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Twinkle, Twinkle, Exploding Star

Nicole Lloyd-Ronning
Physics Department
Stanford University
May 2001

My research focuses on understanding the physics of gamma-ray bursts (GRBs), which, as their name implies, are bursts of high-energy (gamma-ray) radiation that last anywhere from 0.01 seconds to over 1,000 seconds. It is currently believed that GRBs are created when a very massive star collapses into a black hole. In this violent process of collapse, a hot "fireball" is spit out, and subsequently emits gamma-ray radiation for a short time. GRBs are the most energetic explosions in the universe and can be seen in other galaxies billions of light years away.

Studying these events allows us to study physics unachievable in laboratories on Earth; meaning they have the potential to teach us fundamentally new physics and lead us to a better understanding of the physical laws that govern our universe. I find GRBs particularly interesting because the explosions involve many different physical processes and incorporate physics from several sub-disciplines (electromagnetism, fluid mechanics, statistical mechanics, thermodynamics and special relativity, to name just a few). My work focuses specifically on how the radiation from GRBs is produced.

I would first like to give a brief history of the discovery of GRBs in order to emphasize what an important problem they have been in astrophysics for the past thirty years. In the late 1960's, there were a series of satellites put into orbit (the VELA satellites) designed to detect gamma-ray radiation. Their purpose was to see if any country was violating the nuclear test ban treaty, which prohibited nuclear explosions (a process that produces copious amounts of gamma-rays) in the sky above the Earth's atmosphere. But in 1969, one of these satellites saw a burst of gamma-rays that did not come from the Earth or the sun (a strong gamma-ray source). It lasted about 10 seconds and the number of photons (single gamma-ray light "particles") per unit time hitting the detector (otherwise known as the flux) varied quite a bit throughout the event. Scientists had no idea what could be producing these events: Were they from some physical process in our own solar system (such as two comets colliding), or did they occur as a result of some violent event (such as a massive star collapsing under its own gravitational force into a black hole) in another galaxy billions of light years away?

In the 1980's and 1990's, we learned a good deal about GRBs from the many different satellites that were put into orbit to detect them. In particular, we found out they are quite frequent, occurring about once every day. However, unlike radiation of lower energy, high-energy gamma-ray radiation does not carry a signature of the distance to the source, so we had no way of knowing where in our universe they were occurring. This is an extremely important piece of information if one is to understand the physics behind GRBs. Since the flux we measure is proportional to the amount of energy the object releases divided by the square of the distance to the object, the amount of energy required to produce a given flux measured on Earth can vary by a factor of a billion depending on whether the burst occurs in our own galaxy or in another. Hence, knowing the distance to these sources is necessary in understanding how much energy they release, and is a crucial factor in producing a realistic model for these events.

In 1997, just two months after I began working on my thesis, the “Rosetta stone” of GRB physics was found. A Dutch-Italian satellite called BeppoSAX, detected what is called the "afterglow" of the burst. When the fast, hot fireball that initially radiated the gamma-ray energy begins to slow down and cool, it emits lower energy radiation called the afterglow, and it is from this radiation that the distance to the burst can be measured. BeppoSAX, and eventually other telescopes around the world, measured this afterglow for the first time and established that GRBs do in fact occur at cosmological distances (in other galaxies billions of light years away) and are the most energetic explosions in the universe. We could finally define a physical paradigm within which these objects could be studied. As mentioned above, we now believe that GRBs occur when a very massive star (about 25 times the mass of the sun) collapses into a black hole under its own gravitational force. The violent event produces a fireball made of photons and electrons and a magnetic field that releases a billion times more energy in a few seconds than the sun will in its entire lifetime. Knowing the energy required to produce the burst, we could finally investigate the elusive question of how exactly these gamma-ray photons we observe are produced in the fireball, which is the subject of my research.

My work focuses on analyzing the spectrum of a GRB and trying to understand the physics that could have produced it. Not all of the gamma-ray photons that arrive at the detector have exactly the same energy. It is this distribution of photon energies (the burst's spectrum) that carries information about how the photons were produced. For example, the shape of the spectrum depends on a number of things: the magnetic field, the density of the fireball, how fast the fireball is traveling, etc. My most recent work involves the development of physical models (in which all of these parameters play a role) to fit the observed spectral data. In particular, I have shown that models of "synchrotron radiation," radiation produced from electrons spiraling around a magnetic field, can explain the observed spectral data very well, and that this mechanism is most likely producing the gamma-rays in the bursts.

This work has brought up a number of deeper theoretical questions related to fundamental physics at these high energies. For example, this synchrotron radiation model requires the presence of a very strong magnetic field (about a billion times the strength of the Earth's field) and it is an important problem to understand how this field is generated in the fireball to begin with. This research has also challenged conventional wisdom on the nature of the interactions among the various particles in the fireball. In addition, it may eventually lead us to a solution to the important and unanswered question of how the fireball is spit out from the collapsing star to begin with.

As I mentioned earlier, the type of radiation at these energies cannot be produced in laboratories on Earth, which gives me a chance to study physics in conditions never investigated before. This means that we have the opportunity to learn exciting new physics from GRBs. To me, this is motivation enough for studying them, but in addition, as history has proved time and again, breakthroughs in physics have almost always had a direct impact on our everyday lives through improvements in technology, medical research, and so on. But for those who want reasons which have a much more apparent direct impact on our everyday lives, consider the following: I mentioned above that GRBs occur in other galaxies billions of light years away from our own. Since we have massive stars in our own galaxy, the probability that a GRB will occur in the Milky Way is about 1 in 100 million years. In fact, there are theories that a GRB occurred in our galaxy about the time when dinosaurs became extinct, and some believe it was the intense radiation from this nearby burst that caused the mass extinction. So, if nothing else, maybe learning a little more about the nature of these "exploding stars" will help save us from being next!