What Are Gamma-Ray Bursts?

Joshua S. Bloom

• Publisher: Princeton University Press
• Year: 2011
• ISBN:
  9781400837007
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

A brief, cutting-edge introduction to the brightest cosmic phenomena known to science Gamma-ray bursts are the brightest—and, until recently, among the least understood—cosmic events in the universe. Discovered by chance during the cold war, these evanescent high-energy explosions confounded astronomers for decades. But a rapid series of startling breakthroughs beginning in 1997 revealed that the majority of gamma-ray bursts are caused by the explosions of young and massive stars in the vast star-forming cauldrons of distant galaxies. New findings also point to very different origins for some events, serving to complicate but enrich our understanding of the exotic and violent universe. What Are Gamma-Ray Bursts? is a succinct introduction to this fast-growing subject, written by an astrophysicist who is at the forefront of today's research into these incredible cosmic phenomena.Joshua Bloom gives readers a concise and accessible overview of gamma-ray bursts and the theoretical framework that physicists have developed to make sense of complex observations across the electromagnetic spectrum. He traces the history of remarkable discoveries that led to our current understanding of gamma-ray bursts, and reveals the decisive role these phenomena could play in the grand pursuits of twenty-first century astrophysics, from studying gravity waves and unveiling the growth of stars and galaxies after the big bang to surmising the ultimate fate of the universe itself. What Are Gamma-Ray Bursts? is an essential primer to this exciting frontier of scientific inquiry, and a must-read for anyone seeking to keep pace with cutting-edge developments in physics today.

1

INTRODUCTION

Serendipity is jumping into a haystack to search for a needle, and coming up with the farmer’s daughter.

—Julius H. Comroe Jr.

1.1 Serendipity during the Cold War

Before Mythbusters and The A-Team made big explosions cool, big explosions were decidedly uncool. The threat of nuclear war between the United States and the USSR (and, perhaps, China)—made blatantly real during the Cuban Missile Crisis in October 1962—had become a fixture in everyday life. One year after the crisis, seeking to diffuse an escalating arms race and the global increase of radioactive fallout from nuclear weapons testing, Soviet Premier Nikita Khrushchev and U.S. President John F. Kennedy agreed to the Partial Test Ban Treaty. Ratifying nations agreed that all nuclear weapons testing would be conducted underground from then on: no longer would tests be conducted in oceans, in the atmosphere, or in space.

The United States, led by a team at the Los Alamos National Laboratory, promptly began an ambitious space satellite program to test for “non-compliance” with the Partial Test Ban Treaty. The existence of the Vela¹ Satellite Program was unclassified: the rationale, experimental design, and satellite instrumentation were masterfully detailed in peer-reviewed public journals while the program was on going.² The concept for this space-based vigilance endeavor was informed by the physics of nuclear explosions: while the optical flash of a nuclear detonation could be shielded, the X-rays, gamma rays (sometimes written as γ-rays), and neutrons that are produced in copious numbers in the first second of an explosion are much more difficult to hide; we call the measurement of these by-products the “signature” of a nuclear detonation. Going into space for such surveillance was a must: the Earth’s atmosphere essentially blocks X-rays, gamma rays, and neutrons from space.

While the signatures of nuclear detonations were well understood, the background radiation of light and particles in space was not. To avoid false alarms caused by unknown transient enhancements in the background, satellites were launched in pairs—both satellites would have to see the same very specific signatures in their respective instruments for the alarms bells to sound. Widely separated satellite pairs also had the advantage that most of the Earth could be seen at all times. While the Vela orbits provided little vantage point on the dark side of the Moon—a natural location to test out of sight—the gamma rays and neutrons from the expanding plume of nuclear-fission products would eventually come into view. In total, six pairs (Vela 1a,b through Vela 6a,b) were launched between 1964 and 1970.

As evidenced by the Vela Satellite Program, the U.S. was obviously very serious about ensuring compliance. That the capabilities of the program were open was also a wonderful exercise in cold war gamesmanship—you are much less inclined to break the rules if you are convinced you will get caught.

While hundreds of thousands of events were detected by the Velas—mostly from lightning on Earth and charged particles (cosmic rays) hitting the instruments—the telltale signatures of nuclear detonation were thankfully never discovered.³ Those events that were obviously not of pernicious or known origin were squirreled away for future scrutiny.⁴

Starting in 1969, Los Alamos employee Ray Klebesadel began the laborious task of searching, by eye, the Vela data for coincident gamma-ray detections in multiple satellites. One event, from July 2, 1967,^* stood out (figure 1.1). Seen in both the gamma-ray detectors of Vela 4a and Vela 4b (and weakly in the less sensitive Vela 3a and Vela 3b detectors), the event was unlike any known source. Though there was no known solar activity on that day, the event data themselves in one satellite were incapable of ruling out a Solar origin, especially if it was a new sort of phenomenon from the Sun. Over the next several years, other intriguing events similar to the July 2nd event were seen in the Vela data. By 1972, Klebesadel and his colleagues Ian Strong and Roy Olson had uncovered sixteen such events using automated computer codes to aid with the arduous searches.

Figure 1.1. The first gamma-ray burst, GRB 670702, detected by the Vela 3a,b and 4a,b satellites. Shown is the gamma-ray light curve of the event, which is the instrumental brightness (counts per second) versus time measured since the event triggered the on-board instruments. The count rate before the burst is not zero due to persistent gamma-ray sources in the sky and random instrumental events in the detector. But when the event arrives at the detector, it vastly outshines the background. GRB670702 was a long-duration GRB, lasting more than eight seconds and showing variability on timescales less than one second. Adapted from J. Bonnell, A Brief History of the Discovery of Cosmic Gamma-Ray Bursts. http://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/grbhist.html (1995).

