A Search for Time-Variable, Low-Frequency Sources with HERA

The Hydrogen Epoch of Reionization Array (HERA) is a dedicated experiment to measure the power spectrum of the highly redshifted 21 cm emission line of hydrogen. It is a low-frequency radio interferometer, currently under construction in South Africa on the site of the future SKA. When complete, HERA will comprise 350 14-meter antennas operating from 50 - 225 MHz. The data here is taken from the commissioning data from a 19-element hexagonal close-packed subarray operating between 110 and 190 MHz (as described in Kohn et al 2019) over seven nights. This investigation focused on searching for time-variable, low-frequency sources using this well-characterized data. Finding time variable radio sources is not the main goal of HERA, but it is made possible since HERA observes the same patch of sky each night for timescales as long as weeks. Previous searches in the radio frequency may have missed variable sources due to a limited time range. For observing time variability accurately, daily datasets that are perfectly aligned in time and have the same uv-plane coverage are required, so that when images are differenced, the only thing remaining is what has changed night to night. This mostly involved matching bad or flagged data between data sets and ensuring alignment in sampling time was the same night to night. Common examples of flagged data are data resulting from radio frequency interference (RFI) produced by manmade sources or broken antennas on HERA. There are several ways to match flagged data between days which vary the amount of potentially useful data that is thrown out. We applied a logical or method of flagging to all the data sets. Alignment was fixed through interpolation of the data, allowing data points to exist at any given sample time. Thus through rigorous calibration and data preparation, we can hope to achieve noise-limited comparisons between nights. We compare the point spread functions of the data sets between nights to determine the accuracy of the calibration techniques explored in this research project and examine difference images to potentially reveal new detections of time variable sources in the radio frequency.