ARCADE 2 Measurement of the Extra-Galactic Sky Temperature at 3-90 GHz

TL;DR

ARCADE 2 measured the absolute sky temperature across multiple frequencies, revealing an excess at 3.3 GHz and characterizing the background spectrum from 22 MHz to 10 GHz, refining our understanding of the extragalactic radio background.

Contribution

This work provides precise measurements of the sky temperature at multiple GHz frequencies using an innovative balloon-borne cryogenic radiometer, improving calibration accuracy and extending the spectrum of extragalactic background observations.

Findings

Detected a 50±7 mK excess at 3.3 GHz beyond the CMB.

Established a power-law spectrum for the extragalactic background from 22 MHz to 10 GHz.

Refined the CMB temperature measurement to 2.725±0.001 K.

Abstract

The ARCADE 2 instrument has measured the absolute temperature of the sky at frequencies 3, 8, 10, 30, and 90 GHz, using an open-aperture cryogenic instrument observing at balloon altitudes with no emissive windows between the beam-forming optics and the sky. An external blackbody calibrator provides an {\it in situ} reference. Systematic errors were greatly reduced by using differential radiometers and cooling all critical components to physical temperatures approximating the CMB temperature. A linear model is used to compare the output of each radiometer to a set of thermometers on the instrument. Small corrections are made for the residual emission from the flight train, balloon, atmosphere, and foreground Galactic emission. The ARCADE 2 data alone show an extragalactic rise of $50\pm7$ mK at 3.3 GHz in addition to a CMB temperature of $2.730\pm .004$ K. Combining the ARCADE 2 data with data from the literature shows a background power law spectrum of $T=1.26\pm 0.09$ [K] $(\nu/\nu_0)^{-2.60\pm 0.04}$ from 22 MHz to 10 GHz ($\nu_0=1$ GHz) in addition to a CMB temperature of $2.725\pm .001$ K.