The night sky, as our unaided eyes present it to us, obviously contains the sun, the moon, thousands of stars, a few planets, a milky band of light that stretches from horizon to horizon, the occasional meteor or meteor shower, and sometimes a comet. A few centuries of examination with eyes aided by many kinds of instruments have revealed more and more of the nature of these objects—for example, that the planets are more or less like the Earth, orbiting the sun, with some planets having moons of various sizes orbiting them; that the stars are themselves distant suns; and that the Milky Way is a vast pinwheel of stars that we view from the inside, most of its stars being so far away from us that our unaided eyes can’t distinguish them individually. Our instruments also reveal objects previously unseen, such as planets orbiting other stars, and other galaxies like the Milky Way grouped in clusters and superclusters along filaments and walls that surround large spaces a few yottameters across, to name just a few. The instruments were developed gradually, some specifically designed to explore the sky, others found useful for astronomy after being made for different purposes. The discoveries made with earlier instruments informed the design and application of later instruments to investigate those discoveries further and answer questions they raised.
Individually, the discoveries are more or less interesting pieces of information. Combined, they give us an overall concept of the universe’s structure, composition, and history.
While our own eyes see light waves in a narrow range of frequencies of roughly 400-790 trillion vibrations per second, or 400-790 terahertz, the entire set of cosmic light sources emits a much larger frequency spectrum, from the lowest frequencies of radio waves through microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. These radiations have the same nature—electromagnetic waves—but the different frequencies’ interactions with matter are not the same, and each species of atom and molecule interacts most strongly with different portions of the spectrum. Waves of differing frequency thus require different material instruments to detect their presence and show us the objects that emit them.
Many cosmic objects have been found to emit gamma rays—waves whose lowest frequencies are about 10 quintillion vibrations per second, i.e. about 10 exahertz (about, because the differently-named portions of the electromagnetic spectrum are not sharply defined). For light at these frequencies, the amount of energy per photon[Wikipedia] is at least ~6.6 femtojoules, or ~41 kiloelectronvolts (41 keV), as opposed to the few-electronvolt photons of visible light. Other gamma rays are found, including those in gamma-ray bursts, whose sources have yet to be unambiguously identified, but known physical laws suggest what the sources might be. We do find gamma-ray bursts associated with other galaxies, and the bursts’ properties suggest what kinds of objects within those galaxies might be producing them. Gamma-ray bursts were first revealed in the late 1960s and early 1970s, not by astronomical instruments, but by satellites designed to monitor gamma rays produced on or near Earth in nuclear weapons tests. Since information about these satellites was classified at the time, the discovery was only announced in 1973.[SciTech Connect, p. 19 of 20]
Astronomers’ study of gamma-ray bursts has suggested hypotheses about their sources, which further research may disprove or confirm and, in any case, should tell us more about our universe. The results of several investigations partly sponsored by the Energy Department have been published in 2013 alone, many by the SLAC National Accelerator Laboratory, and many of these involving an astronomical instrument launched in 2003, the Fermi Gamma-Ray Space Telescope. This telescope was named for Enrico Fermi, whose theory of cosmic-particle acceleration is the foundation for understanding what the Fermi telescope is expected to discover.[NASA]
The earliest discoveries made with this instrument are described in the report “Fermi Gamma-ray Space Telescope: High-Energy Results from the First Year”[SciTech Connect], which also describes the telescope’s components and the multiple gamma-ray astronomy objectives these components were designed to meet. Some of these objectives relate to gamma-ray bursts. The two main observational components, the Large Area Telescope and the Gamma-ray Burst Monitor, synergize complementary functions. The Gamma-ray Burst Monitor scans the sky in many directions at once, and when it registers a burst, it triggers a reorientation of the Large Area Telescope towards the burst’s source for more detailed scrutiny. The telescope’s objectives relate to answering questions about cosmic gamma rays that were left to answer or were raised for the first time by data from an earlier satellite, the Compton Gamma Ray Observatory, and its Energetic Gamma Ray Experiment Telescope component. The report describes some of what was learned about gamma-ray bursts from this instrument, and how some of its limitations are surpassed by the Fermi telescope’s gamma-burst observing capabilities.
