ROSSI X-RAY TIMING EXPLORER ABSOLUTE TIMING RESULTS FOR THE PULSARS B1821-24 AND B1509-58

Observations with the Rossi X-Ray Timing Explorer and the Jodrell Bank , Parkes, and Green Bank telescopes have enabled us to determine the t ime delay between radio and X-ray pulses in the two isolated pulsars B 1821-24 and B1509-58. For the former we find that the narrow X-ray and radio pulse components are close to being coincident in time, with th e radio peak leading by 0.02 period (60 +/- 20 mu s), while the wide X -ray pulse component lags the last of the two wider radio components b y about 0.08 period. For the latter pulsar we find, using the standard value for the dispersion measure, that the X-ray pulse lags the radio by about 0.27 period, with no evidence for any energy dependence in t he range 2-100 keV. However, uncertainties in the history of the dispe rsion measure for this pulsar make a comparison to previous results di fficult. It is clear that there are no perceptible variations in eithe r the lag or the dispersion measure at timescales of a year or less.