1. Gamma Ray Astronomy
The light we see with our eyes, from red to violet and all the colors in between, is just a narrow sliver of the entire electromagnetic spectrum. Light has properties of both waves and particles. If we think of light as a wave, the color of light is related to its wavelength: red has a longer wavelength than blue. If we think of light as made up of particles, called photons, then red photons carry less energy than blue photons. Gamma-ray photons carry more energy than any other kind of light in the electromagnetic spectrum: millions of times or more energy than photons of visible light.
Figure: The electromagnetic spectrum.
Our eyes are tuned to see the particular range of wavelengths (or photon energies) we call visible light because most of the light produced by the Sun at other wavelengths (including ultra-violet and infrared light) is absorbed in the Earth's atmosphere. For most of history, the field of astronomy was limited to viewing the Universe with telescopes sensitive only to visible light. In 20th century, however, rapid technological advances opened "new windows" allowing astronomers study the Universe in nearly all wavelengths of light, from the longest wavelengths (radio waves) to the shortest (gamma-rays). Every time a given "new window" is opened, astronomers learn something new about the Universe and the astrophysical phenomena within it.
Different methods are used to detect photons depending on the particular part of the electromagnetic spectrum that is explored. In the optical part of the spectrum astronomers use combinations of lenses and mirrors to create an image which can viewed with the eye or recorded using a camera. Most infrared and ultraviolet telescopes also employ lenses and mirrors, but the most energetic ultraviolet photons and X-rays can no longer be reflected by mirrors.
The range of photon energies spanned by visible light is only about a factor of two: blue or violet photons have energies of only about twice that of red photons. The range of energies belonging to the part of the electromagnetic spectrum we call gamma radiation is much larger. Lower energy gamma radiation from astrophysical sources is observed using detectors on satellites. More energetic gamma radiation can be detected indirectly from the ground by observatories like VERITAS.
Gamma-ray photons have many properties similar to photons of visible light and radio waves: they travel in straight lines; they move at the speed of light; and they are not affected by magnetic or electric fields in space. Because gamma rays can traverse great distances in space without absorption by intergalactic dust and gas, they can serve as powerful probes of distant regions of the cosmos as well as otherwise obscured regions of our own Milky Way Galaxy.
Gamma rays are produced on the Earth from the radioactive decay of naturally occurring elements, and oncologists may use gamma rays to treat cancer tumors. Particle physicists can produce gamma rays at accelerators such as Fermilab in Illinois or the Stanford Linear Accelerator in California, where they are used to study the properties of elementary matter. But very high energy gamma rays like those emitted by astronomical bodies do not occur naturally on Earth.
Objects like stars emit light because they are hot. The spectrum of light emitted by a hot object, called thermal radiation, is determined by its temperature. Hotter objects emit more photons with higher energies than cooler objects, but no matter how hot an object is, it will never produce a substantial number gamma rays based only on its temperature.
Gamma rays are produced under different circumstances than photons associated with the glow of a hot object. Gamma rays are produced in certain kinds of radioactive decay, during collisions of subatomic particles, and by charged particles like electrons or protons moving in a magnetic field. The light emitted in these kinds of processes is called non-thermal radiation. In astronomy, non-thermal radiation is usually produced in places with extreme physical conditions: intense gravity, strong magnetic fields, or fast-moving flows of energetic particles.
Since gamma rays are absorbed by the Earth's atmosphere, it was not until shortly after the dawn of the space age in the 1960s when the first astrophysical gamma rays were detected. Before the first gamma-ray observatories existed, scientists had already predicted that certain astrophysical phenomena, such as supernovae remnants, should produce gamma radiation. Development of the field of gamma-ray astronomy was also motivated by a related mystery: the origin of cosmic rays.
Cosmic rays are energetic protons and heavier atomic nuclei from space whose origins are still not fully understood. Cosmic rays arrive at the Earth in equal numbers from all directions. This is because they have electric charge and their paths bend as they traverse interstellar and intergalactic magnetic fields, effectively hiding where they came from.
