Ultraviolet Emission from the Millisecond Pulsar J0437-4715

We observed PSR J0437-4715 with the FUV-MAMA detector of the Hubble Space Telescope Imaging Spectrometer (STIS) to measure the pulsar's spectrum and pulsations. For the first time, UV emission from a millisecond pulsar is detected. The measured flux, $(2.0\pm 0.2)\times 10^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the 1150-1700 \AA range, corresponds to the luminosity $L_{\rm FUV}=(4.7\pm 0.5)\times 10^{27}$ erg 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 $1\times 10^5$ K, above the upper limit on the surface temperature of the younger ``ordinary'' pulsar J0108-1431. For the few-Gyr-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 \AA, which might be a gravitationally redshifted Zeeman component of the Hydrogen Ly$\alpha$ line in a magnetic field $\sim 7\times 10^8$ G. No pulsations are detected, with a $3\sigma$ upper limit of 50% on pulsed fraction.