A BROAD-BAND X-RAY STUDY OF THE GEMINGA PULSAR

We present a comprehensive study of the Geminga pulsar at energies 0.1 -10 keV using data from ASCA, ROSAT, and EUVE. The bulk of the soft X- ray flux can be parameterized as a blackbody of T=(5.6+/-0.6)x10(5) K, occupying a fraction, 0.10-0.64, of the surface area of the neutron s tar at the parallax distance of 160 pc. The ASCA detection of Geminga resolves the nature of the harder X-ray component previously discovere d by ROSAT in favor of nonthermal emission, rather than thermal emissi on from a heated polar cap. The hard X-ray spectrum can be fitted by a power law of energy index 1.0+/-0.5. The hard X-ray light curve has a strong main peak and a weak secondary peak; its total K pulsed fracti on is approximate to 55%. Three ROSAT PSPC observations show significa nt variability of Geminga's light curve. In particular, a peculiar ene rgy dependence of the modulation in the soft X-ray component, dubbed t he ''Geminga effect'' in the original PSPC data, is not present in lat er observations. In addition, fine structure in the soft X-ray light c urve, interpreted as eclipses due to cyclotron resonance scattering by a plasma screen on the closed magnetic held lines, almost disappeared in the most recent observations. All of the variable properties of Ge minga can probably be associated with the nonthermal process that supp lies e(+/-) pairs to its inner magnetosphere.