A Massive White Dwarf Companion to the Eccentric Binary Pulsar System PSR B2303+46

Pulsars in close, eccentric binary systems are usually assumed to have another neutron star as a companion. These double neutron star binaries have proven to be the best laboratories for experimental general relativity and are the most secure candidates for gravitational wave interferometers. We present deep B, V, and R images of the field containing the eccentric binary pulsar system PSR B2303+46. We find a faint, blue object [B=26.60+/-0.09 (B-R)₀=-0.4+/-0.2] coincident with the timing position. We suggest that this object is the optical counterpart to the PSR B2303+46 system. The counterpart is too bright to reflect emission from the pulsar or a neutron star companion. Most likely, the companion of PSR B2303+46 is not a neutron star but a massive white dwarf. We show that the observations are consistent with a hot white dwarf companion (T_eff>~5×10⁴ K) with cooling age equal the characteristic age of the pulsar (t_cool~=30 Myr) and mass within the range set by timing observations and the Chandrasekhar mass (1.2<M_C<1.4 M_solar). Given the eccentric orbit, the white dwarf must have formed before the neutron star, from what was originally the more massive star in the binary. Due to mass transfer, the originally less massive star could become sufficiently massive to end its life in a supernova explosion and form the radio pulsar. We constrain the mass of the pulsar to be in the range 1.24<M_PSR<1.44 M_solar.