One of the Fermi telescope’s objectives was to study the high-energy behavior of gamma-ray bursts and transients. Gamma-ray bursts are thought very likely to result from the formation of stellar-mass black holes[Wikipedia], with a few such bursts occurring every day, a small fraction of which should be detectable by Fermi’s Large Area Telescope. (This instrument did detect 138 gamma-ray bursts in its first year of operation; 10 of these emitted gamma rays of high energy.) The bursts were already known to come in two classes: long-duration bursts of 2 seconds or more, which are associated with galaxies whose stars contain relatively few nuclei that aren’t hydrogen or helium, and which are usually found in regions where stars are forming; and short-duration bursts under 2 seconds long, which may be associated with galactic regions of much lower rates of star formation. Long-duration bursts are thought to result from massive-star collapse and black-hole formation, while short-duration bursts may result from the coalescence of compact objects like black holes, neutron stars[Wikipedia], or white dwarfs[Wikipedia]. In either case it’s thought that an initial fireball produces a plasma jet moving fast enough to have a kinetic energy around a hundred to a thousand times its rest mass[Wikipedia], from which gamma-rays and x-rays may be emitted for a few thousand seconds or more. The jet’s subsequent collision with slower-moving matter surrounding the fireball is thought to produce an afterglow of x-rays and lower-frequency light lasting hours to months. The details of what drives the process, how the energy is transported, why the plasma takes the form of a narrow jet, and the mechanics of light-wave emission are still debated.
“Fermi Gamma-ray Space Telescope: High-Energy Results from the First Year”[SciTech Connect] lists several new findings about high-energy emissions from gamma-ray bursts (pp. 24-27) that include the following:
Table 1. Gamma-ray bursts detected by the Fermi Large Area Telescope between August 2008 and September 2009. Each of these bursts was also detected by the Fermi Gamma-ray Burst Monitor. Redshifts[Wikipedia] have been obtained for half of these bursts. Four of the bursts caused an autonomous repoint request to be generated and initiated. All of the bursts, except GRB 090217, exhibited temporally extended high-energy emission that lasted from tens of seconds to thousands of seconds. (P. 25, “Fermi Gamma-ray Space Telescope: High-Energy Results from the First Year”[SciTech Connect])
Whether an object produces light itself or absorbs, reflects, refracts, or otherwise transmits light that reaches it from somewhere else, the light we see coming from it is a clue to the object’s nature. The object’s composition determines how much light it will produce or affect at each possible frequency and polarization. So if we analyze the light that a particular object produces, reflects, or transmits, we can learn something about the object’s composition. This involves our being in the path of the light coming from the object in question. But in at least one situation, we can even learn about objects from their effects on light waves whose paths miss us entirely.
In our usual experience, two beams of light that cross each other apparently just pass through each other undisturbed. Actually, they do interact a little if the beams both consist of low-frequency waves; they interact a lot more the higher the waves’ frequencies are. We can thus still discover something about light waves that travel along paths that don’t reach us, if those paths are crossed by other light waves that do reach us. From how the light that does reach us is affected, we can get information about the other light waves they interacted with, which in turn tells us something about the objects that interacted with them.
This is the phenomenon behind two reports that examine the universe’s extragalactic background light—the infrared-to-ultraviolet light that crisscrosses the universe and is thought to originate mainly from stars and be shaped by interstellar dust. The character of whatever objects have produced and shaped this light throughout the history of the universe has made the extragalactic background light what it is. While a little of the background light does cross the Earth’s path, most of it goes elsewhere. But it does interact with other light to some degree, most of all with light of the highest frequencies: gamma rays. So analyzing the gamma rays that reach us after interacting with the extragalactic background light can tell us at least something about it, which in turn will tell us something about the objects in the universe that gave the background light the features it has. Seen another way, the exact way in which the universe’s stars and dust clouds have developed has produced specific effects on the gamma rays that reach Earth from across the universe, through the extragalactic background light that links them both. By seeing how gamma rays are affected today, we can test various ideas about how the universe actually developed in the past.
