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تسجيل مستخدم جديدRadio and optical observations of the possible AE Aqr twin, LAMOST J024048.51+195226.9
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Thorstensen (2020) recently argued that the cataclysmic variable (CV) LAMOST J024048.51+195226.9 may be a twin to the unique magnetic propeller system AE Aqr. If this is the case, two predictions are that it should display a short period white dwarf spin modulation, and that it should be a bright radio source. We obtained follow-up optical and radio observations of this CV, in order to see if this holds true. Our optical high-speed photometry does not reveal a white dwarf spin signal, but lacks the sensitivity to detect a modulation similar to the 33-s spin signal seen in AE Aqr. We detect the source in the radio, and measure a radio luminosity similar to that of AE Aqr and close to the highest so far reported for a CV. We also find good evidence for radio variability on a time scale of tens of minutes. Optical polarimetric observations produce no detection of linear or circular polarization. While we are not able to provide compelling evidence, our observations are all consistent with this object being a propeller system.
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AE Aqr objects are a class of cataclysmic variable stars in which the rapidly rotating magnetosphere of the white dwarf (WD) primary centrifugally expels most infalling gas before it can accrete onto the WD. The expulsion of the accretion flow via th
is magnetic propeller extracts angular momentum from the WD and produces large-amplitude, aperiodic flares in optical photometry. The eponymous AE Aqr is the only confirmed member of this class of object, but recently, Thorstensen (2020) discovered a candidate AE Aqr system: LAMOST J024048.51+195226.9. Using survey photometry, we measure a refined orbital period for this system and identify a shallow, previously unrecognized eclipse during which the systems frequent AE Aqr-like flaring episodes cease. A dedicated follow-up study is still necessary to test the proposed AE Aqr classification for LAMOST J024048.51+195226.9, but should it be confirmed, the eclipse of its flare-production region will offer a new means of studying the magnetic propeller phenomenon.
We present optical photometry of the cataclysmic variable LAMOST J024048.51+195226.9 taken with the high-speed, five-band CCD camera HiPERCAM on the 10.4 m Gran Telescopio Canarias (GTC). We detect pulsations originating from the spin of its white dw
arf, finding a spin period of 24.9328(38)s. The pulse amplitude is of the order of 0.2% in the g-band, below the detection limits of previous searches. This detection establishes LAMOST J024048.51+195226.9 as only the second white dwarf magnetic propeller system, a twin of its long-known predecessor, AE Aquarii. At 24.93s, the white dwarf in LAMOST J024048.51+195226.9 has the shortest known spin period of any cataclysmic variable star. The white dwarf must have a mass of at least 0.7MSun to sustain so short a period. The observed faintest u-band magnitude sets an upper limit on the white dwarfs temperature of ~25000K. The pulsation amplitudes measured in the five HiPERCAM filters are consistent with an accretion spot of ~30000K covering ~2% of the white dwarfs visible area, although much hotter and smaller spots cannot be ruled out.
We provide a summary of results, obtained from a multiwavelength (TeV gamma-ray, X-ray, UV, optical, and radio) campaign of observations of AE Aqr conducted in 2005 August 28-September 2, on the nature and correlation of the flux variations in the va
rious wavebands, the white dwarf spin evolution, the properties of the X-ray emission region, and the very low upper limits on the TeV gamma-ray flux.
We have developed a numerical MHD model of the propeller candidate star AE Aqr using axisymmetric magneto-hydrodynamic (MHD) simulations. We suggest that AE Aqr is an intermediate polar-type star, where the magnetic field is relatively weak and an ac
cretion disc may form around the white dwarf. The star is in the propeller regime, and many of its observational properties are determined by the disc-magnetosphere interaction. Comparisons of the characteristics of the observed versus modelled AE Aqr star show that the model can explain many observational properties of AE Aqr. In a representative model, the magnetic field of the star is Bapprox 3.3x10^5 G and the time averaged accretion rate in the disc is 5.5times 10^{16} g/s. Most of this matter is ejected into conically-shaped winds. The numerical model explains the rapid spin-down of AE Aqr through the outflow of angular momentum from the surface of the star to the wind, corona and disc. The energy budget in the outflows, 9x10^{33} erg/s, is sufficient for explaining the observed flaring radiation in different wavebands. The time scale of ejections into the wind matches the short time scale variability in the light curves of AE Aqr.
In the course of a project to study eclipsing binary stars in vinicity of the Sun, we found that the cooler component of LL Aqr is a solar twin candidate. This is the first known star with properties of a solar twin existing in a non-interacting ecli
psing binary, offering an excellent opportunity to fully characterise its physical properties with very high precision. We used extensive multi-band, archival photometry and the Super-WASP project and high-resolution spectroscopy obtained from the HARPS and CORALIE spectrographs. The spectra of both components were decomposed and a detailed LTE abundance analysis was performed. The light and radial velocity curves were simultanously analysed with the Wilson-Devinney code. The resulting highly precise stellar parameters were used for a detailed comparison with PARSEC, MESA, and GARSTEC stellar evolution models. LL Aqr consists of two main-sequence stars (F9 V + G3 V) with masses of M1 = 1.1949$pm$0.0007 and M2=1.0337$pm$0.0007 $M_odot$, radii R1 = 1.321$pm$0.006 and R2 = 1.002$pm$0.005 $R_odot$, temperatures T1=6080$pm$45 K and T2=5703$pm$50 K and solar chemical composition [M/H]=0.02$pm$0.05 dex. The absolute dimensions, radiative and photometric properties, and atmospheric abundances of the secondary are all fully consistent with being a solar twin. Both stars are cooler by about 3.5 $sigma$ or less metal abundant by 5$sigma$ than predicted by standard sets of stellar evolution models. When advanced modelling was performed, we found that full agreement with observations can only be obtained for values of the mixing length and envelope overshooting parameters that are hard to accept. The most reasonable and physically justified model fits found with MESA and GARSTEC codes still have discrepancies with observations but only at the level of 1$sigma$.
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