Probable optical counterpart of the eclipsing millisecond pulsar system, 1957 + 20

Abstract

Following the discovery of an eclipsing, millisecond pulsar 1957+20 (ref. 1) we conducted a series of optical observations to identify the optical counterpart of the system. We report the detection of a probably variable object (hereafter star X) with a mean apparent V magnitude of ∼20 within the astrometric error circle of the equatorial radio position of the pulsar. Star X appears fainter at orbital phase 0.21, the onset of the pulsar eclipse, compared with the quadrature position, suggesting that it is probably the optical counterpart. We argue that the secondary heated by the pulsar wind and not the pulsar itself makes the dominant contribution to the visible radiation from this system. The absolute magnitude of star X is M_v ≈ 10.5, assuming a distance of ∼ 0.8 kpc derived from the dispersion measure. The optical luminosity of the counterpart directly measures the fraction of the pulsar wind power responsible for heating the secondary. Future optical observations may be able to constrain the nature of the pulsar wind.