This concept is examined in two recent reports: “Extragalactic Background Light from Hierarchical Galaxy Formation: Gamma-ray Attenuation up to the Epoch of Cosmic Reionization and the First Stars”[SciTech Connect], which proposes a mathematical model of the background light’s sources, and “Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe”[SciTech Connect], which evaluates such models using data from the Fermi telescope. The reports deal with the attenuation of both gamma-ray bursts and the steady gamma rays from blazars, which are thought to be powered by material falling into supermassive black holes.[Wikipedia]
Mathematical models like the one described in the first report, which take known facts into account to propose how the extragalactic background light was produced over the history of the universe, are essentially hypotheses about the universe’s history themselves. Since the light reaching us from more distant objects started coming our way longer ago, the most recent cosmic history is represented by the appearance of the nearest objects while the earliest history is represented by what we see in the farthest ones. Every new astronomical observation is thus another clue to what that history has been, discrediting some possibilities and eventually suggesting others. The first report’s model of galaxy and star formation, and of the emission of light by stars and dust, improves in certain ways upon earlier models by taking more facts into account, and logically implies that later observations will find certain things and not others. The second report describes upper limits on the sky’s opacity to gamma rays of various frequencies at various times in the universe’s history, and shows how one earlier model of the extragalactic background light is ruled out by Fermi observations.
A flash of light traveling between two places that are stationary with respect to each other and at the same gravitational potential[Wikipedia] will take as long to be received as it did to be emitted: a flash that lasts one second from beginning to end at its source will last one second from beginning to end where it’s received. But to a similarly situated emitter and receiver in relative motion, the emission and reception take different amounts of time. A receiver moving toward the source and into the flash doesn’t wait in one place for the end of the flash to arrive, but meets it sooner, so the flash appears shorter than it did at its source. Actually more is going on than this to shorten the flash: for entities in different states of motion or gravitational potentials, the rate at which time itself passes is different, as can be calculated from the way space and time are interrelated according to the theory of relativity. Indeed, taking the properties of spacetime into account, we find that a flash of light is shorter to the receiver whether the receiver moves toward the source or the source toward the receiver. If the source and receiver move apart, the receiver finds the flashes of light lasting longer. The same causes also make the frequencies of the light higher or lower as the flashes are made shorter or longer.[Wikipedia]
In our expanding universe, the further apart two points are, the more rapidly they separate, and the longer it takes to receive a given flash of light. So we’d expect that gamma-ray bursts from more distant sources would tend to be longer and have lower-frequency waves than those from nearer sources. Even accounting for the fact that gamma-ray bursts might be different, if their sources have the same range of characteristics everywhere in the universe, the distribution of possible burst durations should be the same as seen near their sources and only appear longer with receivers’ increasing distance from the sources.
At first sight, this appears not to be the case—a fact whose implications are examined in the report “On The Lack of Time Dilation Signatures in Gamma-ray Burst Light Curves”.[SciTech Connect] The authors find that more things influence the recorded duration of gamma-ray bursts than their sources’ properties and the universe’s expansion. One of these influences is also known to affect the apparent sizes of distant galaxies: since any light source, like a gamma-ray burst or a galaxy, is dimmer the further away it is, the dimmest portions of it may not send enough light to the recording instrument for the instrument to recognize above the background noise.[Wikipedia] Thus a distant galaxy looks smaller than it is since its dimmer periphery blends into the background noise, and a gamma-ray burst looks shorter than it is since its dimmer beginning and end blend into the background noise. Gamma-ray bursts’ lengthening and dimming with distance may end up competing with each other to the point that their distances and apparent durations may not correlate at all. As it happens, gamma-ray burst data from the Fermi telescope’s predecessor Swift[Wikipedia] does show a lack of such correlation (see Figure 2). Furthermore, assuming the accuracy of a mathematical model for gamma-ray bursts proposed by David Band and others in 1993[Wikipedia], the authors show that the durations measured for gamma-ray bursts near the measuring instruments’ detection threshold should be considered lower limits to the bursts’ actual durations. The authors also conclude that a gamma-ray burst’s temporal properties alone aren’t enough to determine whether it’s intrinsically a short or a long burst.
As accurately as the Band functions describe the distribution of prompt-phase gamma-ray frequencies in other bursts, the gamma-ray burst GRB 090510 discussed above shows strong evidence for deviation from the Band-function pattern. A detailed analysis of GRB 090510 and its deviations are given in “Fermi Observations of GRB 090510: A Short Hard Gamma-Ray Burst with an Additional, Hard Power-Law Component from 10 keV to GeV Energies”.[SciTech Connect] Such differences in gamma-ray bursts’ frequency spectrum have been seen in long-duration gamma-ray bursts observed by other instruments, but GRB 090510 is the first short-duration burst seen by any instrument to exhibit them. The report discusses what the burst’s features imply about the extragalactic background light it passed through on the way to the Fermi telescope; the report also describes details of three proposed mechanisms for producing gamma-ray bursts, but shows that none of these mechanisms fit all of GRB 090510’s features.
