Physics, P-DO
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The Highest Energy Emission from Gamma-Ray Bursts
B.L. Dingus (P-23), M.M. González (P-23/University of Wisconsin), Y. Kaneko, R.D. Preece, M.S. Briggs (University of Alabama, Huntsville), C.D. Dermer (Naval Research Laboratory) IntroductionAstrophysical sources of gamma rays are the most extreme physical laboratories in the universe. Gamma rays, the highest-energy light, are produced by particles that are accelerated to relativistic energies. The highest-energy gamma rays are produced by the highest-energy particles and hence are excellent probes of these extreme environments. Gamma-ray bursts top the list of extreme astrophysical sources. The release of energy in a gamma-ray burst exceeds that of a supernova. Almost all of that energy shows up from within a fraction of a second to a few minutes and emits almost entirely in gamma rays. It is likely that the formation of black holes produces gamma-ray bursts. While gamma-ray bursts occur about once per day in the universe, they are very difficult to detect due to their short duration and unpredictable location. Special wide-field-of-view gamma-ray detectors must be used. These detectors must be above the Earth’s atmosphere except for detecting gamma rays at the very highest energies, such as above about 0.1 TeV (Teraelectronvolt = 1012 eV, which is about one trillion times the energy of visible-light photons). Researchers in Neutron Science and Technology (P-23) have found a new high-energy feature in a gamma-ray burst that was detected by NASA’s Compton Gamma-Ray Observatory. This feature points to interesting possible observations with Milagro, a high-energy detector located at LANL, which is operated by P-23 in collaboration with the University of Maryland, University of California -Irvine, -Santa Cruz, -Riverside, University of New Hampshire, New York University, Michigan State University, and George Mason University. Producing the Highest-Energy Gamma RaysThe easiest way to produce light is by heat. For example, an incandescent light bulb heats up a filament, which then glows. On a larger scale, the light that is detected from the cosmic microwave background is due to the heat left over from the big bang. The wavelength of the light, which is another way of characterizing the energy of the photons, is related to the temperature of the source. A light bulb filament is 3000 K and produces visible photons, whereas the universe is 2.73 K and produces microwave photons. In order to produce gamma-ray photons, the temperature would have to be greater than 1013 K. Such high temperatures are unknown; in addition the distribution of gamma-ray energies is not consistent with such a thermal model. Therefore, gamma rays require a more difficult mechanism to produce them. Charged relativistic particles are the key because they must emit gamma rays when they interact with magnetic fields, photons, or matter. These mechanisms are well studied with accelerators on Earth, such as the one at the Los Alamos Neutron Science Center. We can relate the energy of the gamma ray with the energy and type of the accelerated particle, the type of interaction, and the characteristics of the astrophysical medium. This description has many free parameters, so the more features we can observe, the better we can constrain the models of these sources. The features that we observe are the distribution of the energies of the gamma rays and how they vary with time. The maximum energy of the gamma rays detected is one of the easiest constraints to interpret. For example, the energy of the particle that produces the gamma ray must be larger than or approximately the same as the energy of the gamma ray for all types of these interactions. Gamma-Ray Bursts
The details about how the birth of a black hole can cause a jet of relativistic particles to be accelerated involve fascinating but difficult physics. The conditions of such large gravitational and electromagnetic fields cannot be replicated on Earth. Researchers, such as Chris Fryer and Alex Heger of the Theoretical Astrophysics Group (T-6), are attempting to use computer simulations of the fundamental physics in this extreme environment to replicate these phenomena as shown in Figure 1.1 These simulations cannot be exact due to the complexity of the problem and have yet to produce a complete understanding. These theories need more data from observations both in space and on the ground to provide constraints to the computer models. Detecting Gamma Rays
