Pulsed X‐Ray Emission from the Fastest Millisecond Pulsar: PSR B1937+21 withASCA

Abstract

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) × 10-13 ergs s-1 cm-2, which implies an isotropic luminosity of LX = 4πD2f ~ (5.7 ± 1.0) × 1032(D/3.6 kpc)2 ergs s-1, where D is the distance, and an X-ray efficiency of ~5 × 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 (~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 ± 0.8) × 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.