Confirmation of a Second Propeller: A High-inclination Twin of AE Aquarii

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 periodremains 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. Unified Astronomy Thesaurus concepts: Cataclysmic variable stars (203); Magnetic stars (995); White dwarf stars (1799); DQ Herculis stars (407)

Found: a rapidly spinning white dwarf in LAMOST J024048.51+195226.9

“…Two subsequent studies have added additional weight to the connection between J0240+1952 and AE Aqr. Pretorius et al (2021) detected radio emission, with a radio luminosity close to the highest measured of any CV, higher even than AE Aqr, while Garnavich et al (2021) found that the Balmer emission lines broadened up to ±3000 km/s during flares, and showed that a P Cygni-like component in H𝛼, also seen by Thorstensen (2020), was consistent with Wynn et al (1997)'s magnetic propeller model.…”

“…Pretorius et al (2021) searched for optical pulsations down to a period of 10 s, but found no signals above their detection threshold of 1%. Finally, and most stringently of all, Garnavich et al (2021) presented 𝑔 and 𝑖-band photometry taken with the Hale 5 m telescope and Caltech HIghspeed Multi-color camERA (CHIMERA) which had the cadence and the signal-to-noise to detect spin periods with amplitudes as low as 4 mmag (0.43%) in the 𝑔-band over a period range of 6.3 to 85 s, but again found nothing of significance.…”

VLA Observations of the AE Aqr-type Cataclysmic Variable LAMOST J024048.51+195226.9

“…This means that J0240+19 is a high inclination system and explains why the He I λ6678 Å emission line briefly disappears at orbital phase 0. Additional photometric observations by Garnavich et al (2021) confirmed the high inclination of J0240+19, but did not detect the WD's spin period. The spin period was finally found by Pelisoli et al (2021) to be 24.9328(38) s with an amplitude of 0.2% in the g-band, which is below the detection limits of the previous searches.…”

“…The spectrum of J0240+19 obtained during the first observation is shown in Figure 1. This observation occured between orbital phases 0.70-0.78 using the ephemeris of Garnavich et al (2021). The average flux density and standard deviations for each frequency band are shown in black.…”

“…Unfortunately, J0240+19 was even weaker during the second observation with an average flux density of ≈ 100µJy. Such a low flux density precludes any attempt at searching for, or detecting, a spin period as short as 30 s. We found that the best compromise between time resolution and signal-to-noise ratio is an integration time of 265 s. The observation occurred just after inferior conjunction between orbital phases 0.08-0.32 using the ephemeris of Garnavich et al (2021). The resulting left and right circular polarization light curves are shown in Figure 2.…”