Gamma rays cannot be focused onto small detectors as is done for visible photons with large mirrors. Instead, large detectors are required to cause the gamma rays to interact, turning their energy into ionizing radiation, which can be recorded. Astrophysical sources produce fewer high-energy gamma rays than low-energy gamma rays, thus larger detectors are required for higher-energy gamma rays. Due to the high cost of putting large detectors into space, the maximum energy of detectable gamma rays is limited. However, above 1011 eV, gamma rays produce showers of particles in the Earth’s atmosphere that are detectable on the ground. Milagro, a gamma-ray observatory located at LANL, is the first large-field-of-view gamma-ray detector sensitive down to nearly 1011 eV. The large field of view is crucial to observing short duration, randomly and rarely occurring, gamma-ray bursts. An artist’s conception of the shower of particles impacting Milagro is pictured in Figure 3. A New Feature in the Highest-Energy Gamma Rays from Gamma-Ray BurstsThe largest-field-of-view detector on NASA’s Compton Gamma-Ray Observatory was BATSE (Burst and Transient Source Experiment). BATSE was sensitive to gamma rays of a few times 104 eV up to a few times 106 eV, and detected nearly 3000 gamma-ray bursts during the nine years of this mission. EGRET (Energetic Gamma-Ray Experiment Telescope) was the detector on the Compton Gamma-Ray Observatory that was sensitive to the highest-energy gamma rays, but detected only the brightest burst observed by BATSE. Researchers in P-23 collaborated with researchers at the University of Alabama in Huntsville and the Naval Research Laboratory to combine data from BATSE and EGRET to examine the distribution of the energy of gamma rays, known as the energy spectrum, from the brightest gamma-ray bursts detected by BATSE. Twenty-six bursts were selected and one burst was found to have a new feature at the highest energies. Most gamma-ray burst spectra can be characterized as a single broad bump peaking in the center of the BATSE energy range. The peak of the bump typically evolves with time to lower energies as the brightness of the burst decays. The energy spectrum of one burst, GRB941017, had an additional feature in the spectrum and produced up to the highest energies detectable as shown in Figure 4.2 This feature decays slower than the typical broad bump, which is also detected in this burst. The total energy in this new feature exceeds that of the broad bump by at least a factor of two. The peak energy of this feature is beyond the range of the EGRET detection and suggests researchers may require a more powerful astrophysical source. This observation raises many questions: What fraction of gamma-ray bursts has such high-energy emission? How high in energy does this feature extend? And most importantly, what type of interaction is creating this second feature? Due to the different temporal evolution of the broad bump and the high-energy feature, different types of particles may be responsible for the two types of emission. The lowest-energy broad bump is likely due to electrons interacting with magnetic fields, and the highest-energy feature could result from protons interacting with the gamma rays of the broad bump. However, in order for protons to produce gamma rays, the energy of the protons must be nearly as large as the highest-energy particles known. These particles are known as ultra-high-energy cosmic rays. If this explanation for the high-energy feature of GRB941017 is confirmed by future observations, the long standing mystery of the origin of cosmic rays would be solved. Future ObservationsOn November 20, 2004 NASA launched a new satellite, Swift, which is dedicated to detecting gamma-ray bursts. Swift will only detect gamma rays up to a few times 105 eV but will localize the direction and the distance to their origin very well. Milagro will look for evidence of > 1011 eV gamma rays in coincidence with Swift’s detections of gamma-ray bursts. A prototype version of Milagro was operational during the time when BATSE was detecting gamma-ray bursts and found evidence for one gamma-ray burst which also had much more power released at higher energies than at the energies detected by BATSE.3,4 Milagro is much more sensitive than this early prototype, and P-23 researchers are eagerly awaiting Swift’s operation and more detections of the highest-energy gamma rays from gamma-ray bursts. References
AcknowledgmentThis work was supported by LANL Laboratory Directed Research and Development (LDRD) and Institute of Geophysics and Planetary Physics programs as well as NASA. Milagro has been supported by the National Science Foundation, DOE High Energy Physics, and LANL LDRD. For further information, contact Brenda Dingus, 505-667-0400, dingus@lanl.gov. |
The origin of these powerful explosions is actively being debated. The duration is often as short as a few milliseconds and thus requires a very compact emission region. Also, the total energy released is so large that it exceeds the output of our sun during its entire lifetime. These characteristics have led researchers to believe that the astrophysical source must involve the creation of a black hole.
Gamma rays are photons of light at an energy a few times 104 eV to greater than 1013 eV. This broad energy range requires different types of detectors. Below about 1011 eV gamma rays will not penetrate the Earth’s atmosphere, so a space-based observatory is needed. NASA’s Compton Gamma-Ray Observatory, shown in Figure 2, had four gamma-ray detectors covering different energy ranges with different fields of view.
