Hubble space telescope detection of the double pulsar system J0737-3039 in the far-ultraviolet

We report on detection of the double pulsar system J0737-3039 in the far-UV with the Advanced Camera for Surveys/Solar-blind Channel detector aboard Hubble Space Telescope. We measured the energy flux F = (4.6 ± 1.0) × 10^-17 erg cm^-2 s^-1 in the 1250-1550 Å band, which corresponds to the extinction-corrected luminosity L 1.5 × 10²⁸ erg s^-1 for the distance d = 1.1 kpc and a plausible reddening E(B-V) = 0.1. If the detected emission comes from the entire surface of one of the neutron stars with a 13 km radius, the surface blackbody temperature is in the range T ≃ (2-5) × 10⁵ K for a reasonable range of interstellar extinction. Such a temperature requires an internal heating mechanism to operate in old neutron stars, or, less likely, it might be explained by heating of the surface of the less energetic Pulsar B by the relativistic wind of Pulsar A. If the far-ultraviolet emission is non-thermal (e.g., produced in the magnetosphere of Pulsar A), its spectrum exhibits a break between the UV and X-rays.