Microsecond Timing of PSR B1821−24 withChandraHigh Resolution Camera–S

We perform absolute timing of PSR B1821-24 in M28, using a 50 ksecobservation with Chandra/HRC-S. We have obtained the highest signal-to-noiseX-ray pulsed lightcurve of this source to date, detecting two X-ray pulses, aswell as significant non-pulsed emission -- a persistent X-ray flux whichcomprises 15+/-3% of the total X-ray flux of the pulsar. The Gaussian width ofthe sharp X-ray peak is 34+/-3 micro-sec in time, implying a size of the X-raybeam as it crosses the line of sight of 4.0+/-0.4 deg. We find evidence for asignificant trailing component in both X-ray peaks of the pulse profile.Including three RXTE/PCA observations in our analysis, and tying the phasestogether using a radio ephemeris obtained at Nancay, we find the absolutephases in the X-ray wander with respect to this radio ephemeris by up to 60micro-sec, likely due to the variable dispersion measure, which changes thepulse arrival time in the radio band but not the X-ray band. The presentanalysis makes clear that study of pulsar timing noise properties inmillisecond pulsars such as PSR B1821-24 -- hitherto only studied at radiowavelengths, where variable dispersion measure requires a significantcorrection -- can be studied at X-ray wavelengths, where the effect of variabledispersion measure is negligible. We also examine the known uncertainties inthe absolute Chandra/HRC-S timing accuracy, which amount to +/-12 microsec. Welimit the amount of linear drift in the relative timing accuracy of HRC-S to<3e-10 s s-1.Comment: ApJ, accepted; minor changes from original versio