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Luke Li

Physics 105 Final

4/25/11

On the Origin of Gamma Ray Bursts

Gamma ray bursts (GRB), also known as cosmic ray bursts (CRB), represent the most luminous

and energetic events that occur in the universe (Gamma Ray Bursts: Introduction, 2008). GRBs are

flashes of gamma rays that are associated with extremely energetic explosions in very distant galaxies.

Gamma rays are extremely energetic electromagnetic radiations that have frequencies above a billion

gigahertz and wavelengths of less than ten picometers, equating to energies above 10 keV. Each year,

around a thousand GRBs are detected from earth, almost all of them coming from galaxies billions of

light years away (Massive explosion, 2011). In fact,

one of the farthest detected GRB originated from a

galaxy ten billion light years away, which implies that

the GRB occurred near the very beginning of the

universe itself. The distance that a certain GRB

traveled to reach earth is usually calculated by

studying the amount of redshift in the GRBs (Primer:

The Collapsar, 2011). A redshift in light occurs when

the frequency of the electromagnetic radiation decreases as a result of acceleration away from the

observer. Its name comes from the fact that light that has been redshifted looks “redder,” as its

frequency shifts closer to the red part of the visible spectrum. The accleration of the gamma rays is due

to the fact that space itself is expanding outwards, as described by Hubble’s Law. Therefore, the farther

away the GRB came from (and thus the farther it had to travel), the more it will be redshifted. The

Figure 1 - Gamma Ray Burst (Picture 1) http://www.universetoday.com/1688/longer-lasting-gamma-ray-bursts/

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amount of redshift is then converted to a “redshift parameter,” which is the change in frequency divided

by the original frequency. One of the farthest detected GRB had a redshift parameter of 8.2,

corresponding to a distance of more than thirteen billion light years away (Philips, 2009). Due to the

anisotropic nature of GRBs, many more GRBs occur in the universe than just those detected from earth;

in fact, it is estimated that we only detect about one GRB for every three hundred that occur, due to the

directional nature of GRBs. There are actually two different types of GRBs – long and short. Long GRBs

last at least two seconds, and can be more than a

hundred seconds in length, although most

commonly are around a few seconds (Primer: The

Collapsar, 2011). Short GRBs last on the order of

milliseconds up to two seconds. GRBs are divided

into these two categories because it is believed

that long and short GRBs are caused by totally

different mechanisms. Although the GRB itself is

basically a jet of gamma rays, it usually leaves behind an afterglow, which is composed of

electromagnetic radiation at longer wavelengths. These include X-rays, ultraviolet rays, visible light,

infrared, and radio waves, and can last for hours up to days and sometimes even weeks. These

afterglows are caused by gas and dust in space hit by the gamma rays that heat up, and subsequently

emit radiation for days (SWIFT, 2010). In addition to the GRBs themselves, these afterglows are also

studied by scientists to come up with a comprehensive understanding of GRBs. Since most GRBs

originate from galaxies billions of light years away, they provide an opportunity for scientists to “look

into” the past. Since gamma rays travel at the speed of light, observing a GRB that came from five billion

light years away is kind of like seeing into the past by five billion years, since that was when the

explosion causing the GRB occurred. As stated before, most GRBs do not originate from within our own

Figure 2 - Gamma Ray Burst (Picture 2) http://www.sciencedaily.com/releases/2009/03/090302120108.htm

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galaxy, the Milky Way. In fact, about one GRB occurs in the Milky Way every million years or so, and

about one GRB would occur close enough to the earth to do serious damage every few hundred million

years (Gamma-Ray Bursts, 2008). If one did and hit the earth, there could be disastrous consequences

for life as we know it on earth. In fact, one theory for the mass extinction that happened around 450

million years ago and wiped out close to 80% of all

species on earth and ended the Ordovician era was that a

GRB hit the earth from close range (Stuart, 2010). A

closer look at the possible consequences of a GRB hitting

the earth is studied more in detail later on. Many GRBs

are thought to come from events occurring in nebulas,

the birthplace for stars. A nebula is an interstellar cloud

of dust and gas, the remnants of supernovas or the

collapse of interstellar medium (Types of Nebula, 1997).

