Unbiased flux calibration methods for spectral-line radio observations

TL;DR

This paper discusses and compares unbiased flux calibration methods for spectral-line radio observations, highlighting the importance of accounting for frequency-dependent system temperature and noise diode effects to improve calibration accuracy.

Contribution

It introduces new methods for unbiased calibration using a noise diode and compares them with classical approaches, addressing systematic errors in radio astronomical spectra.

Findings

New calibration methods reduce systematic errors.

Monte Carlo simulations demonstrate improved flux accuracy.

Discussion on noise temperature determination techniques.

Abstract

Position and frequency switching techniques used for the removal of the bandpass dependence of radio astronomical spectra are presented and discussed in detail. Both methods are widely used, although the frequency dependence of the system temperature and/or noise diode is often neglected. This leads to systematic errors in the calibration that potentially have a significant impact on scientific results, especially when using large-bandwidth receivers or performing statistical analyses. We present methods to derive an unbiased calibration using a noise diode, which is part of many heterodyne receivers. We compare the proposed methods and describe the advantages and bottlenecks of the various approaches. Monte Carlo simulations are used to qualitatively investigate both systematics and the error distribution of the reconstructed flux estimates about the correct flux values for the new methods but also the 'classical' case. Finally, the determination of the frequency-dependent noise temperature of the calibration diode using hot-cold measurements or observations of well-known continuum sources is also briefly discussed.