies containing large numbers of very massive stars.

Our conclusion: GRBs happen when an extremely massive star, at the end of its life, runs out of fuel and can no longer support itself. It collapses onto its core, crushing it into a black hole. Matter from the star falls towards the black hole at its center, and before it falls in, some of its energy is focused into powerful jets that pummel out of the north and south poles of the star, making a gamma-ray burst. The rest of the star explodes as a supernova soon afterwards.

Other origins are also possible. For example, some GRBs may be due to two ultra-dense neutron stars smashing into each other; and a small fraction may be magnetic eruptions on neutron stars in very nearby galaxies.
Gamma-Ray Bursts | berkeley.edu