The Optical Spectrum of the Geminga Pulsar

Abstract

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 ~0.5% of the dark sky on Mauna Kea. With particular attention paid to the dominant systematics of sky subtraction in our observing and analysis methods, we attained ~0.1% systematics in heavily binned spectra. The resulting spectrum spanning 3700-8000 Å has a flat power-law shape (f_ν~ν^-0.8) and a broad dip over 6300-6500 Å. Thermal radiation cannot explain the optical spectrum of Geminga. The dominant component can be modeled as either electron synchrotron emission and ion (proton) cyclotron absorption or ion cyclotron emission, the latter in a 10¹¹ G magnetic field.

Based on observations obtained at the W. H. Keck Observatory, which is jointly operated by the California Institute of Technology and the University of California.