PyLMT: A transient detection pipeline for the 4-m International Liquid Mirror Telescope

The International Liquid Mirror Telescope (ILMT) is a 4-m aperture, zenith-pointing telescope with a field-of-view of 22′ , situated in the foothills of the Himalayas. After achieving its first light in April 2022, it is in the preliminary testing and commissioning phase at the time of writing this paper. It uses the time-delay integration technique for imaging due to the fixed-pointing nature of the telescope. The telescope is being used in a continuous survey mode, and it has been surveying the same local sidereal time fields for several consecutive months. This unique feature of the ILMT makes it a useful instrument for time-domain astronomy, especially for detecting transients, variable stars, active galactic nuclei, and asteroids. In this paper, we present the PyLMT transient detection pipeline to detect such transient/varying sources in the ILMT images in near real-time. The pipeline essentially compares a pair of images from the same field using the image subtraction technique to find such sources in
the subtracted images using a convolutional neural network (CNN). It also houses a CNN-aided transient candidate classifier that classifies the transient/variable candidates based on host morphology. It also has the provision to identify catalogued asteroids and other solar system objects using public databases. Relevant CNNs have been trained on data acquired with the ILMT during the cycle of October-November 2022. Subsequent tests on those images have confirmed the detection of numerous catalogued asteroids, variable stars, and other uncatalogued sources. The pipeline has been operational and has detected 12 extragalactic
transients, including 2 new discoveries in the November 2023 - May 2024 observation cycle.