Fermi -LAT constraints on the pulsar wind nebula nature of HESS J1857+026

Context. 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 γ-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 γ-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 γ-ray data acquired by the Large Area Telescope (LAT) aboard Fermi. We also perform a complete γ-ray spectral and morphological analysis. Results. No γ-ray pulsations were detected from PSR J1856+0245. However, significant emission is detected at a position coincident with the TeV source HESS J1857+026. The γ-ray spectrum is well described by a simple power-law with a spectral index of Γ= 1.53 ± 0.11stat ± 0.55syst and an energy flux of G(0.1–100 GeV) = (2.71 ± 0.52stat ± 1.51syst) × 10 −11 erg cm −2 s −1 .T heγ-ray luminosity is L γ (0.1–100 GeV) = (2.5 ± 0.5stat ± 1.5syst) × 10 35 d 9 kpc 2 erg s −1 , assuming a distance of 9 kpc. This implies a γ-ray efficiency of ∼5% for u E = 4.6 × 10 36 erg s −1 , in the range expected for pulsar wind nebulae. Detailed multi-wavelength modeling provides new