As noted above, long-duration gamma-ray bursts are thought to result from massive-star collapse and black-hole formation—one process that results in a supernova.[Wikipedia] The poster “Multigroup Radiation Transport in Supernova Light Curve Calculations”[SciTech Connect] from Los Alamos National Laboratory summarizes a mathematical analysis of supernovae associated with long gamma-ray bursts. These supernovae are always found to be Type Ic,[Wikipedia] whose spectra show very little indication of the presence of helium—interesting, since the fusion of hydrogen into helium is the basic energy source of starlight, at least at the beginning of a star’s existence. Astrophysicists have investigated whether these stars might actually contain more helium than they seem to, with some process either hiding it or else ejecting it before the star becomes a supernova. The poster shows results of a new calculation that suggests such stars may fuse their helium layers into oxygen as they develop, leaving little helium in them when they erupt. Further work under way would produce simulated spectra of supernovae that lost their helium in this manner, which could be compared with the spectra of actual Type Ic supernovae.
The Los Alamos slide set “Optical Observations of Gamma-Ray Bursts: Connections to GeV/TeV Jets”[SciTech Connect] presents an analysis of two gamma-ray bursts in terms of the plasma-jet concept mentioned above. Since a plasma jet’s eventual collision with slower-moving matter produces an afterglow that includes visible light which, unlike gamma rays, our atmosphere is transparent to, a gamma burst’s aftereffects can be observed with ground-based telescopes. The presentation illustrates the “robotic ecosystem” of orbiting and ground-based telescopes used to monitor the sky and quickly focus on transient events, with special emphasis on the RAPTOR-T fast-followup, multicolor-characterization telescope and the RAPTOR-Q full-sky persistence telescopes. One benefit of this ecosystem is evident from a key point this presentation makes: gamma-ray bursts with early peaked visible-light afterglows are a good place to search for gamma-ray afterglow.
According to one slide, the prompt gamma rays of gamma-ray bursts vary simultaneously with their prompt visible-light emissions, these being generated by internal shocks in the bursts’ plasma jets. A burst’s visible afterglow is thought to result from external shocks: either reverse shocks in the jet or forward shock in the surrounding medium. The afterglow may start during a burst’s prompt gamma-ray emission, but will persist hours or days after the gamma rays fade. Details of the gamma-ray bursts GRB 050820a and GRB 130427A are analyzed with this in view, with emphasis on the latter burst.
GRB 130427A was detected by the Fermi telescope as well as by the Swift space telescope,[Wikipedia] which is dedicated to gamma-ray burst studies and is used to observe electromagnetic waves in the visible, ultraviolet, x-ray, and gamma ray parts of the spectrum. GRB 130427A is roughly 4 billion light-years away—close for an observed gamma-ray burst—and the energy per unit area received from it via photons in the 20 keV – 10 MeV range was higher than from any gamma-ray burst ever observed by both Fermi and Swift telescopes. Fermi’s Large Area Telescope detected gamma ray photons with energies above 100 million electronvolts for more than 20 hours, and one gamma-ray photon set a new record for highest energy observed from a burst: 95 billion electronvolts. The gamma-ray and visible afterglow intensities proved to have “striking similarities” for the first 7,000 seconds, suggesting that they were cogenerated in an external shock. Detailed mathematical modeling suggested the afterglow was generated by a reverse shock followed by forward-shock emission after about 3,000 seconds.
“Aided by rapid progress in information technology, current sky surveys are again changing the way we view and study the Universe, and the next-generation instruments, and the surveys that will be made with them, will maintain this revolutionary progress. Substantial progress in the important scientific problems of the next decade (determining the nature of dark energy and dark matter, studying the evolution of galaxies and the structure of our own Milky Way, opening up the time domain to discover faint variable objects, and mapping both the inner and outer Solar System) all require wide-field repeated deep imaging of the sky in optical bands. ... The Large Synoptic Survey Telescope (LSST), a dedicated telecope [sic] with an effective aperture of 6.7 meters and a field of view of 9.6 deg2, will make major contributions to all these scientific areas and more. It will carry out a survey of 20,000 deg2 of the sky in six broad photometric bands, imaging each region of sky roughly 2000 times (1000 pairs of back-to-back 15-sec exposures) over a ten-year survey lifetime. … this book is intended as a first step in a collaboration with the world scientific community to identify and prepare for the scientific opportunities that LSST will enable.”
