We report on the timing properties of the ‘Crab twin’ pulsar PSR B0540−69 measured with X-ray data taken with the Swift telescope over a period of 1100 days. The braking index of the pulsar was estimated to be n = 0.03±0.013 in a previous study performed in 2015 with 500-day Swift data. This small value of n is unusual for pulsars, and a comparison with an old estimate of n ≈ 2.1 for the same target determined ∼10 years earlier suggests a dramatic change in the braking index. To confirm the small value and therefore the large change of n, we used 1100-day Swift observations including the data used in the earlier determination of n = 0.03. In this study we find that the braking index of PSR B0540−69 is n = 0.163 ± 0.001, somewhat larger than 0.03. Since the measured value of n is still much smaller than 2.1, we can confirm the dramatic change in the braking index for this pulsar.

Abstract
1. INTRODUCTION
2. OBSERVATIONS AND DATA REDUCTION
3. TIMING ANALYSIS
    3.1. Direct Frequency Measurements
    3.2. Phase Semi-CoherentMethod
    3.3. Timing Noise
4. DISCUSS
REFERENCES

• Minjun Kim(Department of Astronomy and Space Science, Chungbuk National University)
• Hongjun An(Department of Astronomy and Space Science, Chungbuk National University) Corresponding author