Abstract
Abstract For decades, AE Aquarii (AE Aqr) has been the only cataclysmic variable star known to contain a magnetic propeller: a persistent outflow whose expulsion from the binary is powered by the spin-down of the rapidly rotating, magnetized white dwarf. In 2020, LAMOST J024048.51+195226.9 (J0240) was identified as a candidate eclipsing AE Aqr object, and we present three epochs of time-series spectroscopy that strongly support this hypothesis. We show that, during the photometric flares noted by Thorstensen, the Balmer and He i emission lines reach velocities of ∼3000 km s−1, well in excess of what is observed in normal cataclysmic variables. This is, however, consistent with the high-velocity emission seen in flares from AE Aqr. Additionally, we confirm beyond doubt that J0240 is a deeply eclipsing system. The flaring continuum, He i and much of the Balmer emission likely originate close to the WD because they disappear during the eclipse that is centered on inferior conjunction of the secondary star. The fraction of the Balmer emission remaining visible during eclipse is likely produced in the extended outflow. Most enticingly of all, this outflow produces a narrow P Cygni absorption component for nearly half of the orbit, and we demonstrate that this scenario closely matches the outflow kinematics predicted by Wynn et al. While an important piece of evidence for the magnetic-propeller hypothesis—a rapid WD spin period—remains elusive, our spectra provide compelling support for the existence of a propeller-driven outflow viewed nearly edge-on, enabling a new means of rigorously testing theories of the propeller phenomenon.
Highlights
Intermediate polars (IPs, aka DQ Herculis stars) are a type of cataclysmic variable star (CV) consisting of a cool, nondegenerate companion transferring mass to a magnetized white dwarf (WD) whose spin period is significantly shorter than the binary orbit (Patterson 1994)
If the magnetized WD is spinning sufficiently fast, donated gas may be ejected from the system via a “magnetic propeller” mode (Wynn et al 1997), in which the rapidly rotating magnetosphere of the WD acts as a centrifugal barrier that inhibits accretion onto the WD
A 33 s oscillation is seen in AE Aquarii (AE Aqr) associated with the spin period of its WD; our fast photometry does not detect a coherent oscillation from J0240
Summary
Intermediate polars (IPs, aka DQ Herculis stars) are a type of cataclysmic variable star (CV) consisting of a cool, nondegenerate companion transferring mass to a magnetized white dwarf (WD) whose spin period is significantly shorter than the binary orbit (Patterson 1994). The only confirmed IP in a propeller mode has been AE Aqr (Eracleous & Horne 1996; Wynn et al 1997), which displays unique photometric and spectroscopic properties, including a WD spin period of 33 s (Patterson 1979). Meintjes et al (2015) review the major observational and theoretical studies of AE Aqr. Several additional IPs with WD rotation periods near 30 s have recently been identified (e.g., Ashley et al 2020; Lopes de Oliveira et al 2020), but their magnetic fields are apparently insufficient to power an AE Aqr–like propeller. Thorstensen (2020) pointed to the flaring light curves and spectral properties of LAMOST J024048.51 +195226.9 (J0240 hereafter) as being similar to those of AE Aqr, and suggested that it might be only the second IP in the propeller mode. We present fast-cadence, time-resolved spectroscopy and photometry to compare J0240 with the observational properties of original propeller AE Aqr
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