Sky models have been used in the past to calibrate individual low radio frequency telescopes. Here we generalize this approach from a single antenna to a two element interferometer and formulate the problem in a manner to allow us to estimate the flux density of the Sun using the normalized cross-correlations (visibilities) measured on a low resolution interferometric baseline. For wide fieldof-view instruments, typically the case at low radio frequencies, this approach can provide robust absolute solar flux calibration for well characterized antennas and receiver systems. It can provide a reliable and computationally lean method for extracting parameters of physical interest using a small fraction of the voluminous interferometric data, which can be prohibitingly compute intensive to calibrate and image using conventional approaches. We demonstrate this technique by applying it to data from the Murchison Widefield Array and assess its reliability.

We present the first detection of OH absorption in diffuse gas at z > 0, along with another eight stringent limits on OH column densities for cold atomic gas in galaxies at 0 < z < 0.4. The absorbing gas detected towards Q0248+430 (z q =1.313) originates from a tidal tail emanating from a highly star forming galaxy G0248+430 (z g =0.0519) at an impact parameter of 15 kpc. The measured column density isand T ex are the covering factor and the excitation temperature of the absorbing gas, respectively. In our Galaxy, the column densities of OH in diffuse clouds are of the order of N (OH)∼10 13−14 cm −2 . From the incidence (number per unit redshift; n 21 ) of H I 21-cm absorbers at 0.5 < z < 1 and assuming no redshift evolution, we estimate the incidence of OH absorbers (with log N (OH)>13.6) to be n OH = 0.008 +0.018 −0.008 at z ∼ 0.1. Based on this we expect to detect 10 +20 −10 such OH absorbers from the MeerKAT Absorption Line Survey. Using H I 21-cm and OH 1667 MHz absorption lines detected towards Q0248+430, we estimate (∆F/F ) = (5.2±4.5)×10 −6 , where F ≡ g p (α 2 /µ) 1.57 , α − the fine structure constant, µ − the electron-proton mass ratio and g p − the proton gyromagnetic ratio. This corresponds to ∆α/α(z = 0.0519) = (1.7 ± 1.4)×10 −6 , which is among the stringent constraints on the fractional variation of α.

Low radio frequency solar observations using the Murchison Widefield Array have recently revealed the presence of numerous weakshort-livednarrowband emission features, even during moderately quiet solar conditions. These nonthermal features occur at rates of many thousands per hour in the 30.72 MHz observing bandwidth, and hencenecessarily require an automated approach for their detection and characterization. Here, we employ continuous wavelet transform using a mother Ricker wavelet for feature detection from the dynamic spectrum. We establish the efficacy of this approach and present the first statistically robust characterization of the properties of these features. In particular, we examine distributions of their peak flux densities, spectral spans, temporal spans, and peak frequencies. We can reliably detect features weaker than 1 SFU, making them, to the best of our knowledge, the weakest bursts reported in literature. The distribution of their peak flux densities follows a power law with an index of −2.23 in the 12-155 SFU range, implying that they can provide an energetically significant contribution to coronal and chromospheric heating. These features typically last for 1-2 s and possess bandwidths of about 4-5 MHz. Their occurrence rate remains fairly flat in the 140-210 MHz frequency range. At the time resolution of the data, they appear as stationary bursts, exhibiting no perceptible frequency drift. These features also appear to ride on a broadband background continuum, hinting at the likelihood of them being weak type-I bursts.

The recent availability of fine grained high sensitivity data from the new generation low radio frequency instruments such as the Murchison Widefield Array (MWA) have opened up opportunities for using novel techniques for characterizing the nature of solar emission at these frequencies.Here we use this opportunity to look for evidence for the presence of weak non-thermal emissions in the 100-240 MHz band, at levels weaker than have usually been probed. The presence of such features is believed to be a necessary consequence of nanoflare-based coronal and chromospheric heating theories. We separate the calibrated MWA solar dynamic spectra into a slowly varying and an impulsive, and hence non-thermal, component. We demonstrate that Gaussian mixtures modeling can be used to robustly model the latter and we estimate the flux density distribution as well as the prevalence of impulsive non-thermal emission in the frequency-time plane. Evidence for presence of non-thermal emission at levels down to ∼0.2 SFU (1 SFU = 10 4 Jy) is reported, making them the weakest reported emissions of this nature. Our work shows the fractional occupancy of the non-thermal impulsive emission to lie in the 17-45% range during a period of medium solar activity. We also find that the flux density radiated in the impulsive non-thermal emission is very similar in strength to that of the slowly varying component, dominated by the thermal bremsstrahlung. Such significant prevalence and strength of the weak impulsive non-thermal emission has not been appreciated earlier.