Cosmic rays can produce very energetic gamma rays when they collide with other particles, such as protons in hydrogen gas. In fact, most of the gamma rays from the Milky Way galxay seen at lower energies are from such collisions, and the intensity of the gamma radiation traces differences in the density of hydrogen gas across the plane of the Galaxy.
Although we can not directly see where cosmic rays come from, objects known to produce gamma rays with energies similar to those of cosmic rays are obvious candidates. Astronomers widely believe that supernovae remnants are responsible for producing most of the observed cosmic rays up to a certain particle energy, but the origin of the most energetic cosmic rays remains a mystery.
Early gamma ray satellite missions of the 1960s and the 1970s were able to see diffuse emission of gamma rays from cosmic ray interactions as well as several point sources ("hot spots" of concentrated emission), yet the most exciting observations were of very short, very powerful blasts of gamma rays which are now known as gamma ray bursts (GRB). The first gamma ray bursts were detected unexpectedly by the Vela satellites which were flown by the United States to detect nuclear weapons tests on Earth.
The field of gamma-ray astronomy advanced significantly with the launch of NASA's Compton Gamma Ray Observatory (CGRO) in 1991. The CGRO produced the first detailed map of the diffuse gamma-ray emission in the Milky Way Galaxy, detected hundreds of point sources, and recorded thousands of gamma ray bursts. The CGRO played a crucial role in the advance of our understanding of the origins of GRBs. Thanks in large to part to observations of the CGRO, astronomers believe GRBs occur outside our own galaxy, and that at least some of them may be associated with extreme supernovae involving the formation of a black hole.
Figure: This is an image is the Compton Gamma Ray Observatory.
Figure: EGRET Sky Map
The sensitivity of a telescope is limited by the number of photons it can collect. Usually, the larger a telescope's primary mirror or lens, the fainter are the objects it can detect. The same limitation applies to space-borne observatories. For an instrument like the CGRO, gamma rays must enter and interact within it to be detected, so the number of photons it can collect, and hence its sensitivity, is limited by its size.
The number of gamma rays produced by astrophysical sources drops off rapidly with increasing energy; above a certain photon energy, satellite gamma-ray observatories are not large enough to be sensitive to most gamma-ray sources in the sky. To detect astrophysical gamma rays in this so-called very high energy regime, a new method called the Imaging Air Cherenkov Technique was pioneered in the 1960s-1980s by astronomers at the Fred Lawrence Whipple Observatory in Arizona.
This technique uses the Earth's atmosphere as the sensitive detection medium, along with large optical telescopes on the ground to look for the fleeting signatures of gamma-ray collisions overhead. The 10-meter (34-foot) optical reflector at the Whipple Observatory was built in 1968 to pursue studies in gamma-ray astronomy; it has been in almost continuous nightly use since that time. The technique developed at the Whipple Observatory is the most sensitive method of detecting very high energy gamma rays. It is an important complement to the studies from space using the CGRO.
The Whipple collaboration announced the first ground-based detection of a gamma-ray source, the Crab Nebula, in 1989. A few years later in 1992, a second class of VHE gamma-ray objects was discovered with the detection of gamma-ray flares from the active galaxy Markarian 421. By the year 2003, 18 gamma-ray sources had been detected in the very high energy regime by Whipple and other ground-based observatories.
The field of gamma-ray astronomy has entered a golden age. New generations of both satellite and ground-based observatories are now working together to probe the high energy sky with unprecedented sensitivity.
In addition to VERITAS, three other next-generation ground-based gamma-ray observatories are now in operation: H.E.S.S., located in Nabibia; MAGIC, in the Spanish Canary Islands; and CANGAROO in the Australian outback. Given their range of geographic locations around the globe, the four observatories naturally complement each other by viewing different parts of the sky at different times, making it possible to monitor a single source nearly continuously no matter where it is in the sky.
