Braking Index of GEMINGA Pulsar

The pulsar Geminga, also known as 2CG195+04, IE0630+178 and the faint star G", is a remarkable object in the sense that its energy output is almost all in high energy gamma rays. The pulsar elements of this 237 ms pulsar as given by various authors are surveyed. The braking index, $ n = ftimes ddot f / (dot f)(2) as obtained from the elements given by any one group (Hermsen et al. 1992; Bertsch et al. 1992) based on their own data set appears to be too high or has a very large upper limit compared with 3, the value expected for magnetic dipole radiation. This is largely due to the uncertainty in the value of \ddot f. It is difficult to carry the absolute phase from one set of observations to another for a variety of reasons. Rather than fitting a polynomial in elapsed time to the event phases over different data sets, we have taken a different approach to determine \ddot f. Hermsen et al. (1992) and Mattox et al. (1993) have determined f and \dot f from COS-B and EGRET data respectively at two widely separated epochs. Assuming that there were no glitches, we obtained \ddot f by dividing the difference in \dot f values at the two epochs by the time difference between the two epochs; the resulting value of \ddot f is (4 \pm 2) times 10^{-26} s^{-3} . Combining this with the f and \dot f values we obtained a value of (4.5 \pm 2.3) for the braking index. This value agrees well with the expected. With more observations of Geminga scheduled for the Compton GRO, we expect that the error in \dot f and consequently the errors in \ddot f and n$ will decrease further in future. \leftline{Bertsch et al. (1992) Nature, 357, 306} \leftline{Hermsen et al. (1992) IAU Circular # 5541} \leftline{Mattox et al. Proc. 2nd Compton Symp., (September, 1993), College} \leftline{\quad Park, Md., U.S.A.}