Gregory Aldering, Saul Perlmutter, Peter Nugent, and Stewart Loken, Lawrence Berkeley National Laboratory

Data flow for the SNfactory search and follow-up. Left: Supernovae are discovered on images obtained by the NEAT team at JPL using telescopes on the summits of Haleakala, Hawaii and Palomar Mountain, California. Those images are transferred to LBNL via the Internet. Center: Once at LBNL, the images are archived onto HPSS. They are processed using the PDSF cluster and compared to archived processed images taken a week earlier to look for the light of new supernovae. Right: Follow-up spectroscopic and imaging data are obtained for these new supernovae using the Yale/Lisbon/Ohio (YALO) telescope on the summit of Cerro Tololo, Chile, and the University of Hawaii's 2.2-m telescope on the summit of Mauna Kea, which will have a new instrument—the SuperNova Integral Field Spectrograph (SNIFS)—specifically optimized for the study of supernovae. The search and follow-up flow will be monitored across the SNfactory collaboration, indicated by the salmon-colored "ground-plane." (CENTRA is the Centro Multidisciplinar de Astrofisica at the Instituto Superior Tecnico, Lisbon. FROGS is the French Observing Group for Supernovae.)

Research Objectives
The Nearby Supernova Factory (SNfactory) is an international collaboration between astrophysicists at Lawrence Berkeley National Laboratory and three institutions in France: Laboratoire de Physique Nucléaire et de Haute Énergies de Paris, Institut de Physique Nucléaire de Lyon, and Centre de Recherche Astronomique de Lyon. The aim of the collaboration is to discover nearby supernovae and to study them in detail so that they can be used more effectively as cosmological distance indicators.

Computational Approach
Discovering supernovae as soon as possible after they explode requires imaging the night sky repeatedly, returning to the same fields every few nights, and then quickly processing the data. The most powerful imager for this purpose is the CCD (charge-coupled device) camera built by the Jet Propulsion Laboratory (JPL). This camera delivers 100 MB of imaging data every 60 seconds, and an upgraded version of the camera will more than double this. The new images are compared to images of the same field archived on HPSS using digital image subtraction to find the light of any new supernovae. This digital image subtraction involves numerous steps to align the images and account for blurring by the Earth's atmosphere. Because the amount of data is so large (50 GB per night), the image archive even larger (presently 8 TB and growing), and the computations so extensive, it is critical that the imaging data be transferred to a large computing center (in this case NERSC) as quickly as possible (see figure).

Accomplishments
During the past year we worked to automate the Nearby Supernova Factory image subtraction pipeline. We also began archiving data from the Near Earth Asteroid Tracking (NEAT) team, who built and operate the current JPL camera at Palomar as well as a camera on Haleakala in Hawaii. These efforts have recently come to fruition with the discovery of our first probable supernova from this most recent effort. (Note that in spring 1999 we lead a similar effort which used similar techniques to find over 40 confirmed supernovae.)

Significance
In the past few years, measuring distances to Type Ia supernovae at very high redshifts has allowed astrophysicists to measure the rate of expansion of the Universe over the last 8 billion years. (The Universe is now believed to be about 14 billion years old.) Since all known matter in the Universe is pulled together by gravity, it was expected that these measurements would show that the expansion of the Universe has been slowing down. However, the Type Ia supernova measurements indicate that within the last few billion years, this expected slowdown has been reversed. The cause for this reversal is unknown—it may be related to Einstein's famous Cosmological Constant—and so has been dubbed "dark energy." This discovery, named the most important scientific discovery of 1998 by the journal Science, has revolutionized cosmology. Understanding the physical cause for the dark energy requires more precise measurements, and this should be possible with large numbers of accurately measured Type Ia supernovae. Understanding gained from this project will contribute to the design of future experiments, such as the Supernova/Acceleration Probe (SNAP).

Publications
G. Aldering, "Type Ia supernovae and cosmic acceleration," in AIP Conference Proceeding: Cosmic Explosions, ed. S. S. Holt and W. W. Zhang (Woodbury, New York: American Institute of Physics, 2000).

http://snfactory.lbl.gov/