III. The Electromagnetic Spectrum as a Probe of Gamma-Ray Bursts

To understand gamma-ray bursts, you must first understand that gamma-rays are the most energetic form of light. Light is the familiar word for what physicists call electromagnetic radiation or electromagnetic waves. Light is a form of energy; it can travel through empty space and is in the form of individual wave packets called photons. The waves in packets of visible light are tiny ripples less than a millionth of a meter long. When visible light is split up into its different wavelengths, the result is called a spectrum. Violet light has the shortest wavelength and red light has the longest — about twice as long as violet. Visible light is not the only form of electromagnetic radiation, however. The electromagnetic spectrum extends beyond the colors of the rainbow in both directions — to much shorter wavelengths than the violet and to much longer wavelengths than the red. At the longer wavelengths are radio waves, microwaves, and infrared radiation. At the shorter wavelengths are ultraviolet radiation, X-rays, and gamma-rays.

To understand the Universe, astronomers look at all wavelengths; the cosmic sky has a totally different appearance at different wavelengths of light. At radio wavelengths, astronomers see distant quasars and hot gas in our Milky Way Galaxy. The infrared sky shows mainly tiny dust particles strewn through our Galaxy and other galaxies. Visible and ultraviolet show mainly the light from ordinary stars. X-rays reveal gas heated to millions of degrees lying between galaxies

or falling onto compact objects like neutron stars and black holes. Gamma-rays can be produced only by extremely energetic phenomena, and we see several types of gamma-ray emission in the sky. Gamma-rays seen along the plane of the Milky Way are not from ordinary stars, but from nuclear reactions generated by protons accelerated to nearly the speed of light slamming into gas lying between the stars. Gamma-rays are also seen from blazars -- intense beams of light and particles pointed directly at the Earth produced by massive black holes in distant galaxies. Gamma-rays can be detected in the magnetic flares on the surface of our Sun, and by the radioactive decay of short-lived atomic nuclei produced by supernova explosions in the Galaxy.

All objects in our Universe emit, reflect, and absorb electromagnetic radiation in their own distinctive ways. The way an object does this provides it special characteristics which scientists can use to probe an object’s composition, temperature, density, age, motion, distance, and other chemical and physical quantities. While the night sky has always served as a source of wonder and mystery, it has only been in the past few decades that we have had the tools to look at the Universe over the entire electromagnetic (EM) spectrum and see it in all of its glory. Once we were able to use space-based instruments to examine infrared, ultraviolet, X-ray, and gamma-ray emissions, we found objects that were otherwise invisible to us (e.g., black holes and neutron stars). A "view from space" is critical since radiation in these ranges cannot penetrate the Earth's atmosphere. Many objects in the heavens "light up" with wavelengths too short or too long for the human eye to see, and most objects can only be fully understood by combining observations of behavior and appearance in different regions of the EM spectrum.

We can think of electromagnetic radiation in several different ways:

• From a physical science standpoint, all electromagnetic radiation can be thought of as originating from the motions of subatomic particles. Gamma-rays occur when atomic nuclei are split or fused. X-rays occur when an electron orbiting close to an atomic nucleus is pushed outward with such force that it escapes the atom; ultraviolet, when an electron is jolted from a near to a far orbit; and visible and infrared, when electrons are jolted a few orbits out. Photons in these three energy ranges (X-ray, UV, and optical) are emitted as one of the outer shell electrons loses enough energy to fall down to the replace the electron missing from the inner shell. Radio waves are generated by any electron movement; even the stream of electrons (electric current) in a common household wire creates radio waves ...albeit with wavelengths of thousands of kilometers and of very weak amplitude.

• Electromagnetic radiation can be described in terms of a stream of photons (massless packets of energy), each traveling in a wave-like pattern, moving at the speed of light. The only difference between radio waves, visible light, and gamma-rays is the amount of energy in the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and gamma-rays. By the equation E=hf, energy dictates a photon’s frequency and, hence, wavelength.

The value of the EM radiation we receive from the Universe can be realized by considering the following: Temperatures in the Universe today range from 1010 Kelvin to 2.7 Kelvin (in the cores of stars going supernova and in intergalactic space, respectively). Densities range over 45 orders of magnitude between the centers of neutron stars to the virtual emptiness of intergalactic space. Magnetic field strengths can range from the 1013 Gauss fields around neutron stars to the 1 Gauss fields of planets such as Earth to the 10-7 Gauss fields of intergalactic space. It is not possible to reproduce these enormous ranges in a laboratory on Earth and study the results of controlled experiments; we must use the Universe as our laboratory in order to see how matter and energy behave in these extreme conditions.

As we develop better observing technologies and techniques for gamma-ray astronomy, we can ask and answer fundamental questions about GRBs, such as:

• What are the progenitors of GRBs? Where are the objects which lead to GRBs located within their host galaxies? What is the local environment like at that location?

• Are there different classes of bursts with different underlying physical processes at work?

• Can GRBs be used to probe the early Universe? Can we use the optical/X-ray afterglows as high redshift beacons? Can we use the X-ray emission to probe the intergalactic and intercluster media?