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Gamma-ray Bursts

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Library of Past Questions and Answers

    Gamma-ray Bursts

Tell me more about efforts to detect gamma-rays from nuclear bombs tested in space and what was actually discovered. (Submitted January 07, 1997)

The Question

I have seen shows on my cable that is on the NASA channel. They deal with NASA's programs and also programs that have people that talk about space and the Universe. On one of the shows it had a part dealing with certain light sources that the government wanted to detect to see, if the people were testing bombs in space during WWII. The light source was I believe gamma-rays because of the bombs giving off the light source in that range. It was determined that there were light sources being detected, but not from any place in space from a bomb, but from something else and the energy from this was greater then the energy of the hole Universe. It was mentioned that either it was wrong on the way energy was being assessed or calculated or the size of the Universe is 10x bigger then what they had believed.

The Answer

The search for bombs in space you refer to actually occurred during the height of the cold war, in the 1960s. The United States worried that China or the Soviet Union could explode nuclear weapons in space, hiding this activity from other countries, which would have been a violation of the nuclear test ban treaty. The Vela series of satellites were launched into space to search for gamma-ray emission from such a test in space, even in locations as remote as behind the Moon. No gamma-rays were detected from Earth, but as you know, a number of gamma-ray bursts were detected, and the information gathered from the satellites that observed these events indicated that the bursts occur far from Earth. Explaining the cause of gamma-ray bursts is still an exciting area of research for high-energy astrophysics. You can learn more about this on the Learning Center home pages.

Regards,
Padi Boyd,
for Imagine the Universe!

Question ID: 970107a1

Exactly how energetic are Gamma-ray Bursts? (Submitted May 16, 1997)

The Question

Several individuals have told me that they have read an article(s) in the newspapers recently (either 15 may or 14 May) regarding the detection of a gamma-ray that was a mother of all gamma-rays. According to these individuals the gamma-ray came from a source that that had an energy content larger than that of the energy contained in the known universe! I don't doubt the story (unfortunately I did not see the article) up to a point. I'm sure a powerful gamma-ray was detected but I question the stated energy content. Would you please clarify the stated energy content (was this gamma-ray detected by NASA astronomers?) and tell me where I may obtain up-to-date information regarding this detection of such a powerful gamma-ray.

The Answer

My colleague David Palmer provided the enclosed answer to your question.

Jim Lochner
for Ask an Astrophysicist


I believe the newspaper articles were referring to the recent discovery of a gamma-ray burst counterpart with a high redshift. This source, for a few seconds, produced over a million times as much power as our entire Galaxy, although much less than the rest of the Universe as a whole.

Pictures are at
http://antwrp.gsfc.nasa.gov/apod/ap970521.html

http://antwrp.gsfc.nasa.gov/apod/ap970513.html

The details:

About once a day, there is a sudden flash of gamma-rays coming from some random point in the sky. A Gamma-Ray Burst (GRB) is often the brightest source in the sky, often brighter than everything else combined. When they were discovered a quarter century ago, it was immediately 'obvious' that the sources of these GRBs must be relatively nearby in space, otherwise they would have to produce ridiculous amounts of energy in order to be so bright.

As better and better instruments were flown, more and more data accumulated indicating that these GRBs were distant. Rather than believing that the sources we see were in our little corner of our Galaxy, the astronomical community was split into those who believed that GRB sources were out at the distant fringes of our Galaxy, and those who believed that GRBs were at cosmological distances, far out in the Universe. The Galactic camp required that GRBs were merely ridiculously bright, but the cosmological camp required GRB energies that were completely ludicrous--as much energy as a supernova, released in seconds, and all of it in gamma-rays. (And this ignores the fact that it is very hard to make gamma-rays efficiently, so it may require hundreds to thousands of times more total energy to get that much gamma-ray energy out.)

One reason why GRBs were so mysterious is that nobody had seen them emit anything but gamma-rays, and gamma-rays are hard to work with. You can't focus them with mirrors or lenses, so it's very hard to tell where they are coming from to better accuracy than a degree or so. If you have a 1 degree position, you can look at it with modern telescopes (although you will usually have to look at a small region, move the telescope, look at another small region, etc. since 1 degree is a huge area of the sky in modern terms) and you will see many many different objects. The problem is, you will not be able to tell which of those objects, if any, is the source of the GRB.