As some of the aforementioned documents indicate, seeing more than just the gamma rays in gamma-ray bursts and their environment can help us figure out what causes them. The Large Synoptic Survey Telescope, or LSST, is a ground-based instrument designed to survey the sky in infrared, visible, and ultraviolet light and show us new information about the universe. The LSST Science Book quoted above contains chapters addressing different types of objects to be surveyed and specific questions that the surveys may answer about each of them.
The early sections of the book’s Chapter 8, on using LSST to study transient and variable cosmic phenomena, discuss the study of gamma-ray bursts, with some emphasis on examining the afterglows of the explosions that produce them. If a plasma jet produces a gamma-ray burst, a telescope can register the burst only if the jet is in line with the telescope. The burst’s afterglow, however, is produced when the jet collides with slower-moving material. If the jet spreads out over a wider area during the collision, the resulting afterglow should be visible from many more directions than those aligned with the original plasma jet. If a supernova produced the jet, its own light will be visible from every unobstructed direction. So more gamma-ray burst afterglows should be visible than gamma-ray bursts themselves, and more supernovas that produce gamma-ray bursts should be visible than gamma-burst afterglows. Moreover, the way the afterglows’ brightness varies with time should depend in a particular manner on how large the angle is between the jet and the telescope’s line of sight. Thus a study comparing statistics of afterglow and gamma-ray burst observations should indicate how wide the gamma-ray bursts are and how often gamma-ray bursts, whether visible from Earth or not, actually occur.
The report indicates that our understanding of gamma-ray bursts is expected to be significantly enhanced by study of the associated supernovae, and synoptic surveys like those the LSST is designed to perform are expected to increase the discovery rate for such supernovae to at least 10 times that of dedicated gamma-ray burst missions. Also, the discovery of afterglows not associated with observed gamma-ray bursts is expected to open up entirely new vistas in studies of stellar collapse.
When cosmic gamma-ray bursts were first discovered, about the only thing known about them was that they originated from sources in outer space. Since then, enough has been learned about them to suggest the following picture of the bursts and the objects that produce them, with different details of the picture supported by observation to different degrees:
The distribution of gamma-ray burst durations is bimodal[Wikipedia], with relatively few bursts lasting around two seconds, while many more last for shorter or longer times. The longer bursts are found in galaxies which contain many stars that are relatively poor in elements other than hydrogen and helium, usually in regions of those galaxies where stars are forming. Long gamma-ray bursts are produced by massive stars as they erupt into Type Ic supernovae and collapse to form black holes; these stars may have fused much of their earlier helium content into oxygen before becoming supernovae. Short gamma-ray bursts may be associated with galactic regions where stars form at much lower rates. The bursts themselves appear to come not from supernovae, but from the coalescence of compact objects like black holes, neutron stars, and white dwarfs.
While short and long gamma-ray bursts appear to have different power sources, both sources are thought to produce fireballs with plasma jets that have kinetic energies around 100 to 1000 times their rest masses. A jet emits gamma rays and x-rays for a few thousand seconds or longer, meanwhile undergoing internal shocks that also produce visible light; the jet then collides with slower-moving matter surrounding the fireball to produce an x-ray and lower-frequency afterglow lasting anywhere from hours to months. In a gamma-ray burst’s prompt phase, gamma rays with mega-electronvolt energies appear first, followed by longer-persisting giga-electronvolt gamma rays. A mathematical model of how a burst’s gamma-ray frequencies change from moment to moment was first proposed in 1993, but later observations indicate that actual gamma-ray bursts deviate from the model.
Further observations of gamma-ray bursts may result in changes to this picture of their sources. But such observations also tell us more. Interactions of gamma rays with the universe’s extragalactic background light give us information about the natural history of the galaxies and stars that produced the light. Also, the timing and energy of the highest-energy photons from gamma-ray bursts indicate that the mass scale for some quantum theories of gravity would be somewhat larger than the Planck mass—a clue, not about the universe’s contents, but about its governing laws.
Prepared by Dr. William N. Watson, Physicist
DoE Office of Scientific and Technical Information