A clock stabilization system for CHIME/FRB Outriggers

TL;DR

This paper introduces a minimally invasive clock stabilization system for CHIME/FRB Outriggers, combining GPS and hydrogen maser clocks to improve timing accuracy for VLBI observations of fast radio bursts.

Contribution

It presents a novel clock stabilization method that effectively transfers the CHIME backend clock to a hydrogen maser, enhancing timing stability for VLBI FRB localization.

Findings

Achieved 30 ps rms timing agreement over 9 days

Validated system with VLBI observations of Cygnus A

Met stability requirements for Outriggers

Abstract

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has emerged as the prime telescope for detecting fast radio bursts (FRBs). CHIME/FRB Outriggers will be a dedicated very-long-baseline interferometry (VLBI) instrument consisting of outrigger telescopes at continental baselines working with CHIME and its specialized real-time transient-search backend (CHIME/FRB) to detect and localize FRBs with 50 mas precision. In this paper we present a minimally invasive clock stabilization system that effectively transfers the CHIME digital backend reference clock from its original GPS-disciplined ovenized crystal oscillator to a passive hydrogen maser. This enables us to combine the long-term stability and absolute time tagging of the GPS clock with the short and intermediate-term stability of the maser to reduce the clock timing errors between VLBI calibration observations. We validate the system with VLBI-style observations of Cygnus A over a 400 m baseline between CHIME and the CHIME Pathfinder, demonstrating agreement between sky-based and maser-based timing measurements at the 30 ps rms level on timescales ranging from one minute to up to nine days, and meeting the stability requirements for CHIME/FRB Outriggers. In addition, we present an alternate reference clock solution for outrigger stations which lack the infrastructure to support a passive hydrogen maser.