What were these bursts of gamma rays? To answer that question, the Los Alamos team recognized that it had better determine where on the sky the events came from. Pinpointing the direction of a light source is easy if you can focus it: this is what cameras used for photography and the human eye do well with visible light. But X-rays, and especially gamma rays, are not amenable to focusing: the energies of these photons are so high that they do not readily interact with the free electrons in metals and so cannot be reflected to large angles. The focusing of light without large-angle reflection is exceedingly difficult. The best the X-ray and gamma-ray detectors on the Velas could do was stop those photons, recording both the energy deposited in the detectors and the time that the photon arrived at the satellite.

The arrival time of the photons from specific events held the key to localization. Just as a thunderclap is heard first by those closest to the lightening bolt, an impulsive source of photons would be seen first in the satellite closest to the event and then later, after the light sweeps by, with the more distant satellite. Light (and sound, in the case of thunder) has a finite travel speed. Since the Vela satellites were dispersed at large distances from each other (approximately 200,000 kilometers) the difference in the arrival times of the pulses could be used to reconstruct the origin on the sky, the location on the celestial sphere. As figure 1.2 shows, an event seen in two satellites produces an annular location on the sky, and an event seen in three satellites produces a location in two patches on the sky.^*

Figure 1.2. Triangulation of gamma-ray bursts using the arrival time of light at different satellites. Image (a) shows the geometry of localization in two dimensions. The GRB propagates down through the Solar System as a plane wave depicted here with dashed lines. The event triggers the satellite at left first, then the satellite at right. By measuring the difference in trigger time (Δt) and knowing the distance r between the satellites, the angle θ toward the event can be inferred. Here c is the speed of light. Image (b) shows the localization on the celestial sphere (gray circle) using three satellites. Two independent angles are determined using the difference in arrival times between satellites 1–2 and 2–3. The uncertainty in those angles δθ is directly determined by the uncertainty in the precise time difference between satellite triggers. The direction toward the GRB is determined by the regions on the sky where the annuli overlap.

This triangulation capability, albeit crude, was sufficient to convince the Los Alamos team that it had uncovered a class of events that was not coming from the Earth, Sun, Moon, or any other known Solar System object. In 1973, Klebesadel, Strong, and Olson published their findings in the Astrophysical Journal, one of the venerable peer-reviewed journals used for describing scientific results in astronomy. The paper⁵ titled “Observations of Gamma-Ray Bursts of Cosmic Origin” marked the beginning of the gamma-ray burst (GRB)⁶ enigma that to this day captivates the imagination and keeps astronomers scratching their heads.

The word serendipity is overused and misused in science. Most mistake a serendipitous discovery to be synonymous with an unexpected (and unforeseen) discovery. But, as Julius Comroe’s colorful analogy in the epigraph describes, serendipity demands both an unexpected discovery and an entirely more pleasant discovery than the one being pursued. While GRBs certainly were unexpected and unforeseen,⁷ they were also much more scientifically valuable than what was being sought after: instead of the detection of a nuclear test by an enemy, a discovery that in the 1960s would have set the world down a dangerous and dark path, GRBs were a fresh light from the dark heavens. Indeed, their mysterious nature would captivate a generation of astronomers. The discovery of GRBs—not just their detection but the recognition that the events represented a new phenomenon in nature—was truly a serendipitous moment in modern science.

1.2 A New Field Begins

Members of Klebesadel’s team announced the discovery of GRBs at the June 1973 meeting of the American Astronomical Society, a few days after the publication of their seminal paper. In that meeting (and in the paper) they described their observations testing the hypothesis that GRBs originated from supernovae (SNe) in other galaxies; this was the only physical model for the origin of cosmic bursts of gamma rays available at the time.⁸ By trying to correlate a GRB in time and sky position to all known SNe, the attempt to connect GRBs to the then-brightest explosions in the universe “proved uniformly fruitless.”⁹

Determining what objects and what events on those objects produced GRBs quickly became a hot topic. By the end of 1974, more than one dozen ideas for the origin of GRBs had already been published. The theories spanned an astonishing range of possibilities, from sunlight scattering off fast-moving dust grains to comets colliding with white dwarfs (WDs) to “antimatter asteroids” smashing into distant stars. All viable models necessarily accommodated the available data, but the GRB data were simply too sparse to constrain a talented and imaginative group of eager scientists.

More data would be needed to narrow down the range of plausible models. By the end of 1973, the Los Alamos team had found a total of twenty-three GRBs.¹⁰ Teams working with other satellites equipped with gamma-ray detectors also began reporting detections of GRBs,¹¹ even some of the same events seen by the Vela satellites. New programs were conceived to find more GRBs and observe them with more sensitive detectors. The supposition—if not just a hope—was that with better data some telltale signature of the origin of the events would emerge. Unbeknown to those sprinting to find the answer, for all but a few special events, those telltale signatures would take over thirty years to uncover (a veritab...

Table of contents

• Cover
• Title
• Copyright
• Dedication
• Contents
• Preface
• 1: Introduction
• 2: Into the Belly of the Beast
• 3: Afterglows
• 4: The Events in Context
• 5: The Progenitors of Gamma-Ray Bursts
• 6: Gamma-Ray Bursts as Probes of the Universe
• Notes
• Suggestions for Further Reading
• Glossary
• Index