Wang et al. present the first results from a new ASKAP backend, the CRAFT (Commensal Realtime ASKAP Fast Transient) COherent (CRACO) upgrade. CRACO records millisecond time resolution visibility data, and searches for dispersed fast transient signals including fast radio bursts (FRB), pulsars, and ultra-long period objects (ULPO). With the visibility data, CRACO can localise the transient events to arcsecond-level precision after the detection. that have been removed due to the presence of radio frequency interference.

In many observed fields, there are false positive candidates that are due to a phenomenon called aliasing. These are an artefact of the imaging process and arise when the image is smaller than the FoV of the primary beam. In these conditions, sources outside the imaging region may appear in the image. In the CRACO Pilot Survey, a field of view (FoV) of 1.1 degrees was used to make synthesised images. This is smaller than the primary beam of the antenna which, for an observation centred at 943 MHz, is ~1.8 degrees in diameter. Importantly, real transients outside the imaging region may be detected as alias candidates albeit with a lower SNR, which can increase the effective search FoV. As the visibility data is stored on disc, it is possible to recover the correct positions after the post-processing stage.

The image above is an example of a candidate detection due to aliasing. The top panel shows the detection image of the candidate produced by the pipeline, with a FoV of 1.1 degrees, and image dimensions of 256 x 256 pixels. The bottom panel also shows the detection image, but with the image size doubled to 512 x 512 pixels to yield a 2.2 degree FoV. The detections are marked with cyan bars. The yellow dashed rectangle in the lower panel shows the region covered in the top panel. This illustrates how a candidate can arise from a source whose true sky position lies outside the imaged region in the upper panel, but appears in it due to aliasing. As the team is primarily interested in new sources they have implemented an alias source filter to automatically remove aliases from known sources.