On the absence of jet features in the optical spectra of cataclysmic variables

Blue- and redshifted hydrogen and helium satellite recombination lines have recently been discovered in the optical spectra of at least two supersoft X-ray sources (SSSs), RX J0513−069 and RX J0019.8+2156, and, tentatively, also in one short-period cataclysmic variable star (CV), the recurrent nova T Pyx. These features are thought to provide evidence for the presence of highly collimated jets in these systems. No similar spectral signatures have been detected in the spectra of other short-period CVs, despite a wealth of existing optical data on these systems. Here, we ask if this apparent absence of 'jet lines' in the spectra of most CVs already implies the absence of jets of the kind that appear to be present in the SSSs and perhaps T Pyx, or whether the current lack of jet detection in CVs can still be ascribed to observational difficulties.
To answer this question, we derive a simple, approximate scaling relation between the expected equivalent widths (EWs) of the observed jet lines in both types of systems and the accretion rate through the disc, EW(line)∝˙M^4/3_acc. We use this relation to predict the strength of jet lines in the spectra of 'ordinary' CVs, i.e. systems characterized by somewhat lower accretion rates than T Pyx. Making the assumption that the features seen in T Pyx are indeed jet lines, and using this system as a reference point, we find that, if jets are present in many CVs, they may be expected to produce optical satellite recombination lines with EWs of a few hundredths to a few tenths of an angstrom in suitably selected systems. A similar prediction is obtained if the SSS RX J0513−069 is used as a reference point. Such EWs are small enough to account for the non-detection of jet features in CVs to date, but large enough to allow them to be detected in data of sufficiently high quality, if they exist.