A Southern-Hemisphere all-sky radio transient monitor for SKA-Low prototype stations

TL;DR

This paper introduces the first all-sky radio transient monitor for the southern hemisphere using SKA-Low prototype stations, capable of real-time detection of bright astrophysical and human-made radio transients at low frequencies.

Contribution

It presents a novel all-sky imaging and transient detection system implemented on SKA-Low prototypes, demonstrating real-time monitoring and transient identification at low frequencies.

Findings

Detected bright transients from PSR B0950+08, confirming system capability.

Established a transient surface density upper limit of 1.32e-9 deg^-2.

Proved the system's potential for detecting bright astrophysical transients.

Abstract

We present the first southern-hemisphere all-sky imager and radio-transient monitoring system implemented on two prototype stations of the low-frequency component of the Square Kilometre Array. Since its deployment the system has been used for real-time monitoring of the recorded commissioning data. Additionally, a transient searching algorithm has been executed on the resulting all-sky images. It uses a difference imaging technique, and has enabled identification of a wide variety of transient classes, ranging from human-made radio-frequency interference to genuine astrophysical events. Observations at the frequency 159.4 MHz and higher in a single coarse channel (0.926 MHz) were made with 2s time resolution, and multiple nights were analysed. Despite having modest sensitivity (~few Jy/beam), using a single coarse channel and 2-s imaging, the system detected bright transients from PSR B0950+08, proving that it can be used to detect bright transients of an astrophysical origin. The unusual, extreme activity of the pulsar PSR B0950+08 (up to ~155 Jy/beam) was initially detected in a "blind" search in the 2020-04-10/11 data and later assigned to this specific pulsar. The limitations of our data, however, prevent use from making firm conclusions of the effect being due to a combination of refractive and diffractive scintillation or intrinsic emission mechanisms. The system can routinely collect data over many days without interruptions; the large amount of recorded data at 159.4 and 229.7 MHz allowed us to determine a preliminary transient surface density upper limit of $1.32 \times 10^{-9} \text{deg}^{-2}$ for a timescale and limiting flux density of 2s and 42 Jy, respectively. We plan to extend the observing bandwidth to tens of MHz and improve time resolution to tens of milliseconds in order to increase the sensitivity and enable detections of Fast Radio Bursts below 300 MHz.