DISCOVERY OF A FAINT X-RAY COUNTERPART AND A PARSEC-LONG X-RAY TAIL FOR THE MIDDLE-AGED, gamma-RAY-ONLY PULSAR PSR J0357+3205

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope opened a new era for pulsar astronomy, detecting gamma-ray pulsations from more than 60 pulsars, similar to 40% of which are not seen at radio wavelengths. One of the most interesting sources discovered by LAT is PSR J0357+3205, a radio-quiet, middle-aged (tau(C) similar to 0.5Myr) pulsar standing out for its very low spin-down luminosity (E-rot similar to 6 x 10(33) erg s(-1)), indeed the lowest among non-recycled gamma-ray pulsars. A deep X-ray observation with Chandra (0.5-10 keV), coupled with sensitive optical infrared ground-based images of the field, allowed us to identify PSR J0357+3205 as a faint source with a soft spectrum, consistent with a purely non-thermal emission (photon index Gamma = 2.53 + - 0.25). The absorbing column (N-H = 8 + - 4 x 10(20) cm(-2)) is consistent with a distance of a few hundred parsecs. Moreover, the Chandra data unveiled a huge (9 arcmin long) extended feature apparently protruding from the pulsar. Its non-thermal X-ray spectrum points to synchrotron emission from energetic particles from the pulsar wind, possibly similar to other elongated X-ray tails associated with rotation-powered pulsars and explained as bow-shock pulsar wind nebulae (PWNe). However, energetic arguments as well as the peculiar morphology of the diffuse feature associated with PSR J0357+3205 make the bow-shock PWN interpretation rather challenging.