CHANDRA OBSERVATIONS OF THE ELUSIVE PULSAR WIND NEBULA AROUND PSR B0656+14

PSR B0656+14 is a middle-aged pulsar with a characteristic age tau(c) = 110 kyr and spin-down power E = 3.8 X 10(34) erg s(-1). Using Chandra data, we searched for a pulsar wind nebula (PWN). We found evidence of an extended emission in a 3 ''.5-15 '' annulus around the pulsar, with a luminosity L-0.5-8 keV(ext) similar to 8 x 10(28) erg s(-1) (at the distance of 288 pc), which is a fraction of similar to 0.05 of the nonthermal pulsar luminosity. If the extended emission is mostly due to a PWN, its X-ray effiency, eta(pwn) = L-0.5-8 keV(ext)/E similar to 2 x 10(-6), is lower than those of most other known PWNe, but similar to that of the middle-aged Geminga pulsar. The small radial extent and nearly round shape of the putative PWN can be expained if the pulsar is receding (or approaching) in the direction close to the line of sight. The very soft spectrum of the extended emission (Gamma similar to 8) is much softer than those of typical PWNe; this could be explained by contribution from a faint dust scattering halo, which may dominate in the outer part of the extended emission.