Robust Absolute Solar Flux Density Calibration for the Murchison Widefield Array

Abstract

Sensitive radio instruments are optimized for observing faint astronomical sources, and usually need to attenuate the received signal when observing the Sun. There are only a handful of flux density calibrators that can comfortably be observed with the same attenuation setup as the Sun. Additionally, for wide field-of-view (FoV) instruments like the Murchison Widefield Array (MWA) calibrator observations are generally done when the Sun is below the horizon, to avoid the contamination from solar emissions. These considerations imply that the usual radio interferometric approach to flux density calibration is not applicable for solar imaging. A novel technique, relying on a good sky model and detailed characterization of the MWA hardware, was developed for solar flux density calibration for MWA. Though successful, this technique is not general enough to be extended to the data from the extended configuration of the MWA Phase II. Here, we present a robust flux density calibration method for solar observations with MWA independent of the array configuration. We use different approaches—the serendipitous presence of strong sources; detection of numerous background sources using high dynamic range images in the FoV along with the Sun; and observations of strong flux density calibrators with and without the additional attenuation used for solar observations—to obtain the flux scaling parameters required for the flux density calibration. Using the present method, we have achieved an absolute flux density uncertainty ∼10% for solar observations even in the absence of dedicated calibrator observations.

Tackling the Unique Challenges of Low-frequency Solar Polarimetry with the Square Kilometre Array Low Precursor: The Algorithm

“…We find that inter-picket bandpass phases for MWA can be modelled well by a straight line (Sokolowski et al 2020). Inter-picket bandpass amplitudes show a more complicated variation and are corrected using an independent method described by Kansabanik et al (2022).…”

“…Antennas at larger distances are progressively added. The process of intensity self-calibration with increasing number of distant antennas is similar to that implemented in AIRCARS (Mondal et al 2019) success has already been demonstrated by many studies relying on high dynamic range imaging (e.g, Mohan et al 2019b,a;Mondal et al 2020b,a;Mohan 2021a,b;Kansabanik et al 2022).…”

“…While Mondal et al (2019) have used self-calibration based methods with remarkable success to obtain high dynamic range solar Stokes I images, their algorithm does not include polarimetric imaging. It also does not include absolute flux density calibration, which needs to be done independently (Kansabanik et al 2022). Mc-Cauley et al (2019) have demonstrated polarimetric solar radio imaging with the MWA.…”

“…We obtain the initial flux density calibrated dynamic spectrum using the non-imaging technique mentioned earlier, which is independent of instrumental gains and is computationally much faster. Though precise flux density calibration of MWA solar observations can be done using the method described in Kansabanik et al (2022), it is only applicable to bandpass calibrated visibilities or images, which are not yet available at this stage in the calibration process. Bandpass self-calibration is performed for narrow bandwidths of 1.28 MHz at a time, referred to as a 'picket'.…”

“…Next a 2D polynomial of second order was fit to determine the leakages as a function of direction. It has recently been shown that numerous background sources can be detected in Stokes I with the MWA even in presence of the Sun (Kansabanik et al 2022). However, to pursue a similar approach for determining leakages, a large number of these sources need to be detected in other Stokes parameters as well, which is yet to be demonstrated.…”