It is in nebulas that many stars are formed, as dust and gas combine together under their gravitational

pull and start nuclear reactions. It is also the origin of many GRBs. As previously stated, GRBs are the

most energetic and luminous phenomena in the universe. In fact, one GRB can outshine all the stars in

the Milky Way combined by over a million times in just a few seconds! It also has more energy than the

sun produces over its entire ten billion years lifetime by more than a million times over (Cain, 2005). The

power of a GRB is indeed immense, and nowhere close to anything else scientists have ever seen up to

their discovery.

The history of GRBs is interesting and has significance in its own right. The discovery of GRBs was

somewhat of an accident. The first GRBs were detected in the 1960s by US Vela satellites at the height

of the Cold War between the US and the Soviet Union (Bonnell, 1995). These satellites were sent into

space to detect gamma radiation from nuclear weapons testing, which always emitted characteristic

Figure 3 - A Nebula http://www.spacetelescope.org/images/heic0515a/

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gamma rays. At the time, people in the States were paranoid that the Soviet was secretly testing nuclear

weapons in space, on the far side of the moon. Although this irrational fear may seem ludicrous today, it

was a fear rooted firmly enough for satellites to be sent into

space to detect nuclear weapons testing. After the

deployment of the Vela satellites into space, they started to

pick up gamma radiation—but it was not from any nuclear

weapons testing. In fact, the sources of the gamma rays

seem to come from deep in outer space. This discovery

confounded astronomers, since no one knew where or what

caused the spikes of gamma radiation. Initially, many

theories abounded. Some were more outlandish than

others-- including explanations involving alien warfare and

that the gamma ray bursts were weapons that had missed their targets (Mega Disasters, 2007). One of

the more accepted initial theories was that the rays were caused by neutron stars from within our own

galaxy. Since neutron stars are so dense (dropping a marshmallow on the surface of the star would

result in energy being released equal to atomic weapons), objects that collided with a neutron star

would generate immense energy. The theory was that the collision of asteroids and other intergalactic

matter with a neutron star would release enough energy to account for the gamma ray bursts. All of the

initial theories had the GRBs originating from within the Milky Way, because no one thought it possible

for a GRB to come from other galaxies and still possess the detected amount of energy (Richmond,

2011). Using Einstein’s famous equation E=mc2 as the basis for the theoretical maximum of energy that

can be released from an explosion, it was determined that it was not possible for GRBs to originate from

outside of our own galaxy. Little progress was made until the launch of the Compton Gamma Ray

Observatory and its associated BATSE instrument in 1991, a sensitive gamma radiation detector. The

Figure 4 - Vela Satellite http://www.ph.surrey.ac.uk/astrophysics/files/observational_techniques.html

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data collected from this observatory indicated that the detected gamma rays were isotropic—that is,

they originated from every direction of the night sky uniformly (Richmond, 2011). This was a big

discovery, since it overturned the presumption that the source of GRBs was in our Milky Way. If GRBs

were to come from within our galaxy, then the distribution of gamma rays should be biased towards the

center of the galaxy and closer towards is plane of rotation, since there are a lot more stars and matter

in those directions. This significant discovery implied that GRBs came from galaxies far away, from the

edges of the known universe. This finding led to a lot of controversy at the time, since it implied that

Einstein’s E=mc2 law would be broken. To account for the amount of energy detected in a GRB, the

source mass would have to grossly exceed that which is possible (Mega Disasters, 2007). This problem

was finally resolved when scientists realized that the gamma rays were released in jets—concentrated

bi-directionally—rather than spherically in all directions. Taking this into account, it was determined that

the energy detected were well within the limits of Einstein’s equation (Mega Disasters, 2007). The

gamma rays detected on earth just happened to come from the

jets that were aimed in earth’s direction, and can be fully

explained by conventional physics. With more data from more

sophisticated satellites, it was discovered that many GRBs came

from nebulas, the nursery of stars (Mega Disasters, 2007). At

the time, this was a surprising discovery, since it was postulated

that GRBs was released during the formation of black holes, a

process at the end of a star’s life cycle. Since nebulas were

where stars are born, this finding seemed a little contradictory. Stan Woosley of the University of

California at Santa Cruz solved this mystery by coming up with the term, “hypernova.” Hypernovas are

extremely energetic supernovas that come from the death of stars that are much bigger than that of the

sun—about a hundred times greater at least (Primer: The Collapsar, 2011). These massive stars, despite

