HiFAST : An HI Data Calibration and Imaging Pipeline for FAST II. Flux Density Calibration

TL;DR

This paper presents a flux density calibration model for the FAST II HI data pipeline, analyzing calibration techniques, environmental effects, and stability over time to ensure accurate HI 21-cm spectral measurements.

Contribution

The study introduces a flux calibration model for the HIFAST pipeline and evaluates gain stability and environmental dependencies for FAST II HI observations.

Findings

Gain parameter correlates linearly with atmospheric temperature.

Calibration gain dispersion reduces to <3% after temperature correction.

Receiver response stability allows for moderate calibration errors (~5%).

Abstract

Accurate flux density calibration is essential for precise analysis and interpretation of observations across different observation modes and instruments. In this research, we firstly introduce the flux calibration model incorporated in HIFAST pipeline, designed for processing HI 21-cm spectra. Furthermore, we investigate different calibration techniques and assess the dependence of the gain parameter on the time and environmental factors. A comparison is carried out in various observation modes (e.g. tracking and scanning modes) to determine the flux density gain ($G$), revealing insignificant discrepancies in $G$ among different methods. Long-term monitoring data shows a linear correlation between $G$ and atmospheric temperature. After subtracting the $G$--Temperature dependence, the dispersion of $G$ is reduced to $<$3% over a one-year time scale. The stability of the receiver response of FAST is considered sufficient to facilitate HI observations that can accommodate a moderate error in flux calibration (e.g., $>\sim5\%$) when utilizing a constant $G$ for calibration purposes. Our study will serve as a useful addition to the results provided by Jiang et al. (2020). Detailed measurement of $G$ for the 19 beams of FAST, covering the frequency range 1000 MHz -- 1500 MHz can be found on the HIFAST homepage: https://hifast.readthedocs.io/fluxgain.