The [ITAL]ASCA[/ITAL] Spectrum of the Vela Pulsar Jet

ROSAT observations of the Vela pulsar and its surroundings revealed acollimated X-ray feature almost 45' in length (Markwardt & Ogelman 1995),interpreted as the signature ``cocoon'' of a one-sided jet from the Velapulsar. We report on a new ASCA observation of the Vela pulsar jet at its head,the point where the jet is believed to interact with the supernova remnant. Thehead is clearly detected, and its X-ray spectrum is remarkably similar to thesurrounding supernova remnant spectrum, extending to X-ray energies of at least7 keV. A ROSAT+ASCA spectrum can be fit by two-component emission models butnot standard one-component models. The lower energy component is thermal andhas a temperature of 0.29+/-0.03 keV (1 sigma); the higher energy component canbe fit by either a thermal component of temperature ~4 keV or a power law withphoton index ~2.0. Compared to the ROSAT-only results, the mechanicalproperties of the jet and its cocoon do not change much. If the observedspectrum is that of a hot jet cocoon, then the speed of the jet is at least 800km s^-1, depending on the angle of inclination. The mechanical power drivingthe jet is >10^36 erg s^-1, and the mass flow rate at the head is > 10^-6 M_sunyr^-1. We conclude that the jet must be entraining material all along itslength in order to generate such a large mass flow rate. We also explore thepossibility that the cocoon emission is synchrotron radiation instead ofthermal.Comment: 12 pages, LaTeX in AAS v4.0 preprint style, two PS figures, accepted for publication in the ApJ Letter