Figure 5 – Hypernova http://sensational.gamerdna.com/images/Vec1pdRP/a-hypernova

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their much greater mass, burn out of fuel at a much greater rate than the sun—resulting in a lifetime of

only around a million years compared to the sun’s lifetime of around ten billion years. Since these

massive stars burn out so rapidly, the nebula would not have had a chance to disappear before the star

died, which it normally would have if the star survived longer. Thus, gamma ray bursts that are

associated with these hypernovas would be released during the lifetime of the nebula, and therefore

explains the origin of GRBs from nebulas (Mega Disasters, 2007). Despite the amount of progress made

in the past few decades in understanding GRBs and their emission mechanisms, there are still much

more to learn. For example, the sources of the GRBs are not known with complete certainty. The study

of GRBs is a hot and exciting field, and new discoveries are still being made. Many new satellites have

recently been placed in space, and more will be added in an effort to better understand GRBs. One

particularly successful satellite recently launched into space was SWIFT, named after the bird that is

capable of abrupt changes in flight direction and was launched in 2004 (SWIFT, 2010). It is still

operational today, and is equipped with a very sensitive gamma ray detector as well as X-ray and optical

telescopes, which can observe the afterglow following a GRB. Even more recently, the Fermi mission

was launched with the Gamma ray burst monitor, which can detect and study GRBs. In addition, many

ground telescopes are being built to study GRBs, and the Gamma Ray Burst coordinates network has

been set up. This allows telescopes to rapidly rotate itself to point in the direction of a GRB, within

seconds of its discovery. As more and more information are uncovered about GRBs, scientists are

gaining greater appreciation for one of the truly magnificent wonders of the universe.

One facet of GRBs that require further study is the precise emission mechanism of the gamma

rays, or how the energy from a GRB is converted into radiation (Gamma Ray Burst Emission, 2008). This

is still somewhat of a mystery, since neither the light curves (graph of light intensity vs. time) nor spectra

of GRBs resemble the radiation emitted by familiar physical processes (Gamma Ray Burst Emission,

2008). Therefore, a successful GRB emission model has to explain the physical processes by which

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gamma rays are generated and emitted, within the constraints of the observed light curves and spectra.

Gamma ray bursts are highly efficient, with many converting more than half of its explosion energy into

gamma rays. One theory for GRB emission suggests that

inverse Compton effects may be play a big role

(Kobayashi, 2006). In this model, low energy photons

that already existed are scattered by electrons moving

at relativistic speeds during the explosion, increasing

their kinetic energy and speeds enough to transform

them into gamma rays (Kobayashi, 2006). The emission mechanisms for the lower frequency afterglow

following the initial gamma rays are much better understood. It is well known that the explosion causing

GRBs also cause matter to be expelled away at nearly the speed of light. As the matter collides with

interstellar dust or gas, relativistic shock waves are created. In addition to these shock waves, secondary

shock waves are also produced that propagate back into the GRB source, known as reverse shock waves

(Kobayashi, 2006). The shock waves in turn produce extremely energetic electrons that are accelerated

by strong magnetic and electric fields, just as in a synchrotron, causing the characteristic afterglow

emissions. This model has generally been pretty successful in explaining the features of most GRB

afterglows.

As has been mentioned before, GRBs come in two flavors: short and long. The theory of how

long GRBs are formed is quite well established, with most scientists believing in the so called “collapsar

model” (Primer: The Collapsar, 2011). A collapsar is basically just a star with at least twenty to thirty

times the mass of the sun. When such a star depletes its fuel, the core of the star will start to collapse.

This is because there will no longer be any outward radiation exerting pressure, and the attractive force

of gravity will cause the start to fall in on itself (Primer: The Collapsar, 2011). Since the star is more than

twenty times the mass of the sun, the core will most likely collapse into a black hole. Depending on the

Figure 6 - Inverse Compton Effect http://www.astro.wisc.edu/~bank/index.html

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exact mass of the star, a supernova or hypernova event occurs, with the former blowing the outer layers

of the star into space in a giant explosion. A supernova is one of the most energetic events in the

universe, and is truly a spectacular sight to behold (Supernovae, 2011). A supernova can even briefly

outshine a whole galaxy! A hypernova is just an extreme version of a supernova, with the mass of the

star at least a hundred times the mass of the sun. However, in

the case of a hypernova, the gravitational forces of the star are

so strong that the star does not explode in the sense that a

supernova explodes (Audley, 2005). The energetic explosion

still takes place, but the outer layers are not blown off. Instead,

the outer layers are sucked into the newly formed black hole in

the core, converting its gravitational potential energy to heat

and radiation. This can result in a much greater luminosity than

a supernova, and is why hypernovas are theorized as sources of GRBs (Audley, 2005). Although