Complementing ground-based observatories from Earth orbit, NASA's GLAST mission will soon be viewing the entire gamma-ray sky once every 3 hours. GLAST will be more sensitive to higher energy gamma rays than its predecessor, the Compton Gamma Ray Observatory. Similarly, VERITAS will be more sensitive to lower-energy gamma rays than its predecessor, the Whipple 10 meter telescope. For the first time, the new generation of space and ground-based gamma-ray observatories together will have an almost uninterrupted sensitivity to gamma rays across an incredible range of photon energies, with a factor of nearly one million from the least energetic gamma rays that can be detected by GLAST to the most energetic gamma rays detected by VERITAS.
2 The Extreme Universe
The scientific objectives of VERITAS are remarkably similar to those of the other gamma-ray observatories (including Swift, GLAST, MAGIC, and H.E.S.S.) now (or soon to be) in operation. Recent observations by the Whipple Observatory Gamma-Ray Collaboration and other ground-based groups using the airshower imaging technique have demonstrated the rich scientific content of the very high energy gamma-ray band. The key science projects of VERITAS are described in more detail below.
One of the first tasks VERITAS scientists will undertake will be a deep survey of a patch of the sky in the direction of the Cygnus arm of our Galaxy. The Cygnus arm is a very exciting region of space containing a dense concentration of known gamma-ray sources (shown in the figure below), including five supernova remnants, several pulsars and pulsar wind nebulae, and more than a dozen unidentified gamma-ray sources detected by the EGRET instrument on NASA's Compton Gamma Ray Observatory. Most exciting about the sky survey, however, is the potential for new discoveries, including possible observations of so-called "dark accelerators." A dark accelerator is a gamma-ray source that has no observable counterparts in any other band of the electromagnetic spectrum. The first dark accelerator site to be discovered, TeV J 2032+2130, is in the Cygnus region. Some scientists speculate that dark accelerators like this one could be sites of cosmic ray production, but since only a few such sources have been detected so far, VERITAS has an exciting opportunity to detect new dark acceleration sites and hopefully will begin to shed light on these mysterious objects.
This figure shows the VERITAS sky survey plan for 2007 and 2008 observations of the Cygnus arm of the Milky Way galaxy, along with several known sources of lower-energy gamma-ray emission. Overlaid is a map of molecular gas density.
Exploding stars leave behind a compact core surrounded by a rapidly expanding shell of gas: a supernova remnant (SNR). SNRs are widely believed to be one source of cosmic rays: mysterious radiation from space composed of energetic protons and heavier atomic nuclei. Observations suggest the gamma-ray emission from SNR is due to energetic electrons accelerated in the supernova shockwave. However, since cosmic rays are made of protons and nuclei, observations of electrons accelerated by strong shocks only provide indirect evidence of cosmic ray production.
If Supernova Remnants do contain significant accelerated proton populations these will inevitably interact with the swept up interstellar medium in the SNR to produce gamma rays. VERITAS will be able to make crucial measurements in this area. Higher angular resolution than previous observatories will allow VERITAS to map the emission region, and improved energy resolution will help VERITAS to differentiate proton vs. electron-related contributions to the gamma-ray emission.
At the center of some supernova remnants lies a rapidly spinning neutron star called a pulsar. In some cases, the pulsar itself (rather than the supernova shockwave) can drive the emission of gamma rays. Due their rapid rotation, pulsars can generate incredible electric and magnetic fields capable of accelerating charged particles comprising a so-called "pulsar wind". X-rays and gamma rays are emitted when this wind of charge particles slams into the surrounding material left by the expanding supernova shell. Of the dozen or so very high energy gamma-ray sources discovered prior to 2003, three are pulsar wind nebulae: the Crab Nebula, PSR B1706-44 and Vela.
The EGRET instrument onboard NASA's Compton Gamma Ray Observatory detected pulsed emission from seven young rapidly rotating neutron stars, while ground-based observatories see only steady gamma-ray emission at higher energies. Together, the next generation of both satellite and ground-based observatories will have a better view of intermediate gamma-ray energies. In this way, VERITAS will be in a position to address the question of why pulsed emission dominates in the lower energy gamma-ray domain and steady emission dominates in high-energy domain.
