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

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 × 384pixels. 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 μs and47.1 ± 2.8 μs, 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 μs for FXT-B.