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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.
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.
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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
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