Ultraviolet, X‐Ray, and Optical Radiation from the Geminga Pulsar

We observed the gamma-ray pulsar Geminga with the HST STIS/MAMA detectors tomeasure the Geminga's UV spectrum and pulsations. The slope of the far-UV (FUV)spectrum is close to that of a Rayleigh-Jeans spectrum, suggesting that the FUVradiation is dominated by thermal emission from the neutron star (NS) surface.The measured FUV flux, F_FUV=(3.7+/-0.2)x10^-15 ergs cm^-2 s^-1 in 1155-1702 Aband, corresponds to a brightness temperature T_RJ=(0.3-0.4)(d_200/R_13)^2 MK.The soft thermal component of the Geminga's X-ray spectrum measured with theXMM-Newton observatory corresponds to a temperature T_s=0.49+/-0.01 MK andradius R_s = (12.9+/-1.0)d_200 km. Unlike other NSs detected in the UV-optical,the FUV spectrum of Geminga lies below the extrapolation of the soft thermalcomponent, which might be associated with the Geminga's very low temperature.Surprisingly, the thermal FUV radiation is strongly pulsed, showing a narrowdip at a phase close to that of a broader minimum of the soft X-ray lightcurve. The strong pulsations might be attributed to partial occultations of thethermal UV radiation by regions of the magnetosphere filled withelectron/positron plasma. In contrast with the FUV spectrum, the near-IR (NIR)through near-UV (NUV) spectrum of Geminga is clearly nonthermal. It can bedescribed by a power-law model, F_nu \propto nu^{-alpha} with a slope alpha =0.43+/-0.15, close to alpha = 0.56+/-0.24 of the hard X-ray (E>2.5 keV)magnetospheric component. The observed NIR-optical-NUV spectrum is marginallyconsistent with the extrapolation of the X-ray magnetospheric spectrum. The NUVpulsations, however, do not show a clear correlation with the hard X-raypulsations.Comment: 17 pages including 14 figures, ApJ, 625 in pres