Abstract

The Amsterdam-ASTRON Radio Transient Facility And Analysis Center (AARTFAAC) All-Sky monitor (ASM) will be one of the first and most sensitive image domain transient detection instruments at meter wavelengths. Based on LOFAR, it will operate 24x7 to carry out real time detection of short (~seconds) and bright low frequency radio transients, and will trigger low latency followup at high sensitivity with LOFAR. However, the direction dependent calibration required for imaging its all-sky field of view and addressing refractive eects due to the ionosphere within a hard latency budget pose a significant calibration challenge.Here, we present our real-time wide-field calibration and imaging strategy, and discuss its performance under real observing conditions for fast transient detection. Our approach utilizes a highly efficient O(N²), model sky based multisource self calibration algorithm, with temporal tracking of solutions. In addition, a high resolution Direction of Arrival algorithm estimates model source position offsets due to the ionosphere, thereby achieving confusion noise limited imaging in real-time. Further, we utilize the difference image domain for transient detection, which has a higher sensitivity due to the cancellation of confusion noise. This allows us to generate close to thermal noise limited snapshot images in real-time.