Geminga

W. Hermsen and B. N. Swanenburg, Laboratory for Space Research, Leiden; R. Buccheri, L. Scarsi, and B. Sacco, Istituto di Fisica Cosmica ed Applicazioni dell'Informatica, Consiglio Nazionale delle Ricerche, Palermo; K. Brazier, G. Kanbach, G. G. Lichti, H. A. Mayer-Hasselwander, and A. W. Strong, Max-Planck-Institut fur Extraterrestrische Physik, Garching; I. Grenier, Service d'Astrophysique, Centre d'Etudes Nucleaires de Saclay; K. Bennett and M. Busetta, Space Science Department, European Space Agency, ESTEC, Noordwijk; and J. Mattox, Goddard Space Flight Center, report: "The EGRET and ROSAT telescopes have recently revealed the presence of a periodicity of 237 ms in the emission from the gamma-ray source Geminga (Bertsch et al. 1992, Nature 357, 306) and its soft x-ray counterpart 1E 0630+178 (Halpern and Holt 1992, Nature 357, 222). We have analyzed the COS-B data of five observations (those numbered 0, 14, 39, 54, and 64, performed between 1975 Aug. and 1982 Apr.) with Geminga in the field of view for gamma-ray energies above 100 MeV. Using the Einstein HRI position, we find an extremely stable law of variation of the pulsation frequency, valid for the whole COS-B lifetime, with parameters given by t0 = 1979 Mar. 14.0 UT, f0 = 4.217 750 122 77 +/- 0.000 000 000 24 Hz, f0(dot) = (-0.195 237 9 +/- 0.000 002 4) x 10E-12 Hz sE-1, f0(double dot) = (0.28 +/- 0.16) x 10E-24 Hz sE-2. The accuracy of these parameters is significantly greater than that reported by Bignami and Caraveo (1992, Nature 357, 287) and now allows the detection of pulsations also in the data from COS-B observation No. 54. The stability of the absolute phases of the peaks is within 5 ms for the five COS-B observations. Therefore we recommend use of these parameters for archival searches. When we perform the analysis using the position of G", the proposed optical counterpart for Geminga (Halpern and Tyler 1988, Ap.J. 330, 201; Bignami et al. 1987, Ap.J. 319, 358), the timing solution changes slightly (mainly in the value of f0(double dot)) but remains statistically consistent with that reported above. The COS-B light curve shows, as in the EGRET case, two main peaks separated by 0.5 in phase with a significant bridge of emission between them. Detailed analysis on the variability of the light curve shape and spectra is in progress."