hypernovas can explain the luminosity of GRBs, they have actually not been observed, and whether they

actually exist is still an open question (Audley, 2005). In both supernova and hypernova cases, the black

hole immediately begins to pull in on the stellar material, and a disk of material called an “accretion

disk” is formed (Primer: The Collapsar, 2011). The inner portion of the accretion disk revolves around

the black hole at nearly the speed of light. The rapidly rotation of conducting fluids causes an extremely

strong magnetic field to be produced. Because the inner portion of the disk is spinning faster than the

outside, the magnetic field lines twist fiercely. This in turn causes a jet of material to shoot outward at

nearly light speed, perpendicular to the accretion disk on both sides (Primer: The Collapsar, 2011). The

jet is where the gamma rays originate form, containing matter and antimatter protons and electrons.

So far, only the first step of GRB creation has been explained within the collapsar model. The

second step is called the relativistic fireball model, and explains how the gamma rays are actually

Figure 7 - A Supernova http://schools-wikipedia.org/wp/s/Supernova.htm

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created (Piran, 1999). Since the jet of material created from a GRB is traveling at close to the speed of

light, relativistic effects as outlined in Special Relativity become important (Gamma-Ray Bursts, 2004). In

this fireball mode, the jet stream is the “fireball,” but it is actually more like a fire hose. The fireball acts

like a shock wave when it blasts outwards, colliding into other matter in its path. Within the fireball,

pressure, density, and temperature vary, and many internal shock waves are produced within the

fireball that bounce back and forth (Primer: The Collapsar, 2011). Faster moving blobs of material within

the fireball overtake slower moving blobs (although

through the frame of reference of the faster moving

blobs, the slower moving blobs appear to move at

relativistic speeds backwards). Scientists believe that

the gamma rays are produced as a result of the

collisions of the blobs of matter. However, light

cannot escape from the fireball until it has cooled

enough to become somewhat transparent (Primer: The Collapsar, 2011). At that point, light rays shoot

outward from the jet. From the earth’s perspective, the photons have been accelerated, which results in

what is known as a blueshift (Primer: The Collapsar, 2011). This is when the frequency of

electromagnetic radiation has been increased as a result of its increase in velocity. As a result, the light

rays are seen as gamma rays, electromagnetic radiation with extremely high frequencies. As the fireball

continue on its path towards us, collisions with interstellar material cause emissions of less energetic

radiation, first X-rays and then ultraviolet all the way until radio waves, as the photons lose their energy

as they undergo collisions.

In

another

variant of the

Figure 8 - Fireball Model http://www.swift.ac.uk/grb.shtml

Figure 9 - A Magnetar http://www.scienceimage.csiro.au/mediarelease/mr06-169.html

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collapsar model, the black hole produced at the center of the massive star is replaced with a magnetar

(Vieru, 2010). A magnetar is a type of neutron star with an extremely strong magnetic field, thousands

of times stronger than a normal neutron star and over a hundred trillion times stronger than the earth’s

or the sun’s. The production of the magnetar is associated with the emission of great amounts of high

energy radiation, in the form of X-rays and gamma rays (Vieru, 2010). Compared to the black hole

version of the collapsar model, this magnetar model is pretty similar in most respects except for the

destiny of the star core. Scientists used to believe that magnetars do not have sufficient mass to explain

some of the more energetic GRBs observed, but it is now believed that magnetars can indeed explain at

least a subcategory of GRBs (Vieru, 2010).

Long GRBs are well modeled by the collapsar and magnetar model, but short GRBs on the other

hand are a little more mysterious (Zimmerman, 2005). One reason for this is that they are harder to

study, due to their shorter duration. They are not thought to be related to the collapse of massive stars,

since short GRBS have been observed to originate from regions with no star formation or massive stars

present (Halber, 2005). The most popular theory for short GRBs is as the result of the merger between

two binary neutron stars (Halber, 2005). Neutron stars are

composed of some of the densest materials in the universe,

with a teaspoonful weighing five billion tons (Halber, 2005). Although mostly found in isolation,

occasionally neutron stars come in pairs orbiting each other. Over hundreds of millions or billions of

years, the two stars start to spiral toward each other at ever increasing speeds, almost to the speed of

light. This spiral is due to the release of energy via gravitational waves, or the ripples in the fabric of

space-time caused by gravity (Cain, 2005). The resulting crash of the two neutrons are so huge, it

eclipses the energy of a quadrillion suns (Halber, 2005). As the two stars collide in a hundredth of a

second, a black hole is born. Conventionally, black holes suck in everything and do not allow anything to

escape. However, in the split second just before the black hole is formed, the debris from the explosion

Figure 10 - Binary Neutron Stars Colliding http://wn.com/Colliding_Binary_Neutron_Stars

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is expelled outwards as superheated jets of gas and dust (Halber, 2005). The two jets, occurring on

opposite sides just as in the collapsar model, travel off at nearly the speed of light. It is also these jets

where the gamma rays are produced.