Recently, the BeppoSAX spacecraft was launched by ESA. This spacecraft has an instrument that can locate GRBs (if they fall in its 40 x 40 degree field of view) to within about 10 arcminutes. It also has an X-ray telescope which, if pointed to an accuracy of 10 arcminutes, will see a dim X-ray source and locate it with an accuracy of an arcminute.

Three times this instrument has seen a GRB with its first instrument and, within 8 hours, pointed its X-ray telescope and seen a fading source which it could locate to within an arcminute. On two of these occasions, people using big optical telescopes have seen points of light flare up and die down at the right location. These points of light are very faint (20th magnitude) but with large telescopes such as the Hubble Space Telescope, Mount Palomar, and Keck Observatory they can be studied.

The latest optical source has lines in its spectrum which imply that it is at a redshift of z = 0.8, about halfway across the Universe. The BATSE instrument on The Compton Gamma-Ray Observatory measured this GRB as well, and reports a flux (at Earth) which reached a maximum of 1.7 x 10-7ergs/cm2/s. This gives a total peak power of a few x 1050 ergs/second, and over the 35 second duration of the burst, it produced a few x 1051 ergs.

To get a feeling for this, our entire galaxy produces about 3 x 1043 ergs/second at all wavelengths (mostly optical). Therefore, this gamma-ray burst object was millions of times as bright as our galaxy. That's pretty bright, but there are maybe 10 to 100 billion galaxies in the Universe. So the gamma-ray burst was about a 10,000th as bright as the entire Universe.

So it was millions of times brighter than our Galaxy, the brightest thing in the entire Universe for a few seconds, and was a significant fraction of the brightness of the Universe, but the press is once again guilty of exaggeration.

David Palmer

Question ID: 970516d

What is "hypernova"? (Submitted February 16, 1998)

The Question

I just heard the term 'hypernova'. Do hypernovae really exist? Is it true that there was a recently discovered one? How does a hypernova form, in contrast to supernovae & black holes?

The Answer

A hypernova is a possible explanation for gamma-ray bursts. It can be thought of as a "failed supernova" -- a massive star whose core collapses but which doesn't quite blow itself apart. The idea is that the star's core collapses because it has run out of fuel and can no longer produce enough pressure to withstand gravity. The central part of the star collapses, forming either a neutron star or a black hole. In a supernova the resulting shockwave blows off the outer parts of the star. In the case of a hypernova the shock wave doesn't blow off the outer layers of the star. The material of the outer layers falls onto the central black hole or neutron star converting its gravitational potential energy to heat and radiation. This can result in a much higher luminosity than a supernova. This is why hypernovae were proposed as a possible explanation for gamma-ray bursts. The X-ray afterglow from a gamma-ray burst has been found to be more luminous than a supernova. Whether hypernovae actually exist is still an open question.

Damian Audley
for Ask an Astrophysicist

Question ID: 980216d

Could the Gamma-ray burst we are seeing be the effects of the formation of a new black hole? (Submitted August 08, 1997)

The Question

Could the Gamma-ray burst we are seeing be the effects of the formation of a new black hole(s)? Can it be that we are seeing the creation of an event horizon - similar to the breaking of a rubber band? Since the bursts last from about 30ms to over 1000s, could they be related to the size of the star (3km = 30ms) and possibly to a cascaded black hole creation event (1000sec.+) involving several massive stars like in the center of a galaxy?

The Answer

In theory, the formation of an event horizon can occur without any excitement whatsoever. It is not like the breaking of a rubber band, but more like an animal descending below the surface of the water. This can occur quietly, like a whale swimming down to look for squid, or violently, like the winner of the 1997 Texas Belly Flop Competition.

Although the creation of an event horizon and the formation of a black hole need not necessarily create a burst of gamma-rays, most models of gamma-ray bursts end up with a black hole. This is because the energy required (assuming these sources are cosmological) is a large fraction of the mass of a star and the time scales are indeed comparable to the size of an event horizon for such a mass. Specifically, the currently most popular GRB model involves two neutron stars colliding with each other, a process which forms a black hole. (Nobody quite knows how such an event would produce gamma-rays, but the formation of a black hole at the end is thought to be inevitable.)

