TeV Gamma Rays from Geminga and the Origin of the GeV Positron Excess

The Geminga pulsar has long been one of the most intriguing MeV-GeV $\gamma$-ray point sources. We examine the implications of the recent Milagro $\gamma$-ray Observatory detection of extended, multi-TeV $\gamma$-ray emission from Geminga, finding that this reveals the existence of an ancient, powerful cosmic-ray accelerator that can plausibly account for the multi-GeV positron excess that has evaded explanation. We explore a number of testable predictions for $\gamma$-ray and electron or positron experiments (up to $\sim 100\mathrm{TeV}$) that can confirm the first “direct” detection of a cosmic-ray source.

Figure 1

The cosmic-ray positron fraction. Shown are data compiled from Refs. 7, 8, 31, 34 and scenarios based on the secondary model of Ref. 9 (shaded) and a plausible Geminga contribution (solid, dashed, and dotted lines, respectively) dependent upon distance and energetics (see text for details).

Figure 2

Minimal inverse-Compton $\gamma$-ray spectrum of the extended emission from Geminga (shaded) and the Milagro measurement at 20 TeV (left axis). Also, the energy distribution (dotted line) of the associated $e^{\pm }$ (right axis).

Figure 3

Cosmic-ray $e^{-}+e^{+}$ data from Fermi [16], HESS [17, 18], and Refs. 48, 49 and AMS $e^{-}$ data [34]; with the Galactic $e^{-}$ model modified from Ref. 9 (solid line), contributions from Geminga (lower lines), and total (upper lines).