Other theories for the origin of the short GRBs have also been proposed. These include the

merger of a neutron star and a black hole, the collapse of a neutron star, and even the evaporation of

primordial black holes (Zimmerman, 2005). In the

case of the merger of the black hole and neutron star,

the black hole swallows the neutron star and grows in

size, emitting gamma rays as a result (Cain, 2005). X-

rays are also emitted later on, as the black hole

finishes swallowing the remnants of the neutron star.

A black hole-neutron star merger can be distinguished

from a merger of two neutron stars through studying

the gravitational waves generated (Cain, 2005). A black hole-neutron star merger generates stronger

gravitational waves than binary neutron stars. For the theory of neutron star collapse, a neutron star

that collapses into a black hole may also produce gamma rays. Primordial black holes are hypothetical

black holes posited by Stephen Hawking that is formed from the extremely dense matter present near

the beginning of the universe (Fegan, 1978). These primordial black holes evaporate energy through

thermal radiation. It is hypothesized that some of the higher mass primordial black holes are now in

their final stages of evaporation, which ends in an explosion. This may also account for the origin of

short GRBs.

Figure 11 - Primordial Black Hole http://www.space.com/8057-primordial-black-holes-formed-big-bang.html

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Gamma ray bursts are some of the most spectacular events that occur in the universe. Their

energy and brightness are unparalleled. To fully appreciate the power of GRBs, one can imagine a

hypothetical situation where a GRB strikes the earth from close distance. This is extremely unlikely, as

only about one GRB occurs every million years in the Milky Way. However, the consequences can be

severe, and one theory for the Ordovician mass extinction

is that a GRB hit the earth (Stuart, 2010). If a short GRB

was to hit the earth from 3,500 light years away, earth

would basically be subjected to 300,000 megatons of TNT,

or thirty times the world’s nuclear weaponry

(Zimmerman, 2005). When a GRB hits the earth, the first

things people would notice is an extremely bright flash of

light, kind of like a second sun. The gamma rays would

strip the earth of its ozone layer, the shielding device that

protects the earth from harmful radiation (Stuart, 2010).

The gamma rays would tear apart the ozone, creating

destructive nitrogen radicals that further the damage. Without the ozone layer, the extremely high

levels of radiation will eventually kill off all life on earth. Initially, all the plankton in the ocean that

comprises the bottom of the food chain will be killed off by the radiation (Stuart, 2010). It is actually the

ultraviolet rays produced by the scattering of the gamma rays in the atmosphere that most potently

damages the plankton. The radiation targets an enzyme essential in photosynthesis. Since these

organisms produce 40% of the ocean’s photosynthesis, such an event will have dire consequences for

both ocean life and the carbon dioxide cycle (Stuart, 2010).

In the past forty years, few phenomena in astronomy have captured the attention of scientists

quite like gamma ray bursts. This is because nothing is quite like GRBs – in scope, distance, or power.

Figure 12 - Artist's rendition of earth under a close GRB strike http://www.greaterspams.com/Home9.php

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GRBs come from the farthest edges of the observable universe, and can briefly outshine entire galaxies

millions of times over. The root progenitors and emission mechanisms of GRBs are still not fully

understood, although research has come a long way in this area. Long GRBs are thought to originate

from the core collapse of massive stars in hypernovas, with jets blasting out perpendicularly to the plane

of a newly formed black hole at nearly light speed. Short GRBs are more mysterious, but the general

consensus is that it is formed through the merger of two binary neutron stars or the merger of a neutron

star and a black hole. If a GRB was to strike earth from close distances, there would be disastrous effects

as all life on earth will be wiped out. There are still a lot more to learn about gamma ray bursts, and

every day scientists are learning new information and taking steps towards a greater understanding of

GRBs. In the near future, science may one day elucidate the true nature and cause of gamma ray bursts.

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