Fermi-LAT Constraints on the Pulsar Wind Nebula Nature of HESS J1857+026

Since its launch, the Fermi satellite has firmly identified 5 pulsar wind nebulae plus a large number of candidates, all powered by young and energetic pulsars. HESS J1857+026 is a spatially extended gamma-ray source detected by H.E.S.S. and classified as a possible pulsar wind nebula candidate powered by PSR J1856+0245. Aims. We search for -ray pulsations from PSR J1856+0245 and explore the characteristics of its associated pulsar wind nebula. Methods. Using a rotational ephemeris obtained from the Lovell telescope at Jodrell Bank Observatory at 1.5 GHz, we phase.fold 36 months of gamma-ray data acquired by the Large Area Telescope (LAT) aboard Fermi. We also perform a complete gamma-ray spectral and morphological analysis. Results. No pulsation was detected from PSR J1856+0245. However, significant emission is detected at a position coincident with the TeV source HESS J1857+026. The gamma-ray spectrum is well described by a simple power law with a spectral index of Gamma = 1.53 +/- 0.11(sub stat) +/- 0.55(sub syst) and an energy flux of G(0.1 C100 GeV) = (2.71 +/- 0.52(sub stat) +/- 1.51(sub syst) X 10(exp -11) ergs/ sq cm/s. This implies a gamma.ray efficiency of approx 5 %, assuming a distance of 9 kpc, the gamma-ray luminosity of L(sub gamma) (sub PWN) (0.1 C100 GeV) = (2.5 +/- 0.5(sub stat) +/- 1.5(sub syst)) X 10(exp 35)(d/(9kpc))(exp 2) ergs/s and E-dot = 4.6 X 10(exp 36) erg /s, in the range expected for pulsar wind nebulae. Detailed multi-wavelength modeling provides new constraints on its pulsar wind nebula nature.