Thermal X-ray emission from a baryonic jet: a self-consistent multicolour spectral model

We present a publicly available spectral model for thermal X-ray emission from a baryonic jet in an X-ray binary system, inspired by the microquasar SS 433. The jet is assumed to be strongly collimated (half-opening angle Theta similar to 1 degrees) and mildly relativistic (bulk velocity beta = V-b/c similar to 0.03-0.3). Its X-ray spectrum is found by integrating over thin slices of constant temperature, radiating in optically thin coronal regime. The temperature profile along the jet and corresponding differential emission measure distribution are calculated with full account for gas cooling due to expansion and radiative losses. Since the model predicts both the spectral shape and luminosity of the jet's emission, its normalization is not a free parameter if the source distance is known. We also explore the possibility of using simpleX-ray observables (such as flux ratios in different energy bands) to constrain physical parameters of the jet (e.g. gas temperature and density at its base) without broad-band fitting of high-resolution spectra. We demonstrate this approach in application to Chandra High Energy Transmission Grating Spectrometer spectra of SS 433 in its 'edge-on' precession phase, when the contribution from non-jet spectral components is expected to be low. Our model provides a reasonable fit to the 1-3 keV data, while some residuals remain at higher energies, which may be partially attributed to a putative reflection component. Besides SS 433, the model might be used for describing jet components in spectra of other Galactic X-ray binary systems (e.g. 4U 1630-47), ULXs (e.g. Holmberg II X-1), and candidate SS 433 analogues like S26 in NGC 7793 and the radio transient in M82.