Geminga

The Compton Observatory EGRET Team (J. R. Mattox, Computer Sciences Corporation and Goddard Space Flight Center; D. L. Bertsch, B. L. Dingus, C. E. Fichtel, R. C. Hartman, S. D. Hunter, P. W. Kwok, P. Sreekumar, and D. J. Thompson, Goddard Space Flight Center, NASA; D. A. Kniffen, Hampden-Sydney College; J. Chiang, J. Fierro, Y. C. Lin, P. F. Michelson, and P. L. Nolan, Stanford University; E. Schneid, Grumman Aerospace Corporation; and K. Brazier, G. Kanbach, H. A. Mayer-Hasselwander, C. von Montigny, K. Pinkau, H. Rothermel, and M. Sommer, Max-Planck-Institut fur Extraterrestrische Physik) communicates: "In addition to the 5 weeks of Geminga pulsar exposure in 1991, we have now analyzed 260 gamma-ray events with energies between 0.07 and 30 GeV from a low-sensitivity observation (33 deg off-axis) in the interval 1992 June 11-25. Using first and second frequency derivatives obtained from the COS-B ephemeris (IAUC 5541) and a frequency determined from the 1991 EGRET data, we predict a phase for 1992 June which differs from the observed phase by 0.06 of a revolution (the uncertainty of the prediction is 0.08 due to the limited precision of the frequency). This result, combined with the COS-B finding (IAUC 5541) that the timing residuals are 5 ms or less for a single ephemeris over 7 years, suggests that Geminga has not glitched between 1975 and 1992, and that the rotation is very well fit by a quadratic ephemeris. The frequency in the following ephemeris has been refined so that the 1991 and 1992 phases are consistent; numbers in parentheses are approximately the 1-sigma error of the last digit given: T0 = 1991 May 24.0 UT, f0 = 4.2176749957(6) sE-1, f1 = -1.9508(5) x 10E-13 sE-2, f2 = +4(1) x 10E-25 sE-3. The phase of peak 1 (preceding the bridge of emission which links peak one and peak two; see Bertsch et al. 1992, Nature 357, 306, for the light curve) at T0 is 0.64(1) at the solar system barycenter, and 0.21(1) at the geocenter. We expect that this ephemeris will give the phase to within 0.1 of a revolution for the interval 1988.8-1994.0. The position used for the barycenter correction is that of the G" star (Halpern and Tytler 1988, Ap.J. 330, 201)."