We report on the Fermi-LAT observations of the Geminga pulsar, the second brightest non-variable GeV source in the γ-ray sky and the first example of a radio-quiet γ-ray pulsar. The observations cover one year, from the launch of the Fermi satellite through 2009 June 15. A data sample of over 60,000 photons enabled us to build a timing solution based solely on γ-rays. Timing analysis shows two prominent peaks, separated by Δphi = 0.497 ± 0.004 in phase, which narrow with increasing energy. Pulsed γ-rays are observed beyond 18 GeV, precluding emission below 2.7 stellar radii because of magnetic absorption. The phase-averaged spectrum was fitted with a power law with exponential cutoff of spectral index Γ = (1.30 ± 0.01 ± 0.04), cutoff energy E 0 = (2.46 ± 0.04 ± 0.17) GeV, and an integral photon flux above 0.1 GeV of (4.14 ± 0.02 ± 0.32) × 10–6 cm–2 s–1. The first uncertainties are statistical and the second ones are systematic. The phase-resolved spectroscopy shows a clear evolution of the spectral parameters, with the spectral index reaching a minimum value just before the leading peak and the cutoff energy having maxima around the peaks. The phase-resolved spectroscopy reveals that pulsar emission is present at all rotational phases. The spectral shape, broad pulse profile, and maximum photon energy favor the outer magnetospheric emission scenarios.

El PDF auto-archivado en Digital.CSIC es el PRE-PRINT del autor.-- arXiv:1007.1142. -- 32 páginas, 12 figuras, 3 tablas. -- Autores: Abdo, Aous A. ... [et al.]