David Palmer and Jim Lochner
for Ask an Astrophysicist

Question ID: 970808b

How can two merging Neutron Stars make a Gamma-Ray Burst? (Submitted April 01, 1998)

The Question

I'm a 16 year old amateur astronomer, and I'd like to know (in layman's terms, preferably) how merging neutron stars can result in GRBs.

The Answer

The gravitational potential energy of two 1.4 neutron stars almost in contact is about 3 x 10^53 ergs, which may enough to power gamma-ray bursts even at the cosmological distances where we now know them to occur, at least if the event is allowed to be non-isotropic and concentrate most of the energy release in narrow beams. The gravitational energy would be primarily released via the production of neutrino-antineutrino pairs, a small proportion of which would interact to produce electron-positron pairs, which would in turn annihilate generating a gamma-ray fireball. Theoreticians are having a very hard time modeling this process in detail though!

Paul Butterworth
for the Ask an Astrophysicist team

Question ID: 980401a

Could Gamma-ray Bursts (GRBs) be caused by evaporating mini black holes as suggested by Stephan Hawking? (Submitted May 19, 1997)

The Question

Could gamma-ray bursts be due to evaporating mini-black holes as suggested by Stephen Hawking (1974)?

The Answer

This is a very good question, one that gamma-ray astronomers have thought seriously about. They have concluded that evaporating mini-black holes are unlikely to be the origin of (at least the majority of) gamma-ray bursts (GRBs).

Here are some of the reasons:

Every GRB is different: some are complex, some are simple, some are long, some are short, some are hard, some are soft. But every decaying black hole is the same (except for angular momentum and surrounding material).

Decaying black holes have well predicted time and energy behavior. What is expected from a black hole decay is a brief, rapidly hardening flash which would be most observable by Comptel or EGRET. Comptel and EGRET have looked for such, and not found any.

Decaying black holes don't decay unless they are relatively small, so they don't have very much energy, thus they must be in our immediate neighborhood to be seen. In contrast, the distribution of observed GRBs on the sky implies that they are either at cosmological distances or at least rather far out in the Galactic halo.

David Palmer and Koji Mukai
for Ask an Astrophysicist

Question ID: 970519a

What did the optical afterglow light curve of GRB990510 tell us? (Submitted June 14, 1999)

The Question

Please explain the phenomena associated with the luminosity of GRB990510 that has been explored recently. Thank you.

The Answer

GRB 990510's optical afterglow was well-placed to be viewed continuously for many hours during its first day after the burst. As a result, we have an unprecedented multi-color, continuous light curve for it.

The light curve shows that the light does not follow a straight power law decay, but falls off more rapidly than would be extrapolated from the early observations.

This is what would be predicted if the burst were not isotropic but was, rather, in the form of a jet. As the jet slows down it spreads out, and so it no longer shines as brightly in the direction of the jet as it would otherwise, letting some emission go off to the side.

The afterglow from this GRB was also polarized by 1.6%, which also suggests a jet.

If the emission from GRBs is in the form of jets, then that means that there is less energy in the burst than a straightforward calculation from observations would suggest.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

Question ID: 990614a

What is the danger of a close GRB to Earth? (Submitted September 10, 1999)

The Question

I have studied with fascination the massive gamma ray burst event (GRB990123) that has shocked astronomers with the unimaginable power of the burst that occurred on the 23 of January this year. One press release claimed that if the same event happened a mere 2000 light years away, it would appear twice as bright as the Sun for the brief time of the burst. My question relates to the impact of the gamma rays on earth if such an event occurred within a relatively close proximity to earth, would it compare to the electromagnetic pulse (EMP) effect of a high-altitude nuclear explosion on earths delicate electronic hardware? (How close would such a burst have to be to create such an effect?)

The Answer

If a gamma-ray burst occurred near to us, it would be Bad.

For a description of what a mere supernova could do, see http://stupendous.rit.edu/richmond/answers/snrisks.txt

The gamma-rays from 990123 had 1000 times the energy flux of the optical light, so at 2000 light years the gammas would deposit 2000 times as much energy as the Sun (in addition to twice as much visible light). Furthermore, this gamma-ray energy would interact in the upper atmosphere, producing nitrogen oxides that would rapidly catalyze the destruction of the ozone layer.

