2001ApJ...554..316T

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 ∼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)x10^–13 ergs.s^–1.cm^–2, which implies an isotropic luminosity of L_X=4πD²f~(5.7±1.0)x10³²(D/3.6 kpc)² ergs.s^–1, where D is the distance, and an X-ray efficiency of ∼5x10^–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 (∼100 µs), near the time resolution limit (61 µs) of the observation. The instantaneous flux in the primary peak (1/16 phase interval) is found to be 4.0±0.8x10^–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.