The case for jets in cataclysmic variables

Abstract

For decades cataclysmic variables (CVs) were thought to be one of the few classes of accreting compact objects to not launch jets, and have consequently been used to constrain jet launching models. However, recent theoretical and observational advances indicate that CVs do in fact launch jets. Specifically, it was demonstrated that their accretion-outflow cycle is analogous to that of their higher mass cousins -the X-ray Binaries (XRBs). Subsequent observations of the CV SS Cygni confirmed this and have consistently shown radio flaring equivalent to that in the XRBs that marks a transient jet. Based on this finding and the emission properties, several studies have concluded that the radio emission is most likely from a transient jet. Observations of other CVs, while not conclusive, are consistent with this interpretation. However, the issue is not yet settled. Later observations have raised a number of questions about this model, as well as about potential alternative radio emission mechanisms. CVs are non-relativistic and many have well-determined distances; these properties would make them ideal candidates with which to address many of our outstanding questions about fundamental jet physics. Here we review the case for jets in CVs, discuss the outstanding questions and issues, and outline the future work necessary to conclusively answer the question of whether CVs launch jets.

Accretion flows in nonmagnetic white dwarf binaries as observed in X-rays

“…SS Cyg is the brightest DN observed readily over the entire electromagnetic spectrum. Radio emission from SS Cyg during several outbursts is interpreted as synchrotron emission originating from a transient jet and more recently other radio detections (15-80 Jy in the range 8-12 GHz) of about 6 DN in outburst suggest existence of collimated jet-like outflows or flares as in XRBs (Körding et al, 2008;Russell et al, 2016;Coppejans et al, 2016b;Coppejans and Knigge, 2020) known to have connection to advective hot flows. In addition, the outburst spectra of WZ Sge obtained by the Chandra ACIS data show two component spectrum of only hard X-ray emission, one of which may be fitted with a power law suggesting thermal Comptonization of the optically thick disk photons and/or scattering from an existing wind during the outburst (photon index varies from 0.8-2.0 and evolves in time).…”

Extended X-ray emission from the classic nova DQ Her -- On the possible presence of a magnetized jet

“…White dwarfs can act as the compact object for jet collimation, and indeed disk+jet systems have been found in symbiotic stars in which a white dwarf accretes material from a main sequence or red companion, for example, the well-studied R Aqr (Ramstedt et al 2018;Schmid et al 2018;Melnikov et al 2018) or MCW 560 (Stute & Sahai 2009). On the other hand, the conspicuous absence of jets in CVs has been explained in terms of the particular physical conditions in these systems (Soker & Lasota 2004), although recent observational and theoretical results have found some evidence for transient jets (Coppejans & Knigge 2020).…”

Eclipses of jets and discs of X-ray binaries as a powerful tool for understanding jet physics and binary parameters

“…Over the past decade, it has become increasingly clear that cataclysmic variables show evidence for synchrotron emitting jets (e.g., Körding et al 2008;Russell et al 2016;Coppejans & Knigge 2020). Radio emission from these sources has been associated with both the compact jets and the transient jets.…”