Timing Calibration of the Follow-up X-Ray Telescope On Board the Einstein Probe Satellite

Abstract The Follow-up X-ray Telescope (FXT) is one of the main scientific instruments on board the Einstein Probe astronomical satellite, which was launched in 2024 January. FXT consists of two Wolter I type nested telescopes (FXT-A and FXT-B) with a focal length of 1600 mm. The focal plane detector employs a PNCCD with 384 × 384 pixels. The timing mode of FXT serves as the primary operating mode for fast X-ray timing observations. To evaluate and validate the timing performance of FXT prior to launch, a comprehensive timing calibration was performed at the 100 m X-ray test facility. By simulating various periodic Crab-like profiles using the Grid Controlled X-ray Tube (GCXT) in conjunction with a pulsar simulation module, it was verified that the relative time accuracy of FXT exceeds 5 × 10 −9 . Furthermore, employing GCXT with a voltage pulse generation module enabled the determination of the time resolutions for FXT-A and FXT-B, recorded as 45.6 ± 2.7 μ s and 47.1 ± 2.8 μ s, respectively. An absolute timing calibration for FXT-B was carried out using the GCXT and a time interval analyzer, revealing a measured time delay of 3.9 ± 2.1 μ s for FXT-B.