An Observing Strategy to Overcome Challenges in Observing the Sun with MeerKAT
MeerKAT is an exceptionally sensitive radio telescope, built to observe faint astronomical sources. However, at GHz frequencies, the Sun is the brightest source in the sky with a flux density a few orders of magnitude higher than the brightest astronomical source in the sky. Hence the signal must be attenuated during Solar observations to ensure smooth operations of the instrument. To handle this problem, a novel observing approach was taken in this study. These observations were done in MeerKAT L-band. Instead of pointing directly at the Sun, the Sun was kept ∼2°.5 away from the pointing center. This allows for attenuation of the solar signal sufficient to observe the Sun with MeerKAT. However, this alignment causes the instrumental effects to vary in a non-trivial manner, and hence special care is required to calibrate and image this data.
High-fidelity Spectroscopic Snapshot Solar Imaging with MeerKAT: The First Demonstration
Radio interferometric imaging is a Fourier imaging technique, hence the fidelity and sensitivity of radio interferometric images for emissions at different angular scales depend on the sampling density in the Fourier plane, commonly known as the UV-plane. The MeerKAT array is centrally condensed with 39 dishes lying within 1 km and the remaining dishes distributed within a radius of ∼8 km. This provides MeerKAT with extremely good surface brightness sensitivity and also allows the generation of radio images with an extremely high dynamic range (DR) and image fidelity. To calibrate the solar observations taken in the sidelobes of the primary beam of MeerKAT, we need special care to account for the chromaticity of the MeerKAT primary beam as well as the spectral variability of the solar emission. After applying the basic calibration, obtained from the standard astronomical calibrators, we performed self-calibration towards the solar center in 20 MHz spectral chunks. Once self-calibration converges, each 20 MHz spectroscopic image is corrected for primary beam response to obtain the final flux density calibrated images.
An example of spectroscopic images on 26th September 2020 is shown in the bottom panel of Figure 1. At a single spectral slice, the sensitivity also varies across the solar disk due to variations in the primary beam gain across the solar disk, which is evident from the spectroscopic images shown in Figure 2. The entire solar disk is visible once images over the full band are stacked together. We find that the solar disk is detected at ∼50σ signal-to-noise ratio (SNR), where σ is the rms noise close to the Sun. This wide-band stacked image is shown in the top panel of Figure 2 and used for a qualitative assessment of the fidelity of the solar features seen across the observing band with the simulated images.
Figure 1: Spectroscopic images of the Sun on 2020 September 26, 09:07 UTC. Top panel: normalized average image over the entire MeerKAT L band. Bottom panels: four sample images at different 20 MHz spectral chunks across the observing band.
Qualitative and Quantitative Comparison with Simulation to Demonstrate the Fidelity of the Images
To understand the fidelity of the features detected in MeerKAT image, it is very important to compare the structures detected across the full spectral band with those seen in the simulated radio maps. The simulated images were generated only assuming thermal bremsstrahlung from multi-thermal plasma. The multi-thermal plasma was characterized by the differential emission measure distribution, which was obtained by inverting the images at different extreme ultraviolet wavelengths from the AIA/SDO using the DEM inversion method described by Hannah & Kontar 2013.
We generated synthetic MeerKAT radio images from the simulated radio maps at the same observing frequency at the same time of observation and compared them with the true MeerKAT images. One such example is shown in the top panels of Figure 2. The left panel shows the synthetic MeerKAT map and the right panel shows the observed map from MeerKAT. The qualitative similarities between the synthetic and true observed images are very evident. The most striking similarities are the locations and relative intensities of the various bright points; some of them have been marked by cyan circles in both panels of the same figure. This qualitative analysis demonstrates fidelity in detecting even the small and faint features in the solar images.
Figure 2: Top panels: Comparison between synthetic and observed MeerKAT radio images on 2020 September 27, 10:45 UTC. The left panel shows a synthetic MeerKAT solar radio image and the right panel shows the observed MeerKAT solar radio image. In both images, multiple bright regions are detected. Some of them are marked by cyan circles. Bottom panels: Comparison of observed spectra with synthetic spectra. The left panel shows spectra for two sample regions. Solid lines represent the simulated spectra considering only thermal emission. Unfilled diamonds represent spectra from synthetic MeerKAT maps obtained from the simulation. Filled circles represent the measured spectra from the true MeerKAT observation. The right panel shows the missing flux fraction is shown as a function of frequency.
For a quantitative understanding, we compared true observed spectra (unfilled diamonds) for two sample regions with both the simulated spectra (solid lines) and synthetic spectra (filled circles) in the bottom left panel of Figure 2. We notice that while there are large discrepancies between simulated and observed spectra, synthetic and observed spectra show excellent similarities within the uncertainties. This demonstrates that the large discrepancy between the observed and simulated spectra is primarily due to the missing flux density in MeerKAT solar maps. The missing flux density fraction is plotted in the bottom right panel of Figure 2, which decreases with the decrease in observing frequency. This missing flux fraction can potentially be estimated by a comparison between simulated and synthetic maps and corrected in true observed spectra.
Conclusion and Future Work
Given the well-behaved spectroscopic snapshot PSF and the precise calibration, the first solar spectroscopic images using MeerKAT presented here are of the highest quality at these frequencies available to date. The quality of these images is demonstrated both qualitatively and quantitatively by comparison with realistic simulations. While this is adequate for demonstrating the feasibility of MeerKAT for solar observations and evaluating the quality of the images it can deliver, the preferable approach for solar observing will be to keep the Sun in the main lobe of the primary beam and attenuate the signal using electronics to the required levels. Some members of this team are currsently working with the MeerKAT engineering team to identify suitable attenuation for solar observations and to develop a calibration strategy for solar observations performed along these lines. Once enabled, we are convinced that, with its high-quality spectroscopic snapshot solar imaging capability, MeerKAT solar observations will open a new frontier in solar radio physics.
Based on a recent paper by Kansabanik, D., et al., 2024 ApJ 961 96, https://doi.org/10.3847/1538-4357/ad0b7f
References
- Jonas, J. & MeerKAT Team 2016, in Proc. of Science 277, MeerKAT Science: On the Pathway to the SKA, ed. R. Taylor (Trieste: SISSA), 1
- Chen, W., Barr, E., Karuppusamy, R., Kramer, M., & Stappers, B. 2021, JAI, 10, 2150013
- Santander-Vela, J., Bartolini, M., Miccolis, M., & Rees, N. 2021, arXiv:2110. 13329
- Fox, N. J. 2017, AGUFM, 2017, SH 21C–02
- Hannah, I. G., & Kontar, E. P. 2013, A&A, 553, A10
