How ARTS Finds FRBs - I: Innovative Hierarchical Beamforming

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The Apertif Radio Transient System (ARTS) is a supercomputing instrument that performed real-time detection and localisation of Fast Radio Bursts on Apertif at Westerbork. In a series of images we highlight some of the cutting-edge algorithms and instrumentation that allowed it to become the most productive FRB search system in the world. The overview paper, including the description of the first 5 FRB discoveries is submitted to A&A and available on arxiv (van Leeuwen, Kooistra et al. 2022).

Today: An overview of the beam-forming hierarchy in ARTS. Wide-field transient surveys at both full (coherent-addition) sensitivity and high time resolution are almost always limited by the large computational demands involved. One unique feature of the WSRT is its linear and regularly-spaced, E-W layout. This produces full sensitivity Tied-Array Beams (TABs) that are not single, small circular beams, but have a large area in the N-S direction, and multiple sensitive sidelobes. Exploiting this fact allowed us to overcome the computing limitations of full-FoV surveys with e.g., LOFAR and SKA.

In the today's image (Fig. 4 in the ARTS overview paper), panel a) shows the 40 Compound Beams (CBs) formed by each Apertif PAF. The output signals of these CBs can be combined into Incoherent-Array Beams (IABs) or coherent TABs, whose grating responses fill the entire CB as shown in panel b). The frequency-dependence of these grating responses shown in panel c) can be exploited to disambiguate them by combining signals from multiple TABs to form a Synthesized Beam (SB) as illustrated in panel d). All ARTS FRBs were discovered and localised in these SBs. Finally, a Tracking Beam (TB) can optionally be formed towards a specific locus as shown in panel e).