Generation of radiative knots in a randomly pulsed protostellar jet. II. X-ray emission

Abstract

Context. Protostellar jets are known to emit in a wide range of bands, from radio to IR to optical bands, and to date also about ten X-ray emitting jets have been detected, with a rate of discovery of about one per year. Aims. We aim at investigating the mechanism leading to the X-ray emission detected in protostellar jets and, in particular, at constraining the physical parameters that describe the jet/ambient interaction by comparing our model predictions with observations available in the literature. Methods. We perform 2D axisymmetric hydrodynamic simulations of the interaction between a supersonic jet and the ambient medium. The jet is described as a train of plasma blobs randomly ejected by the stellar source along the jet axis. We explore the parameter space by varying the ejection rate, the initial Mach number of the jet, and the initial density contrast between the ambient medium and the jet. We synthesized from the model the X-ray emission as it would be observed with the current X-ray telescopes. Results. The mutual interactions among the ejected blobs and of the blobs with the ambient medium lead to complex X-ray emitting structures within the jet. The X-ray sources consist of several components: irregular chains of knots; isolated knots with measurable proper motion; apparently stationary knots; reverse shocks. The predicted X-ray luminosity strongly depends on the ejection rate and on the initial density contrast between the ambient medium and the jet, with a weaker dependence on the jet Mach number. Conclusions. Our model represents the first attempt to describe the X-ray properties of all the X-ray emitting protostellar jets discovered so far. The comparison between our model predictions and the observations can provide a useful diagnostic tool necessary for a proper interpretation of the observations. In particular, we suggest that the observable quantities derived from the spectral analysis of X-ray observations can be used to constrain the ejection rate, a parameter explored in our model that is not measurable by current observations in all wavelength bands.