We observed a 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 { \AA } ) = ( 2.5 \pm 0.3 ) \times 10 ^ { -16 } erg s ^ { -1 } cm ^ { -2 } and F ( 3800 - 6000 { \AA } ) = ( 6.4 \pm 0.4 ) \times 10 ^ { -17 } erg s ^ { -1 } cm ^ { -2 } , which correspond to luminosities of \approx 5.8 \times 10 ^ { 27 } and 1.4 \times 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 , f _ { \nu } \propto \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 \times 10 ^ { 5 } and 2.1 \times 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 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 FUV .