First X-Ray Observations of the Young Pulsar J1357-6429

The first short Chandra and XMM-Newton observations of the young andenergetic pulsar J1357-6429 provided strong indications of a tail-likepulsar-wind nebula associated with this object, as well as pulsations of itsX-ray flux with a pulsed fraction above 50% and a thermal component dominatingat lower photon energies (below 2 keV). The elongated nebula is very compact insize and may be interpreted as evidence for a pulsar jet. The thermal radiationis most plausibly emitted from the entire neutron star surface of a 10 kmradius and a 1.0+/-0.1 K temperature, covered with a magnetized hydrogenatmosphere. At higher energies the pulsar's emission is of a nonthermal(magnetospheric) origin, with a power-law spectrum of a photon index of1.1-1.3. This makes the X-ray properties of PSR J1357-6429 very similar tothose of the youngest pulsars J1119-6127 and Vela with a detected thermalradiation.Comment: Minor changes, including the reference to Esposito et al. (2007 A&A, 467, L45). Accepted by ApJ Letter