Timing the Geminga pulsar with high-energy gamma rays.

The EGRET phase 1 & 2 observations of Geminga have been used to determine an optimum contemporary ephemeris using the "Z_n² timing method". The residuals for this linear ephemeris over 2.1 years are less than 5 ms, suggesting that Geminga has not glitched. A position estimate is found to be consistent with the G″optical counterpart. The comparison of f˙between this ephemeris and that of COS-B yields an estimate of f which is an order of magnitude smaller than that determined from the COS-B data. It is estimated that the anticipated EGRET phase 3 Geminga exposure will allow for the comparison of Geminga phase between EGRET, COS-B, and SAS-2.