THE OPTICAL-SPECTRUM OF THE GEMINGA PULSAR

We obtained an optical spectrum of the isolated pulsar Geminga at the Keck Observatory. The optical object is at the limit of spectroscopic capability of any telescope, with a continuum flux that is approximate to 0.5% of the dark sky on Mauna Kea. With particular attention paid to the dominant systematics of sky subtraction in our observing and an alysis methods, we attained approximate to 0.1% systematics in heavily binned spectra. The resulting spectrum spanning 3700-8000 Angstrom ha s a flat power-law shape (f(nu) proportional to nu(-0.8)) and a broad dip over 6300-6500 Angstrom. Thermal radiation cannot explain the opti cal spectrum of Geminga. The dominant component can be modeled as eith er electron synchrotron emission and ion (proton) cyclotron absorption or ion cyclotron emission, the latter in a 10(11) G magnetic field.