Li - 1
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 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 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 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=mc2as 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 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=mc2law 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 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 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 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 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.