The Magnetic Scenario for the SW Sextantis Stars
The physical geometry of a cataclysmic variable (CV) containing a weakly magnetic white dwarf (i.e., an “intermediate polar” or IP, which contains a truncated inner accretion disk with magnetically-controlled accretion curtains through which material flows from the inner disk edge to the white dwarf) could account for many of the peculiar observational properties that originally defined the SW Sex stars, if:
- the single-peaked line profiles arise in a magnetic accretion curtain close to the white dwarf (WD);
- the shallow eclipse of low excitation lines results because material following the field lines initially rises above the disk plane; and
- self-absorption in the accretion curtain accounts for the transient absorption feature.
Thus, it is little surprise that recently there has been an increasing trend in the relevant literature to invoke a magnetic scenario for the SW Sex stars. In the interest of providing a reference for future discussion, we summarize here key points of evidence culled from various literature sources that support a magnetic scenario for the SW Sex stars:
- Perhaps the most blatant piece of evidence is that the observational properties of some CVs (e.g., TT Ari, V533 Her, V795 Her, EX Hya, BT Mon, V348 Pup) have caused them to be independently classified as both possible IPs/magnetic CVs and as possible SW Sex stars.
- The detections (albeit somewhat marginal) of circular polarization in LS Peg and V795 Her (see 2001ApJ...548L..49R and 2002ASPC..261..533R) points to the presence of a magnetic WD.
- “Flaring” in optical emission lines on time scales of tens of minutes, which is suggestive of the asynchronous spin of the WD in an IP, is present in DW UMa, V533 Her, BT Mon, and LS Peg (see 2002MNRAS.337..209R and references therein). The circular polarization observed in LS Peg and V795 Her is also modulated on similar time scales.
- Superhumps and kilosecond QPOs have been observed in the optical light curves of numerous SW Sex stars. These features can be ascribed to (but do not require) the presence of a WD magnetic field (2002PASP..114.1364P; note, however, that these authors suggest that the magnetic fields in SW Sex stars are comparable in strength to the highly magnetic “polars,” rather than being the weakest of the IPs, as is commonly asserted in other literature sources.)
- Hot spots in the disks of SW Sex stars that are revealed by eclipse mapping and/or Doppler tomography might be produced when the overflowing accretion stream encounters the WD magnetosphere; for example, as speculated in 2001ApJ...548L..49R for the observations of SW Sex in 2001A&A...368..183G.
- The typical Doppler tomograms of SW Sex stars (e.g., 1994ApJS...93..519K) might also be explained by a magnetic propeller mechanism in which some material in the overflowing accretion stream is ejected from the inner disk by the spinning WD magnetosphere (see 1999ASPC..157..349H; note that this is a somewhat extreme variant of the magnetic scenario for SW Sex stars – to our knowledge, little additional work has been presented in support of it.)
- Optical eclipse mapping studies of DW UMa (2000A&A...364..573B), SW Sex (2001A&A...368..183G), V1315 Aql, and other SW Sex stars (e.g., 1992A&A...260..213R) show that their disk temperature profiles are best reproduced by assuming that the inner disk is suppressed or missing, as in the IPs. These results are somewhat ambiguous, as the temperature profiles could also be attributed to obscuration by the rim of a flared accretion disk. For example, the V-shaped eclipses seen in DW UMa and other SW Sex stars can be accounted for by a flared accretion disk (2000ApJ...539L..49K).
- The low inclination SW Sex stars V795 Her (1996MNRAS.278..219C), LS Peg (1999MNRAS.305..661M, 1999PASP..111..184T), and V442 Oph (2000ApJ...537..936H) display Balmer emission line components extending to large velocity offsets from the line centers (Δv ≈ 1500–2000 km s-1), which produce prominent orbital S-waves in trailed spectra of these systems. It has been suggested (e.g., 1999MNRAS.305..661M) that these components are present in the high inclination systems also, but at smaller velocity offsets such that they do not detach from the emission line cores and are, instead, visible as wings of the Balmer lines. This behavior would require the emitting material responsible for the narrow components to have a vertical extent that allows a larger radial velocity to be seen at lower inclination. While this is consistent with the expected structure for the magnetically-controlled accretion flow in a polar or IP, it also might be explained by a non-magnetic model involving emitting material located out of the disk plane (e.g., 2003AJ....126.2473H).
- Several SW Sex stars (including DW UMa) are also VY Scl stars. The presence of a magnetic WD can suppress dwarf nova outbursts during the characteristic low states of the VY Scl stars (when the mass transfer rate is small enough that the disk instability mechanism is no longer quenched; 2002A&A...394..231H). We note, however, that (as described in 2002A&A...394..231H) the presence of a low mass (MWD < 0.4Msun) and/or very hot (TWD > 40,000 K) WD can also suppress dwarf nova outbursts during VY Scl low states. The system parameters for DW UMa (2003ApJ...583..437A) show that it does not satisfy the former condition (MWD ≈ 0.8Msun), but it does satisfy the latter (TWD = 50,000 K). Thus, a magnetic field is not necessarily required to suppress dwarf nova outbursts in DW UMa or other SW Sex stars with hot WDs. In addition, the analysis in 2002A&A...394..231H does not seem to have accounted for the fact that hotter WDs are larger (e.g., 1986A&A...154..125K), which (according to Equation 3 in 2002A&A...394..231H) would cause dwarf nova outbursts to be suppressed at a lower WD temperature. (Perhaps a more fundamental issue is that the low states of SW Sex stars may not have been well-enough observed to rule out the presence of dwarf nova outbursts. For example, the long-term optical light curve of DW UMa presented in 1993PASP..105..922H has only sparse sampling of the low state. The light curve shows several isolated data points during low states that are brighter by a magnitude or more compared to the typical low state brightness. These might correspond to poorly sampled outbursts.)
- (added on 30 Sep 2011) There has been substantial recent work (expanding on suggestions originally made several decades ago) on exploring the origin of viscosity in accretion disks by focusing on the magnetorotational (Balbus-Hawley) instability created by a dynamo-generated magnetic field produced by, and entangled in, the disk material itself. This scenario is also interesting from the point-of-view of considering a magnetic origin to the SW Sex syndrome, since it eliminates the necessity of having a magnetic WD (and, hence, the associated observational trappings of polars or intermediate polars), while retaining the potential for a magnetically-influenced accretion explanation for the SW Sex observational characteristics, as described above. Observational evidence points to the SW Sex stars having hot, geometrically large accretion disks fed by high mass transfer rates, which could produce optimum conditions necessary for the generation of a magnetic dynamo. Possibly the SW Sex stars, instead of being the intermediate polars with the weakest WD magnetic fields and highest mass transfer rates, are the high mass transfer rate CVs with non-magnetic WDs that have the strongest dynamo-generated disk magnetic fields.
For early work on this topic, see
For more recent work, see
Also see Hoard et al. 2003, Astronomical Journal, v.126, p. 2473 – 2003AJ....126.2473H.