We observed the gamma-ray pulsar Geminga with the FUV-MAMA and NUV-MAMA detectors of the Space Telescope Imaging Spectrometer to measure the Geminga ’ s spectrum and pulsations in the ultraviolet .
The slope of the far-ultraviolet ( FUV ) spectrum is close to that of a Rayleigh-Jeans spectrum , suggesting that the FUV radiation is dominated by thermal emission from the neutron star ( NS ) surface .
The measured FUV flux , F _ { FUV } = ( 3.7 \pm 0.2 ) \times 10 ^ { -15 } ergs cm ^ { -2 } s ^ { -1 } in 1155–1702 Å band , corresponds to a brightness temperature T _ { RJ } \approx ( 0.3 – 0.4 ) ( d _ { 200 } / R _ { 13 } ) ^ { 2 } MK , depending on the interstellar extinction ( d = 200 d _ { 200 } pc and R = 13 R _ { 13 } km are the distance and the NS radius ) .
The soft thermal component of the Geminga ’ s X-ray spectrum measured with the XMM-Newton observatory corresponds to a temperature T _ { s } = 0.49 \pm 0.01 MK and radius R _ { s } = ( 12.9 \pm 1.0 ) d _ { 200 } km .
Contrary to other NSs detected in the UV-optical , for which the extrapolation of X-ray thermal component into the optical underpredicts the observed flux of thermal radiation , the FUV spectrum of Geminga lies slightly below the extrapolation of the soft thermal component , which might be associated with the Geminga ’ s very low temperature .
Surprisingly , the thermal FUV radiation is strongly pulsed , showing a narrow dip at a phase close to that of a broader minimum of the soft X-ray light curve .
The strong pulsations might be attributed to partial occultations of the thermal UV radiation by regions of the magnetosphere filled with electron/positron plasma .

In contrast with the FUV spectrum , the near-infrared ( NIR ) through near-ultraviolet ( NUV ) spectrum of Geminga is clearly nonthermal .
It can be described by a power-law model , F _ { \nu } \propto \nu ^ { - \Gamma + 1 } , with a photon index \Gamma = 1.43 \pm 0.15 , close to the slope \Gamma = 1.56 \pm 0.24 of the hard X-ray ( E > 2.5 keV ) magnetospheric component .
The extrapolation of the X-ray magnetospheric spectrum into the optical is marginally consistent with ( perhaps lies slightly above ) the observed NIR-optical-NUV spectrum .
The NUV pulsations , however , do not show a clear correlation with the hard X-ray pulsations .