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 \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 \gamma -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 \gamma -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 \gamma -ray spectrum is well described by a simple power-law with a spectral index of \Gamma = 1.53 \pm 0.11 _ { stat } \pm 0.55 _ { syst } and an energy flux of G ( 0.1 – 100 GeV ) = ( 2.71 \pm 0.52 _ { stat } \pm 1.51 _ { syst } ) \times 10 ^ { -11 } ergs cm ^ { -2 } s ^ { -1 } .
The \gamma -ray luminosity is L _ { PWN } ^ { \gamma } ( 0.1 – 100 GeV ) = ( 2.5 \pm 0.5 _ { stat } \pm 1.5 _ { syst } ) \times 10 ^ { 35 } \left ( \frac { d } { 9 kpc } \right ) ^ { 2 } ergs s ^ { -1 } , assuming a distance of 9 kpc .
This implies a \gamma - ray efficiency of \sim 5 \% for \dot { E } = 4.6 \times 10 ^ { 36 } erg s ^ { -1 } , in the range expected for pulsar wind nebulae .
Detailed multi-wavelength modeling provides new constraints on its pulsar wind nebula nature .

Conclusions :