X-ray telescopes such as @NASANuSTAR record the time an X-ray photon hits the detector, its position on the detector, and its energy. The time reference (“clock”) that records this time needs to be precise. The spacecraft quartz oscillator that provides this time reference, however, drifts by a few milliseconds every day. It doesn’t seem like a lot, but this effect alone was able to heavily limit the satellite’s scientific impact. One of the things NuSTAR couldn’t do was finding fast pulsars. These stars are cosmic lighthouses, emitting a beam of radiation that sweeps across the sky as the star rotates. The fastest pulsars make one rotation in slightly more than a millisecond. The only way to discover some of these fast pulsars is running a periodicity search on ~day-long observations. This cannot work if the “clock” we use as a reference is drifting by milliseconds during a given observation.

In this paper, we demonstrate that the spacecraft’s reference oscillator has a predictably temperature-dependent frequency. In practice, we find a very precise model to describe the clock drift. We refine the model further, by comparing the arrival time of pulsar impulses measured by NuSTAR and other satellites. We find a couple of other effects, such as the oscillator’s aging and other unmodeled electronic delays.

NuSTAR’s Science Operations Center now uses this model to produce clock correction files. NuSTAR is now an excellent pulsar finder. Enjoy!

The paper was published in The Astrophysical Journal and can be downloaded here