Identification of the Optical Counterpart of Fermi Black Widow Millisecond Pulsar PSR J1544+4937

We report the optical identification of the companion to the Fermi black widow millisecond pulsar PSR J1544+4937. We find a highly variable source on Keck Low Resolution Imaging Spectrometer images at the nominal pulsar position, with 2 mag variations over orbital period in the B, g, R, and I bands. The nearly achromatic light curves are difficult to explain with a simply irradiated hemisphere model, and suggest that the optical emission is dominated by a nearly isothermal hot patch on the surface of the companion facing the pulsar. We roughly constrain the distance to PSR J1544+4937 to be between 2 and 5 kpc. A more reliable distance measurement is needed in order to constrain the composition of the companion.