Caesar source finder: Recent developments and testing

Abstract

A new era in radioastronomy will begin with the upcoming large-scale surveys planned at the Australian Square Kilometre Array Pathfinder (ASKAP). ASKAP started its Early Science program in October 2017 and several target fields were observed during the array commissioning phase. The SCORPIO field was the first observed in the Galactic Plane in Band 1 (792-1032 MHz) using 15 commissioned antennas. The achieved sensitivity and large field of view already allow to discover new sources and survey thousands of existing ones with improved precision with respect to previous surveys. Data analysis is currently ongoing to deliver the first source catalogue. Given the increased scale of the data, source extraction and characterization, even in this Early Science phase, have to be carried out in a mostly automated way. This process presents significant challenges due to the presence of extended objects and diffuse emission close to the Galactic Plane. In this context we have extended and optimized a novel source finding tool, named CAESAR, to allow extraction of both compact and extended sources from radio maps. A number of developments have been done driven by the analysis of the SCORPIO map and in view of the future ASKAP Galactic Plane survey. The main goals are the improvement of algorithm performances and scalability as well as of software maintainability and usability within the radio community. In this paper we present the current status of CAESAR and report a first systematic characterization of its performance for both compact and extended sources using simulated maps. Future prospects are discussed in light of the obtained results.

Evolutionary map of the Universe (EMU): Compact radio sources in the scorpio field towards the galactic plane

“…Compact sources were extracted from the SCORPIO ASKAP map with the CAESAR source finder (Riggi et al 2016(Riggi et al , 2019) using an iterative flood-fill algorithm in which the detection threshold is initially set to 5σ and the flooding threshold to 2.5σ . At each iteration the background and noise maps are recomputed, excluding the sources found in the previous iteration and the detection threshold is lowered in steps of 0.5σ .…”

“…Two iterations were used in this work, corresponding to a final 4.5σ seed threshold level. Algorithm parameter values, reported in Table A1 (available in the online version of this article), resulted from a fine-tuning procedure carried out both on the simulated data sample described in Riggi et al (2019) and on the simulated maps described in Section 3.3.…”

“…False detections are largely due to sidelobes around bright sources (compact or extended) and overdeblending of extended sources. Reliability can be improved by ∼10 per cent after applying a neural network classifier previously trained to identify 'good' sources from the data (see Riggi et al 2019 for more details). Results are shown in Fig.…”

“…It is now known from simulation studies (Hopkins et al 2015;Riggi et al 2019) that many source finders produce a biased flux density measurement as they approach the detection threshold. CAESAR, like other widely used finders (e.g.…”

“…In the first paper (Umana et al, in preparation), we discuss the ASKAP's potential for the discovery of different classes of Galactic objects in comparison with original ATCA observations, and described the data reduction strategies adopted to produce the final mosaic. The goal of this paper is to report a first catalogue of the compact sources present in SCORPIO. This work will also serve as a validation on real data of the designed source extraction algorithms tested so far with simulated data (Riggi et al 2016(Riggi et al , 2019.…”