In-flight Timing Calibration of the Follow-up X-Ray Telescope on Board the Einstein Probe

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Abstract After the launch of the Einstein Probe, pulsars are observed to calibrate the timing system of the onboard Follow-up X-ray Telescope (FXT) by measuring the times of arrival (TOAs). The FXT has two independently operating modules, named FXTA and FXTB, each equipped with Wolter-I optics and a pnCCD camera. For scientific observations, each pnCCD has three distinct readout modes that differ in frame cycles, the full-frame (FF) mode (50 ms frame –1 ), the partial-window (PW) mode (2.2 ms frame –1 ), and the timing mode (TM; 23.68 μ s row –1 , 9.4352 ms frame –1 ). Thus, 1RXS J170849.0–400910 with a spin period of 11 s is observed in FF mode, PSR B1509–58 (150 ms) in PW mode, and the Crab pulsar (33 ms) in TM as well as PSR B1821–24 (3.05 ms) and PSR B1937+21 (1.56 ms). The TOAs of these pulsars measured by FXTA and FXTB are compared with the results of the Neutron Star Interior Composition Explorer (NICER), and for the Crab pulsar with the Jodrell Bank monthly ephemeris (JBE). The results verify that the timing systems of FXTA and FXTB in the FF, PW, and TM modes are consistent with each other, and are in good alignment with NICER. The measured advance of the Crab pulsar’s main peak relative to JBE is 35 7 ± 54 ( sys . ) ± 14 ( stat . ) μ s in FXTA and 394 ± 67 ( sys . ) ± 14 ( stat . ) μ s in FXTB. Based on the measurements of PSR B1821–24 and PSR B1937+21, the preliminary absolute timing accuracy is estimated to be 6 μ s for FXTA and 31 μ s for FXTB in TM. Additionally, the systematic delay of FXTA timing results relative to FXTB is found to be 20 ± 21 ( sys . ) ± 19 ( stat . ) μ s.

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