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 Å) = (2.5 ± 0.3) ×10^-16 erg s^-1 cm^-2 and F(3800-6000 Å) = (6.4 ± 0.4) ×10^-17 erg s^-1 cm^-2, which corresponds to luminosities of ≈5.8 ×10²⁷ and 1.4 ×10²⁷ 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, F_v,∞ V^α with slope α = 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 α < 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 ×10⁵ and 2.1 ×10⁵ K for α 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 Hα bow shock, making PSR J2124-3358 the second pulsar (after PSR J0437-4715) with a bow shock detected in the FUV.