Pulsed X-ray emission from the fastest millisecond pulsar:: PSR B1937+21 with ASCA

We have detected pulsed X-ray emission from the fastest millisecond pulsar known, PSR B1937+21 (P = 1.558 ms), with ASCA. The pulsar is detected as a point source above similar to 1.7 keV, with no indication of nebulosity. The source flux in the 2-10 keV band is found to be f= (3.7 +/- 0.6) x 10(-13) ergs s(-1) cm(-2) which implies an isotropic luminosity of L-X = 4 piD(2)f similar to (5.7 +/- 1.0) x 10(32)(D3.6 kpc)(2) ergs s(-1), cm where D is the distance, and an X-ray efficiency of similar to5 x 10(-4) relative to the spin-down power of the pulsar. The pulsation is found at the period predicted by th radio ephemeris with a very narrow primary peak, the width of which is about 1/16 phase (similar to mus), near the time resolution limit (61 mus) of the observation. The instantaneous flux in the primary peak (1/16 phase interval) is found to be (4.0 +/- 0.8) x 10(-12) ergs s(-1) cm(-2). Although there is an indication for the secondary peak, we consider its statistical significance too low to claim a definite detection. The narrow pulse profile and the detection in the 2-10 keV band imply that the X-ray emission is caused by the magnetospheric particle acceleration. Comparison of X-ray and radio arrival times of pulses indicates, within the timing errors, that the X-ray pulse is coincident with the radio interpulse.