Number 2 November 1992
On the cover
This cover figure is from the paper "SAS 2 Observation of Pulsed High Energy Gamma Radiation from Geminga" by J.R. Mattox, D.L. Bertsch, C.E. Fichtel, R.C. Hartman, D.A. Kniffen & D.J. Thompson, Ap.J. Letters, in press.
The recent discovery by Halpern and Holt (1992, Nature, 357, 222) of 237ms pulsations from Geminga using ROSAT prompted a reexamination of archival COS B and SAS 2 data. The front cover shows the gamma-ray phase plot for Geminga for E 70 MeV. Arrival times of the archival SAS 2 and COS B events were epoch folded with the COS B ephemeris. Phase 0 corresponds to the epoch time at the solar system barycenter. COS B events were selected from the energy-dependent cone (Buccheri et al. 1983, A&A, 128, 245) . The dashed line shows the level of diffuse gamma radiation expected if no source were present. For comparison the EGRET data (Bertsch et al. 1992, Nature, 357, 306) are shown in the figure. The phase of the EGRET data has been adjusted to match COS B, because the uncertainties in the timing fits for COS B and EGRET data do not allow an absolute phase comparison. The EGRET histogram contains 5322 events (approximately 36 days of exposure), the COS B histogram 1897 events (approximately 251 days of exposure), and the SAS 2 histogram 82 events (19 days of exposure).
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