2012A&A...544A...3R

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. We search for γ-ray pulsations from PSR J1856+0245 and explore the characteristics of its associated pulsar wind nebula.Using a rotational ephemeris obtained from the Lovell telescope at Jodrell Bank Observatory at 1.5GHz, 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.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±0.11_stat±0.55_syst and an energy flux of G(0.1-100GeV)=(2.71 ±0.52_stat±1.51_syst)x10^–11erg/cm2/s. The γ-ray luminosity is L_PWN^γ(0.1-100 GeV)=(2.5±0.5_stat±1.5_syst)x10³⁵((d)/(9kpc))²erg/s, assuming a distance of 9kpc. This implies a γ-ray efficiency of ∼5% for {dot}(E)=4.6x10³⁶erg/s, in the range expected for pulsar wind nebulae. Detailed multi-wavelength modeling provides new constraints on its pulsar wind nebula nature.