Are the Unidentified EGRET Sources Geminga-Type Pulsars?

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Session 9 -- Galactic X-Ray/Gamma-Ray Sources
Display presentation, Monday, 30, 1994, 9:20-6:30

[9.02] Are the Unidentified EGRET Sources Geminga-Type Pulsars?

D.J. Thompson, D.L. Bertsch, C.E. Fichtel, R.C. Hartman, S.D. Hunter (NASA/GSFC), B.L. Dingus, J.A. Esposito, R. Mukherjee, P. Sreekumar, J.R. Mattox (USRA/GSFC), G. Kanbach, H.A. Mayer-Hasselwander (MPE), C. von Montigny (NAS/NRC), J.M. Fierro, Y.C. Lin, P.F. Michelson, P.L. Nolan (Stanford), D.A. Kniffen (Hampden-Sydney), E.J. Schneid (Grumman)

Observations with the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Observatory show more than 30 unidentified sources of E$>$100 MeV gamma rays concentrated along the galactic plane (Fichtel et al., 1994, ApJS, in press). Based on their spatial distribution, their typical distances are 2-6 kpc. Combining these distances with the observed gamma-ray fluxes gives luminosities above 100 MeV in the range (0.1 - 1) x 10$^{36}$ ergs/s. These derived luminosities are higher than those of most of the observed gamma-ray pulsars, even when substantial beaming is considered. It is unlikely, therefore, that most of these sources closely resemble Geminga, which is an older, nearby pulsar with a total spin-down luminosity $\dot E$ of only 3 x 10$^{34}$ ergs/s and an approximate gamma-ray luminosity of 2 x 10$^{33}$ ergs/s for an assumed distance of 250 pc. If these sources are undiscovered pulsars, they are probably younger and more distant, with the principal simil! arity to Geminga being that they h ave no observable radio pulse. For most of these, finding an unknown short-period pulsation with the gamma-ray data alone will be extremely difficult.

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