The prototypical pulsar wind nebula is the Crab Nebula: the remains of a supernova observed by Chinese astronomers in the year 1054. As the brightest and best-studied source of high-energy gamma rays, the Crab Nebula remains very important to VERITAS scientifically and as a point of comparison for measuring the amount of gamma-ray radiation from other sources.
Many galaxies harbor a supermassive black hole (with a billion to a trillion solar masses) at the center surrounded by a swirling disk of matter called an accretion disk. In most cases, the light from this central region, the active galactic nucleus (AGN), far outshines the light from all the stars in the rest of the galaxy. In a process that is little understood, the black hole and accretion disk interact to drive the formation of two powerful jets of highly-energetic particles and radiation that move outward from near the black hole and extend across distances much greater than the size of the host galaxy itself. Viewed from the side, the jets of active galaxies can been seen at radio frequencies, as shown in this optical and radio composite of the active galaxy 3C 219.
In some cases the jets produced by these Active Galaxies point directly at the Earth allowing VERITAS scientists to study gamma-ray emission originating very close to the black hole. Objects belonging to this class of active galaxies are called blazars. Observing blazars with VERITAS will lead to a greater understanding of the central environment near the black hole, the role of the black hole in creating fast moving jets, and the processes occurring inside the jet that give rise to the observed emission.
A more thorough understanding of active galaxies will allow astronomers to make measurements of the Extragalactic Background Light (EBL). The EBL is made up of infrared light produced by stars and hot dust, and the intensity of the EBL contains information on the rate of star formation when the Universe was much younger than it is today. Blazars can be used to measure the intensity of the EBL by looking for how much of the gamma-ray emission is lost between the source and the Earth due to collisions with EBL photons. To take full advantage of this technique will require better understanding of the emission of blazars as a class. VERITAS is expected to discover 50 to 100 new blazars, a few of which are likely to be greater than 10 times more distant than those cataloged by the Whipple 10 meter telescope.
Cosmic rays are energetic protons and heavier atomic nuclei from space whose origins are not yet fully understood. Cosmic rays arrive at the Earth in equal numbers from all directions. This is because they have electric charge and their paths bend as they traverse interstellar and intergalactic magnetic fields, effectively hiding where they came from. Astronomers widely believe that supernovae remnants are responsible for producing most of the observed cosmic rays up to a certain particle energy, but the origin of the most energetic cosmic rays remains a mystery. Since AGN are known to produce gamma rays with energies similar to those of the most energetic cosmic rays, it is thought that they could also be where very high energy cosmic rays are accelerated.
The closest active galaxies observed at very high energies are hundreds of millions of light years distant, so we can not directly see their inner structure. Based on multi-wavelength observations, astronomers have constructed an AGN model with the following components (pictured at right): a supermassive black hole and accretion disk emitting X-rays; a surrounding dusty torus which blocks emission from the accretion disk when viewed from the side, clouds of gas orbiting the black hole producing atomic line emission, and two fast moving jets of energetic particles and radiation. Correlations between the intensity of the emission at different wavelengths over time provide clues about the physical conditions and processes occurring in the jets. The left hump of a blazar’s spectral energy distribution (above) is attributable to energetic electrons moving in magnetic fields, but the exact origin of the second hump, seen in the gamma-ray regime, is still debated. The observed gamma-ray emission is likely caused by very energetic electrons scattering low-energy photons up to gamma-ray energies, but the source of scattered photons is not fully settled. The gamma-ray emission could also be consistent with radiation processes involving energetic protons. Future AGN observations with VERITAS and its multi-wavelength partners will help solidify our understanding of AGN.
With current technology, we can only detect matter in the Universe if it emits light (or bends the light from another object) that we can collect with a telescope. For more than 70 years, a body of evidence has been growing that suggests there is an unseen "Dark Matter", invisible in all wavelengths of light, comprising most of the mass (as much as 90%) in the Universe.
