Detection of Pulsed X-ray Emission from The Fastest Millisecond Pulsar PSR B1937+21 with ASCA

Abstract

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 $L_{\mathrm{x}}=4\pi D^{2}f\sim (5.7\pm 1.0)\times 10^{32}(D/3.6\mathrm{kpc}{)}^2$ 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.

14 pages with 5 figures. Ap. J. in press