Hubble Space Telescope Detection of the Millisecond Pulsar J2124-3358 and its Far-ultraviolet Bow Shock Nebula

We observed the nearby millisecond pulsar J2124-3358 with the Hubble Space Telescope in broad far-UV (FUV) and optical filters. The pulsar is detected in both bands with fluxes F (1250-2000 angstrom) = (2.5 +/- 0.3) x 10(-16) erg s(-1) cm(-2) and F (3800-6000 angstrom) = (6.4 similar to 0.4). x. 10-17. erg s-1 cm-2, which corresponds to luminosities of similar to 5.8. x. 1027 and 1.4 x 10(27) erg s(-1), for d = 410 pc and E(B-V) = 0.03. The optical-FUV spectrum can be described by a power-law model, n similar to n f nu alpha nu(alpha), with slope alpha = 0.18-0.48 for a conservative range of color excess, E(B - V) = 0.01-0.08. Since a spectral flux rising with frequency is unusual for pulsar magnetospheric emission in this frequency range, it is possible that the spectrum is predominantly magnetospheric (power law with alpha < 0) in the optical, while it is dominated by thermal emission from the neutron star surface in the FUV. For a neutron star radius of 12 km, the surface temperature would be between 0.5 x 10(5) and 2.1 x 10(5)K for alpha ranging from-1 to 0, E(B - V) = 0.01-0.08, and d = 340-500 pc. In addition to the pulsar, the FUV images reveal extended emission that is spatially coincident with the known H alpha bow shock, making PSR J2124-3358 the second pulsar (after PSR J0437-4715) with a bow shock detected in the FUV.