One clock to rule them all
A new IACHEC-endorsed method unifies the absolute timing calibration of ~15 X-ray space missions using thousands of Crab pulsar observations.
Precision timing is the backbone of pulsar science in X-rays. But every satellite carries its own clock, with its own quirks — temperature drifts, aging oscillators, unmodelled electronic delays. We already tackled NuSTAR’s clock in a previous paper; this time we go broader.
Working within the International Astronomical Consortium for High Energy Calibration (IACHEC), we developed a flexible method for cross-calibrating the absolute timing of any X-ray space mission. As many other works in the past, we used the Crab pulsar as a clock reference. This is the brightest, most steadily timed source in the X-ray sky and has been observed thousands of times over decades.
In the paper, we use a novel method to perform this pulsar timing with X-ray data that have been pre-processed (“barycentered”) using different time scales and reference frames. Normally, this would imply systematic errors in the order of a few milliseconds, which is a lot for precise timing, but our method decreases these effects to a few micro seconds.
We used the method on a library of Crab observations from roughly fifteen missions, spanning 1996 to 2025. We verified that most missions complied with their own requirements, and found a number of issues that the mission teams are addressing.
To make this accessible, we are releasing TOAExtractor, an open-source tool that automates the whole pipeline.