Microsecond timing of PSR B1821-24 with Chandra high resolution camera-S

We perform absolute timing of PSR B1821-24 in M28, using a 50 ks observation with Chandra/HRC-S. We have obtained the highest signal-to-noise ratio X-ray pulsed light curve of this source to date, detecting two X-ray pulses, as well as significant nonpulsed emission: a persistent X-ray flux that comprises 15% ± 3% of the total X-ray flux of the pulsar. The Gaussian width of the sharp X-ray peak is 34 ± 3 μs in time, implying a size of the X-ray beam as it crosses the line of sight of 4°.0 ± 0°.4. We find evidence for a significant trailing component in both X-ray peaks of the pulse profile. Including three RXTE/PCA observations in our analysis and tying the phases together using a radio ephemeris obtained at Nançay, we find that the absolute phases in the X-ray wander with respect to this radio ephemeris by up to 60 μs, likely as a result of the variable dispersion measure, which changes the pulse arrival time in the radio band but not the X-ray band. The present analysis makes clear that pulsar timing noise properties in millisecond pulsars such as PSR B1821-24 - hitherto only studied at radio wavelengths, where variable dispersion measure requires a significant correction - can be studied at X-ray wavelengths, where the effect of variable dispersion measure is negligible. We also examine the known uncertainties in the absolute Chandra/HRC-S timing accuracy, which amount to ±12 μs. We limit the amount of linear drift in the relative timing accuracy of HRC-S to less than 3 × 10^-10 s s^-1.