Closure Traces: Novel Calibration-Insensitive Quantities for Radio Astronomy

TL;DR

This paper introduces closure traces, new calibration-insensitive quantities in radio astronomy that are robust against station gains and polarimetric leakage, enabling clearer polarization measurements.

Contribution

The paper presents the first closure quantities from visibilities that are insensitive to station gains and leakage, extending existing closure concepts and capturing all residual information in the data.

Findings

Closure traces are insensitive to station gains and leakage.

Products of closure traces reveal source polarization structure.

Closure traces contain all remaining information in the visibility data.

Abstract

Closure phases and closure amplitudes have proven critical to modern radio interferometry due to their insensitivity to the uncertain station gains. We present the first set of closure quantities constructed from parallel-hand and cross-hand visibilities that are insensitive to both station gains and to polarimetric leakage. These complex "closure traces" are a natural extension of closure amplitudes and closure phases, are independent of all station-based linear corruptions of the polarized visibilities, and are complete in the sense that they contain all remaining information present in the visibility data. Products of closure traces on so-called "conjugate" quadrangles are sensitive only to structure in the source polarization fraction -- independent of variations in the Stokes $I$ structure -- and thereby provide unambiguous probes of polarization in astronomical sources.