The discovery of X-ray emission from the protostellar jet L1551 IRS5 (HH 154)

We have for the first time detected X-ray emission associated with a protostellar jet, on the jet emanating from the L1551 IRS5 protostar. The IRS5 protostar is hidden behind a very large absorbing column density, making the direct observation of the jet's emission possible. The observed X-ray emission is likely associated with the shock ``working surface'', i.e. the interface between the jet and the circumstellar medium. The X-ray luminosity emanating from the jet is moderate, at $L_{\rm X} \simeq 3 \times 10^{29}$ erg/s, a significant fraction of the luminosity normally associated with the coronal emission from young stars. The spectrum of the X-ray emission is compatible with thermal emission from a hot plasma, with a temperature of $\simeq 0.5$ MK, fully in line with the temperature expected (on the basis of the jet's velocity) for the shock front produced by the jet hitting the circumstellar medium.