The Light Curve of the Companion to PSR J2051−0827

We have obtained new photometry of the companion to the eclipsing binary millisecond pulsar PSR J2051-0827. The data are modeled by a gravitationally distorted, low-mass secondary star that is irradiated by the impinging pulsar wind. The best-fit models require that greater than 30% of the incident energy is absorbed by the secondary star and reradiated as optical emission. Unless the companion is significantly farther away than its dispersion measure distance, it underfills its Roche lobe by at least 30%. A small companion makes it difficult to drive a wind from its surface and thus explain the mass loss in the system. Our models show that the system is inclined by more than 30°, and thus the companion mass is in the range of 0.025-0.055 M (for a 1.4 M pulsar). The unilluminated side of the companion is cool, with a best-fit temperature of 3000 K.