2005ApJ...625..307K

We observed the γ-ray pulsar Geminga with the FUV-MAMA and NUV-MAMA detectors of the Space Telescope Imaging Spectrometer to measure 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±0.2)x10^–15 ergs/cm2/s in the 1155-1702 Å band, corresponds to a brightness temperature T_RJ~(0.3-0.4)(d₂₀₀/R₁₃)² MK, depending on the interstellar extinction (d=200d₂₀₀pc and R=13R₁₃ km are the distance and the NS radius, respectively). The soft thermal component of Geminga's X-ray spectrum measured with the XMM-Newton observatory corresponds to a temperature T_s=0.49±0.01 MK and radius R_s=(12.9±1.0)d₂₀₀ km. Contrary to other NSs detected in the UV-optical, for which the extrapolation of the 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 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 to 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_ν∝ν^–Γ+1, with a photon index Γ=1.43±0.15, close to the slope Γ=1.56±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 (or 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.