This Legacy journal article was published in Volume 2, November 1992, and has not been updated since publication. Please use the search facility above to find regularly-updated information about this topic elsewhere on the HEASARC site.
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Number 2					       November 1992

cover image

The Geminga

237ms Pulsar

Goddard Space Flight Center

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 greater than or equal to 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) complex equation. 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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