Ultraviolet emission from the millisecond pulsar J0437-4715

We observed PSR J0437-4715 with the FUV-MAMA detector of the Space Telescope Imaging Spectrometer (STIS) to measure the pulsar's spectrum and pulsations. For the first time, UV emission from a millisecond pulsar has been detected. The measured flux, (2.0 ± 0.2) × 10^-15 ergs s^-1 cm^-2 in the 1150-1700 Å range, corresponds to the luminosity L_FUV = (4.7 ± 0.5) × 10²⁷ ergs s^-1, for the distance of 140 pc and negligible interstellar extinction. The shape of the observed spectrum suggests thermal emission from the neutron star surface with a surprisingly high temperature of about 1 × 10⁵ K, above the upper limit on the surface temperature of the younger "ordinary" pulsar J0108-1431. For the few-Gigayear-old J0437-4715, such a temperature requires a heating mechanism to operate. The spectrum of J0437-4715 shows marginal evidence of an emission line at 1372 Å, which might be a gravitationally redshifted Zeeman component of the hydrogen Lyα line in a magnetic field of ∼7 × 10⁸ G. No pulsations are detected, with a 3 σ upper limit of 50% on the pulsed fraction.