And then, a few centuries later, it gets worse, if current models are correct. A storm of cosmic rays would pretty much wipe out everything that wasn't beneath a few hundred meters of rock.

See Sky and Telescope for February 1998 (in most good libraries) for more details.

In answer to your EMP question, I believe that the typical scenarios involve a a big bomb, call it 10 megatons, at a high altitude, call it 1000 km. 10 megatons is 4.2 x 1023 ergs. 990123 produced about 4 x 54 ergs of gamma-rays, so it produced 1 x 1031 times as much, and would be as vicious at 3 x 1015 times the distance. 3 x 1015 x 1000 km is about 300,000 light years.

So by this analysis, from anywhere in our Galaxy, the gamma rays would cause a massive EMP event and smite all of the electronics on that side of the planet. (And maybe on the other side as well, I don't know how EMP propagates over the horizon.)

However, there are probably mitigating effects. A bomb produces a very fast release of gamma-rays (microseconds to milliseconds) causing a fast rise in electric field, the same amount of energy over a shorter time means more power in the pulse. The slower GRB 990123 (lasting about a minute) would probably cause a corresponding decrease in the EMP. There are, however, GRBs with rise times of less than a millisecond.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

Question ID: 990910a

Has the Earth evern been hit by a GRB in the past? (Submitted April 28, 2010)

The Question

Is it possible that Earth may have encountered GRB radiation in the distant past? If so, would there be any remnant (geological evidence, chemical evidence) to know?

The Answer

Thanks for your question. This is a good question, and it certainly is possible that a nearby GRB beamed in the direction of the Earth has happened since life arose here.

It has been suggested that the Ordovician-Silurian extinction event about 440 million years ago could have been caused by a GRB. There is no evidence falsifying this hypothesis, but there is no evidence establishing it either..
http://en.wikipedia.org/wiki/Ordovician-Silurian_extinction_event

It may be possible to detect historical GRB exposure from rocks on the moon, but it's much harder on the Earth due to tectonic activity and so forth.

We hope that helps.

-Kevin and Hans,
for "Ask an Astrophysicist"

Question ID: 100428a

Would a Gamma-ray burst from Eta Carina going Hypernova kill us? (Submitted March 23, 2004)

The Question

If the star eta Carina exploded as a Gamma-Ray Burst during a Hypernova event, would it be dangerous for human beings?

The Answer

It depends where the human beings are of course!

It's believed by some that gamma ray bursts, produced by the implosion of a massive star to a black hole, produce most of their emission in jets which are directed along the rotational axis of the imploding star. Most people who study Eta Car think that the rotational axis is along the symmetry axis of the homunculus nebula which surrounds eta Car, and this axis is tilted by about 45 degrees to our line of sight, which means that most of the dangerous emission would not be directed at us. So there's probably little danger to humanity. However there might be enough emission to disable communication and other satellites.

Mike Corcoran

Question ID: 040323a

Do we only see a fraction of the GRBs because they are beamed? (Submitted May 09, 2006)

The Question

Is it possible that there are many more gamma-ray bursts than the approximate 360 a year seen, because they are beamed towards us?

The Answer

Yes, the gamma-ray bursts are almost certainly beamed, and so there are likely thousands that we don't see, for every burst that we do detect. The exact ratio depends on how strongly beamed they are.

Hope this helps,

Koji & Kevin
for "Ask an Astrophysicist"

Question ID: 060509a

How are GRB distances measured? (Submitted September 12, 2009)

The Question

How can we confirm the redshift of a Gamma-ray brust?

The Answer

The redshift of gamma-ray bursts can be measured in several different ways. For relatively nearby ones, we can wait for the gamma-ray bursts themselves to fade, and obtain long-exposure spectra of the host galaxies. There are now so many gamma-ray bursts with known host galaxies, so this method is as solid as it gets in cosmology.

For more distant gamma-ray bursts, for which the host galaxy is too faint for the current generation of instruments, things are a bit trickier. In such a case, astronomers often are able to detect absorption features in the gamma-ray afterglows themselves (which, in the optical, persist for hours to days). Such absorption may occur in the host galaxies, or in the intergalactic space between us and the host galaxies. Therefore, the measured redshift of the absorbers is the minimum redshift for the gamma-ray bursts themselves.


Hope this helps,
Koji & Barb
for "Ask an Astrophysicist"

Question ID: 090912a


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