Astronomers know very well that the intrinsic brightness of a star is related to its mass. This allows astronomers to calculate the total mass in a galaxy based on its brightness. Using the laws a gravity, astronomers can then predict the motions of stars within a galaxy, or the motions of galaxies within clusters of galaxies. In 1933, astronomer Fritz Zwicky noted that galaxies of the Coma Cluster of galaxies (shown below right) moved much faster than expected based on calculations using the visible mass of the cluster. This and subsequent observations of the motions galaxies within clusters can only be understood by invoking an extra, unseen source of additional mass.
A similar phenomena is seen in the motions of individual stars within a galaxy. Stars orbiting the center of a galaxy move too fast based on only the amount of visible matter influencing their motions.
Yet more evidence comes from observations of gravitational lensing. According to Einstein, the gravity of an object results from the bending of the fabric of space in the vicinity of a massive object. Massive objects like galaxies or clusters of galaxies have enough gravity that the path of a beam of light traveling near the galaxy is actually bent. The more massive the galaxy, the more the light beam bends. Measurement of gravitational lensing effects also indicate the presence of up to five time more invisible "Dark Matter" than visible matter.
MACHO is an acronymn for "MAssive Compact Halo Objects." MACHOs are objects made up of normal matter that don't emit enough light to be detected: namely brown dwarf stars and black holes. Brown dwarfs are balls of hydrogen gas to small to ignite the process of nuclear fusion. MACHOs are thought to inhabit the halo region of the Milky Way galaxy: a roughly spherical region around the center of the galaxy which bulges out of the galactic disk, home of very old globular star clusters. Astronomers search for MACHOs mainly by looking for the effects of their gravitational pull using techniques like graviational lensing. So far not nearly enough MACHOs can be inferred based on measurements in order to conclude that this category of dark-matter candidates can account for all, or even most, of the missing matter.
The second possibility is that dark matter is composed of a theoretical type of matter called "Weakly Interactive Massive Particles," or WIMPs. The term "weakly interacting" means that such dark-matter particles can be extremely long-lived, but also extremely difficult to detect. One possible way to detect them would be too look for the rare signature of WIMP annihilation: the collsion of two WIMPs in which two gamma rays result. If they exist, WIMPs will tend to concentrate near objects with intense gravity, VERITAS will search for signs of WIMP annihilation coming from the direction of objects like galaxy superclusters.
In the lower-energy gamma-ray sky the most mysterious events are short but extremely powerful bursts of gamma rays appearing to be distributed uniformly in the sky. NASA's Compton Gamma Ray Observatory (CGRO) played a crucial role in advancing our understanding of GRBs in the 1990s. The CGRO had the ability to notify a network of other space-based and ground-based observatories shortly after the detection of a GRB. This resulted in the detection of a handful of associated afterglows observed at optical, infrared, and radio frequencies, allowing astronomers to gauge the distance to these few GRB events. The most distant GRBs observed so far are more than 12 billion light years distant, which means the light we observe from them was produced when the Universe was very young.
The GRBs recorded by the CGRO are distributed uniformly in the sky. This fact, along with observations of optical afterglows associated with a handful of GRBs, have lead astronomers to conclude that GRBs originate outside our own Galaxy. The fact that GRBs are so bright despite their extreme distances from the Earth means that they release an enormous amounts of energy. If a typical GRB emits all its energy isotropically (equally in all directions), the total amount of energy released over the brief time of the burst would be equivalent to more than the the amount of energy stored in the form of the mass of our Sun! Alternatively, GRBs might emit less energy if the emission we observe is focused —or beamed— in a particular direction. In fact, this latter possibility is now favored by observational evidence.
GRBs come in two general categories: short bursts lasting fractions of second, and long bursts which can last seconds or minutes. Long GRBs are thought to occur when extraordinarily massive stars (greater than 40 solar masses) collapse and form a black hole. The orgin of short-duration GRBs is less certain, but could be associated with the merger of two neutron stars to form a black hole. So far, GRBs have only been observed in lower-energy gamma rays observed by satellites. VERITAS will search for evidence of these strange events at high energies.