The Light Curve of the Companion to PSR J2051−0827

Abstract

We have obtained new photometry of the companion to theeclipsing binary millisecond pulsar PSR J2051-0827. The data aremodeled by a gravitationally distorted, low-mass secondary star that isirradiated by the impinging pulsar wind. The best-fit models require thatgreater than 30% of the incident energy is absorbed by the secondarystar and reradiated as optical emission. Unless the companion issignificantly farther away than its dispersion measure distance, itunderfills its Roche lobe by at least 30%. A small companion makes itdifficult to drive a wind from its surface and thus explain the massloss in the system. Our models show that the system is inclined bymore than 30 deg, and thus the companion mass is in the range of0.025-0.055 M_solar (for a 1.4 M_solar pulsar). The unilluminated sideof the companion is cool, with a best-fit temperature of 3000 K.