Follow-up observations at 16 and 33 GHz of extragalactic sources from WMAP 3-year data: I - Spectral properties

TL;DR

This study presents follow-up high-frequency observations of extragalactic sources from WMAP data, revealing diverse spectral behaviors and highlighting variability effects, especially in fainter sources, with implications for understanding source spectra at microwave frequencies.

Contribution

It provides new simultaneous measurements at 16 and 33 GHz for WMAP sources, revealing spectral diversity and the impact of variability on flux density estimates.

Findings

44% of sources have rising spectra between 13.9 and 33.75 GHz

93% of sources have spectral index alpha < 0.5

Fainter sources show lower flux densities with VSA compared to WMAP, likely due to variability

Abstract

We present follow-up observations of 97 point sources from the Wilkinson Microwave Anisotropy Probe (WMAP) 3-year data, contained within the New Extragalactic WMAP Point Source (NEWPS) catalogue between declinations of -4 and +60 degrees; the sources form a flux-density-limited sample complete to 1.1 Jy (approximately 5 sigma) at 33 GHz. Our observations were made at 16 GHz using the Arcminute Microkelvin Imager (AMI) and at 33 GHz with the Very Small Array (VSA). 94 of the sources have reliable, simultaneous -- typically a few minutes apart -- observations with both telescopes. The spectra between 13.9 and 33.75 GHz are very different from those of bright sources at low frequency: 44 per cent have rising spectra (alpha < 0.0), where flux density is proportional to frequency^-alpha, and 93 per cent have spectra with alpha < 0.5; the median spectral index is 0.04. For the brighter sources, the agreement between VSA and WMAP 33-GHz flux densities averaged over sources is very good. However, for the fainter sources, the VSA tends to measure lower values for the flux densities than WMAP. We suggest that the main cause of this effect is Eddington bias arising from variability.