Timing Calibration of the NuSTAR X-ray Telescope

TL;DR

This paper details the timing calibration process for the NuSTAR X-ray telescope, achieving microsecond-level timestamp accuracy crucial for studying fast astrophysical phenomena.

Contribution

It introduces a method to correct temperature-dependent oscillator frequency response and characterizes spacecraft clock offsets for precise timing calibration.

Findings

Achieved ~65 microsecond timestamp accuracy.

Developed a temperature correction method for the oscillator.

Characterized spacecraft clock behavior during downlinks.

Abstract

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission is the first focusing X-ray telescope in the hard X-ray (3-79 keV) band. Among the phenomena that can be studied in this energy band, some require high time resolution and stability: rotation-powered and accreting millisecond pulsars, fast variability from black holes and neutron stars, X-ray bursts, and more. Moreover, a good alignment of the timestamps of X-ray photons to UTC is key for multi-instrument studies of fast astrophysical processes. In this Paper, we describe the timing calibration of the NuSTAR mission. In particular, we present a method to correct the temperature-dependent frequency response of the on-board temperature-compensated crystal oscillator. Together with measurements of the spacecraft clock offsets obtained during downlinks passes, this allows a precise characterization of the behavior of the oscillator. The calibrated NuSTAR event timestamps for a typical observation are shown to be accurate to a precision of ~65 microsec.