Optimal tiling of SPOTLIGHT field-of-view with multi-beam synthesis

Abstract

Fast Radio Bursts (FRBs) are a source of a very short-duration transient signal of an order of milliseconds, the highly energetic cosmological event, origin of which is not fully understood yet and that has become the center of attention in astronomy recently. SPOTLIGHT is a real-time multi-beam commensal system that will run in parallel with the regular GMRT observations and will search FRBs and pulsars in the field of view (FoV). For real-time search of FRBs, a state-of-the-art parallel multi-node computer cluster is utilized to form visibilities, post-correlation beam-formed high-time resolution time data, and machine learning algorithms to classify the event of interest from the unwanted noise (e.g., radio frequency interference).

The survey speed for the FRBs can be increased by meeting two requirements viz. 1. Increasing FoV : One way to acheive this is by forming large number of beams. 2) Precise localisation: While many FRBs are detected in the time domain, a very small fraction of FRBs have been localized, and hence, new techniques for localization are in high demand across the world. SPOTLIGHT is targeted to search FRBs in the observation field by splitting it into about 2000 post-correlated beams steered across FoV. Digital technology allows the formation of thousands of such synthetic beams tiled in a given region in the sky. Array configuration, i.e. antennas included, frequency of observation and the source directions (declination of source, hour angle of observation) with respect to the array, determines the post correlated beam size and its orientation. With the earth's rotation, the configuration of the array changes systematically over observation time and for different sources in the sky (e.g., calibrators, target field) for a given observing session. So, dynamic and efficient computing of post correlated beams and tiling thousands of such beams becomes crucial in source localisation.

Multi-beam simulation package is developed which models the post correlated beam for a given observational setup and optimally arranges the beams in a given region of sky. It supports three modes : 1. Sky survey mode : Covering maximum FoV (with moderate length of baseline ) 2. Targeted mode - Covers given FoV (for specific search e.g. globular cluster observation, size few arc-min). 3. Multi-source mode : Observes different sources in given FoV.

In Sky Survey mode beams are tiled such that it covers the maximum FoV within primary beamwidth. Selects the optimum overlap ratio of the beam to give maximum coverage for the given observing session. However in Targeted mode, the number of beams and the overlap ratio is calculated based on the given FoV. Multi-Source mode is extended version of targeted mode, where multiple sources within given FoV can be observed.

We carried out functionality checks for modeling technique for post correlation beam shape and verification of tilling arrangement with variety of antenna configuration, source directions, and observing frequencies. We tested simulation of post correlation beam pattern by observing a field with 800 steered beams with source away from the phase center and compared the folded SNR map (fig. 1) for these beams with simulated beam patterns (fig. 2). The results showed that the source observed was located well within the accuracy of 1HPBW.

The simulation package simulates the post correlation beam shape and optimally tiles the beams such that it effectively covers a larger fraction of the field of view while keeping the sensitivity of detection fairly high so as not to miss any faint FRB event occurring across the field of view.