An accurate absolute flux density scale from 1 to 50 GHz

Summary form only given. The flux density scale utilized by most of radio astronomy is based on the 1977 paper by Baars et al. which provides convenient polynomial expressions for a number of discrete sources, ultimately based on absolute measurements of Cassiopeia A and Cygnus A. These two primary sources are not suitable as calibrators for high-resolution intereferometers, so the small-diameter secondary sources listed in their paper, particularly 3C48, 3C286 and 3C147, are commonly utilized to calibrate the amplitude scale. However, it has long been known that many of these secondary sources are variable. Furthermore, the Baars et ai. polynomial expressions for these sources are not valid at frequencies higher than 15 GHz, while interferometers such as the Very Large Array routinely observe at frequencies up to 50 GHz. There is thus a need to establish a flux density standard not dependent upon variable sources, is based on an absolute standard, and is valid to frequencies at least as high as 50 GHz. To address these needs, we have utilized the Very Large Array to accurately measure, over a period of 28 years, the flux density ratios of a number of calibrator sources to determine their variabilities, and have tied these sources to the emission of the planet Mars, as prediced by the model of Rudy et ai., adjusted to the absolute WMAP emission scale published by Weiland et ai.. Fourteen sources have been monitored by this program, including the extragalactic sources 3C48, 3C123, 3C138, 3C147, 3C196, 3C286, and 3C295, the planetary nebulae NGC7027 and NGC6572, the evolved star MWC349, and the planets Venus, Mars, Uranus and Neptune, at ten frequency bands spanning the range from 74 MHz to 50 GHz. Our proposed flux density scale spans the range from 1 to 50 GHz, and is based on the emission of Mars at frequencies above 5 GHz, and on the existing Baars scale at frequencies below 5 GHz. Three sources 3C196, 3C286, and 3C295 are shown to be non-variable over the monitor- - ing period, and we will present polynomial expressions, accurate to ~1% at most bands, for their spectral flux density from 1 to 50 GHz.