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

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Abstract
ROSAT observations of the Vela pulsar and its surroundings have previously revealed a collimated X-ray feature almost 45' in length, interpreted as the signature "cocoon" of a one-sided jet from the Vela pulsar. 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. The head is clearly detected, and its X-ray spectrum is remarkably similar to the surrounding supernova remnant spectrum, extending to X-ray energies of at least 7 keV. A ROSAT plus ASCA spectrum can be fitted by two-component emission models but not by standard one-component models. The lower energy component is thermal and has a temperature of 0.29 ± 0.03 keV (1 σ); the higher energy component can be fitted by either a thermal component of temperature ~4 keV or a power law with photon index ~2.0. Compared with the ROSAT-only results, the mechanical properties of the jet and its cocoon do not change much. If the observed spectrum is that of a hot jet cocoon, then the speed of the jet is at least 800 km s-1, depending upon the angle of inclination. The mechanical power driving the jet is ≥1036 ergs s-1, and the mass flow rate at the head is ≥10-6 M☉ yr-1. We conclude that the jet must be entraining material all along its length in order to generate such a large mass flow rate. We also explore the possibility that the cocoon emission is synchrotron radiation rather than thermal.
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