Abstract
Geminga is a radio-quiet, rotation-powered pulsar with a 237-msec period observed in X-ray and gamma-ray bands. We conduct a periodicity search with its ASCA data and XMM-Newton data in the period range of 0.1 sec to 100 sec. The H-test and the method of epoch folding are employed with period search steps at one fifth of the corresponding Fourier widths. Periods other than the already known pulsar period, if found, may indicate the phenomenon of neutron star oscillations, which in a broad sense include waves on the stellar surface. We have found no evidence for such oscillations in our search. Upper limits to the pulsed component of these possible oscillations at the 3-sigma level inferred from the 1994 and 1999 ASCA data in 0.7-5 keV are 3.5x10^-14 erg/cm^2/sec and 1.9x10^-14 erg/cm^2/sec respectively, which are 12.4\% and 6.9\% of the source flux. 3-sigma upper limits inferred from 2002 XMM-Newton data in energy bands of 0.2-0.7 and 0.7-5 keV are 7.7x10^-13 erg/cm^2/sec and 1.7x10^-14 erg/cm^2/sec, which are 1.3\% and 3.3\% of the source flux respectively.