PSR J1856+0245: Arecibo Discovery of a Young, Energetic Pulsar Coincident with the TeV γ-Ray Source HESS J1857+026

We present the discovery of the Vela-like radio pulsar J1856+0245 in the Arecibo PALFA survey. PSR J1856+0245 has a spin period of 81 ms, a characteristic age of 21 kyr, and a spin-down luminosity Ė = 4.6 × 10 36 ergs s −1 . It is positionally coincident with the TeV γ-ray source HESS J1857+026, which has no other known counterparts. Young, energetic pulsars create wind nebulae, and more than a dozen pulsar wind nebulae have been associated with very high energy (100 GeV-100 TeV) γ-ray sources discovered with the HESS telescope. The γ-ray emission seen from HESS J1857+026 is potentially produced by a pulsar wind nebula powered by PSR J1856+0245; faint X-ray emission detected by ASCA at the pulsar's position supports this hypothesis. The inferred γ-ray efficiency is γ = L γ /Ė =3.1 % (1-10 TeV, for a distance of 9 kpc), comparable to that observed in similar associations.