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National Aeronautics and Space Administration

Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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ARCADE 2006 Results

ARCADE compares the sky to an on-board calibration target whose temperature can be adjusted to match the sky. The figure below shows data from a few minutes of the 2006 flight, as the 3 GHz radiometer alternately views the sky, then the calibrator, then returns to the sky.

3 GHz data

The instrument rotates once every 100 seconds, sweeping the antenna beam in a circle 60 degrees in diameter centered on the zenith. The "spikes" in the sky data (blue) show the increased signal as the antenna beam sweeps over the Galactic plane. Note that the calibrator temperature matches the observed range of sky values. To determine the temperature of the sky at any point, we can simply find data from the calibrator (red) with the same radiometer voltage as the sky data, then read off the calibrator temperature using the embedded thermometers. This nulled design greatly increases the accuracy of the measurement.

< 3 GHz data

The figure above shows the extragalactic temperature measured by ARCADE from the 2006 flight. The ARCADE data are consistent with a blackbody CMB spectrum with temperature T_0 = 2.729 +/- 0.004 K. This is in agreement with the value 2.725 +/- 0.001 K measured by the Cosmic Background Explorer at much shorter wavelengths, and demonstrates that a 4-hour flight from a balloon-borne instrument can yield sensitivities comparable to a year-long space mission.

The surprising result from ARCADE is the detection of an extragalactic radio background much brighter than expected. Although a radio background composed of the combined radio emission from distant alaxies had been predicted, the detected signal is five to ten times too bright to come from such galaxies. The origin of the detected background is not known.

A set of papers describing the ARCADE instrument, and scientific results from the 2006 flight can be downloaded.
  • The ARCADE 2 Instrument, J. Singal et al. 2008, ApJ, submitted

  • ARCADE 2 Observations of Galactic Radio Emission, A. Kogut et al. 2008, ApJ, submitted

  • The Extra-Galactic Sky Temperature At 3--90 GHz, D.J. Fixsen et al. 2008, ApJ, submitted

  • Detection and Interpretation of the Extragalactic Radio Background at 3 GHz, M. Seiffert et al. 2008, ApJ, submitted