Particle Astrophysics with the Milagro and STACEE Telescopes

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

Among the amazing results emerging from the present "Golden Age" of astronomy are those involving Very High Gamma-ray telescopes. These detect light packets of energies trillions of times larger than ordinary room light. What objects in the cosmos can emit such packets? Astrophysicists believe that ordinary particles accelerated to tremendous speeds near the centers of large galaxies, or emitted by material falling into large black holes at the centers, are likely responsible. There are also huge, short bursts of gamma rays from semi-random places in the sky, thought to be from some especially large far-away supernova (hypernova) or a black hole merger with a star. Only a few special telescopes can "see" these gamma rays, and two of them are sponsored in part by the UC Santa Cruz group funded by this NSF project. The two telescopes are Milagro and STACEE, both located in New Mexico. The gamma rays are so elusive that no space telescope can detect them, and a large ground-based device is necessary. Milagro uses no glass lenses (the gamma-ray light would not focus), but uses instead a 6 million gallon reservoir of specially pure water to convert a shower of particles produced by a single gamma ray into ultraviolet light of ordinary energies. The arrival time of that light on photodetectors in the water is then used to create an image of the gamma-ray sky. Milagro works day and night in all weather and "points" everywhere in the visible sky simultaneously. In its first few years, it was successful enough that the NSF provided funds to enlarge it substantially by the addition of outlying water tanks, each of which improves its pointing ability and sensitivity to far-away sources. By contrast, STACEE is a nighttime telescope with mirrors to direct light to a central camera. It sees blue and ultraviolet light produced in the atmosphere from those same type of showers used by Milagro, but is sensitive down to a lower range of energies of gamma-ray (only 50 billion times the energy of ordinary light!). STACEE uses the mirrors of a solar energy collector belonging to Sandia Laboratories, after sunset when the skies are clear. The two telescopes are complementary to one another in the study of these spectacular sources. The goal of this Project is to perfect these two telescopes and to contribute to a better understanding of how the engines-galactic centers, black holes and hypernovae-really create the high-energy particles. It is not yet known even if the bursts of gamma-rays involve the highest energies or if they are confined to a lower range. Thus far, one event from Milagro is the only evidence for energies of the "trillion-times" range. As usual with science, the big discovery may be one the researchers don't even expect: no one has yet confirmed the 1974 groundbreaking predictions of Stephen Hawking concerning the explosive evaporation of small black holes, but Milagro could do this if one were close enough. So along with the bread-and-butter projects and the merely spectacular, the truly extraordinary discovery holds the imagination and dedication of the researchers. Those same researchers make a concerted effort to communicate the excitement of the search and their discoveries to the taxpaying public and to their students-the next generation of astrophysicists.