An X-Ray Pulsar Timing Observatory Testing Whether Photon Travel Times Follow the Stated Transit Relation on Interplanetary Paths

This paper presents a space mission design, computed from stated assumptions and not measured, built to measure one quantity: the systematic timing residual predicted by the transit relation over a one-year baseline, for which the design case adopts 145 nanoseconds. The instrument is a free-flying observatory carrying a 2-square-metre silicon-drift X-ray detector array with sub-microsecond time tagging referenced to an onboard atomic clock, observing a fixed list of twelve millisecond pulsars as its host spacecraft moves around the Sun. It operates from a heliocentric Earth-trailing orbit, so the observatory samples photon paths from many solar elongations for 4 years. The one-day measurement uncertainty is 900 nanoseconds; averaged over the full mission of 1461 days it falls to 23.5 nanoseconds, giving a signal-to-noise ratio of 6.16 for the design case and a five-sigma detection limit of 118 nanoseconds. The spacecraft has a dry mass of 820 kilograms, a wet mass of 990 kilograms including propellant, and a total electrical load of 1210 watts supplied by a solar array of 3.49 square metres at the operating distance. Every performance figure in this paper comes from the deposited computation; the design is simulated, not measured, and no hardware described here has been built or flown.