Abstract
The recently discovered millisecond pulsar PSR1957 + 20 is eclipsed for ∼50 min out of each 9.16-h orbit1; several authors suggested1–4 that spin-down radiation from the pulsar is evaporating material from the surface of the ˜0.02Mṁ companion. Here we report observations intended to find the optical counterpart of the pulsar and to study luminosity variations with orbital phase. We show that the candidate optical counterpart5 is actually two stars, one the true counterpart, the other an unrelated background star. The luminosity of the system varies by at least a factor of five, with a maximum of magnitude 20.3 when the pulsar hides the companion. The phase of the variations implies that the light comes mostly from the companion star, and suggests that it is tidally locked, with the bright side constantly illuminated by the pulsar. The companion has a low colour temperature, ∼5,500 K, which with its magnitude and the pulsar's dispersion-measure distance indicates a radius of 0.15R⊙, about the size of a 0.02M⊙ hydrogen white dwarf.
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Fruchter, A., Gunn, J., Lauer, T. et al. Optical detection and characterization of the eclipsing pulsar's companion. Nature 334, 686–689 (1988). https://doi.org/10.1038/334686a0
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DOI: https://doi.org/10.1038/334686a0
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