We have detected pulsed X-ray emission from the fastest millisecond pulsar known , PSR B1937 + 21 ( P = 1.558 msec ) , with ASCA .
The pulsar is detected as a point source above \sim 1.7 keV , with no indication of nebulosity .
The source flux in the 2–10 keV band is found to be f = ( 3.7 \pm 0.6 ) \times 10 ^ { -13 } erg s ^ { -1 } cm ^ { -2 } , which implies an isotropic luminosity of L _ { x } = 4 \pi D ^ { 2 } f \sim ( 5.7 \pm 1.0 ) \times 10 ^ { 32 } ~ { } ( D / 3.6 { kpc } ) ^ { 2 } erg s ^ { -1 } , where D is the distance , and an X-ray efficiency of \sim 5 \times 10 ^ { -4 } relative to the spin-down power of the pulsar .

The pulsation is found at the period predicted by the radio ephemeris with a very narrow primary peak , the width of which is about 1/16 phase ( \sim 100 \mu s ) , near the time resolution limit ( 61 \mu s ) of the observation .
The instantaneous flux in the primary peak ( 1/16 phase interval ) is found to be ( 4.0 \pm 0.8 ) \times 10 ^ { -12 } erg 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 .