Identification of Orbital Eclipses in LAMOST J024048.51+195226.9, a Candidate AE Aqr-type Cataclysmic Variable Star
DDraft version October 2, 2020
Typeset using L A TEX
RNAAS style in AASTeX63
Identification of Orbital Eclipses in LAMOST J024048.51+195226.9,a Candidate AE Aqr-type Cataclysmic Variable Star
Colin Littlefield
1, 2 and Peter Garnavich Department of Physics, University of Notre Dame, Notre Dame, IN 46556 Department of Astronomy, University of Washington, Seattle, WA
ABSTRACTAE Aqr objects are a class of cataclysmic variable stars in which the rapidly ro-tating magnetosphere of the white dwarf (WD) primary centrifugally expels mostinfalling gas before it can accrete onto the WD. The expulsion of the accretion flowvia this “magnetic propeller” extracts angular momentum from the WD and produceslarge-amplitude, aperiodic flares in optical photometry. The eponymous AE Aqr isthe only confirmed member of this class of object, but recently, Thorstensen (2020)discovered a candidate AE Aqr system: LAMOST J024048.51+195226.9. Using sur-vey photometry, we measure a refined orbital period for this system and identify ashallow, previously unrecognized eclipse during which the system’s frequent AE Aqr-like flaring episodes cease. A dedicated follow-up study is still necessary to test theproposed AE Aqr classification for LAMOST J024048.51+195226.9, but should it beconfirmed, the eclipse of its flare-production region will offer a new means of studyingthe magnetic propeller phenomenon.INTRODUCTIONAE Aqr objects are among the rarest subtypes of cataclysmic variable stars, and atpresent, only one has been confirmed. These short-period binaries contain a rapidlyrotating, magnetized white dwarf (WD) and a late-type companion star that losesmass via Roche lobe overflow. The defining feature of this class of object is a “mag-netic propeller,” a process in which the WD’s magnetosphere centrifugally expelsnearly all of the infalling matter that comes from the donor star, greatly inhibitingaccretion onto the WD (Eracleous & Horne 1996; Wynn et al. 1997). The expulsionof the accretion flow extracts angular momentum from the WD’s rotation, causingits spin period to gradually lengthen.The only known AE Aqr star is AE Aqr itself, and its singular nature has motivateda voluminous body of observational and theoretical work; for reviews of AE Aqrspecifically, see Welsh (1999) and Meintjes et al. (2015). The rotational period of theWD in AE Aqr is a mere 33 sec (Patterson 1979) and gradually increasing (de Jager
Corresponding author: Colin Littlefi[email protected] a r X i v : . [ a s t r o - ph . S R ] S e p Littlefield & Garnavich et al. 1994), as expected for the magnetic-propeller scenario. AE Aqr is famous forits erratic, large-amplitude flares, which are thought to occur when the accretion flowfrom the secondary is shocked—either when it encounters the WD’s magnetosphere(Eracleous & Horne 1996) or when blobs of expelled matter collide (Welsh et al. 1998).Thorstensen (2020) reported a candidate AE Aqr object, LAMOSTJ024048.51+195226.9, with a 7.34-hour orbital period. He noted several key prop-erties that resembled AE Aqr, including the presence of large-amplitude flares,the absence of He II emission, and unusually weak He I emission. Moreover, itsCatalina Real-Time Transient Survey (CRTS; Drake et al. 2009) light curve (Fig. 1in Thorstensen 2020) looks very similar to that of AE Aqr (Fig. 6 in ˇSimon2020), with one curious exception: a shallow dip between 0 . < φ orb < .
0, where φ orb = 1 . σ uncertainty on the final digit. Combining this new period with the epoch of in-ferior conjunction from Table 3 in Thorstensen (2020), we obtain an updated orbitalephemeris of T conj [ BJ D ] = 2458836 . . × E, (1)where T conj is the predicted Barycentric Julian Date (BJD) in Barycentric DynamicalTime (TDB) of inferior conjunction and E is the integer cycle count. In Fig. 1, we present the CRTS and ASAS-SN light curves, phased with Eq. 1. TheCRTS photometry is unfiltered and therefore dominated by the contribution of theM1.5 secondary, the ellipsoidal variations of which are readily apparent in the lightcurve (Thorstensen 2020). With the refined period, the dip that Thorstensen (2020)observed near orbital phase 0.9 shifts in phase and becomes nearly centered on thesecondary’s inferior conjunction—exactly as would be expected of an eclipse of theWD by the secondary. Moreover, in the combined CRTS/ASAS-SN dataset, not a We converted the time standard of the Thorstensen (2020) epoch of inferior conjunction from BJDin Coordinated Universal Time to BJD in TDB (Eastman et al. 2010). clipses in a candidate AE Aqr object m a g ( C V - b a nd ) CRTS orbital phase V , g m a g ASAS-SN eclipseg-bandV-band
Figure 1.
CRTS (top panel) and ASAS-SN (bottom panel) light curves of LAMOSTJ024048.51+195226.9, phased to Eq. 1. The CV band for the CRTS data refers to anunfiltered bandpass using a Johnson V zeropoint. ASAS-SN non-detections have beenomitted for clarity, as they do not meaningfully constrain the eclipse depth. The shadedregion indicates an interval in the CRTS light curve, centered on inferior conjunction, duringwhich the high-amplitude flares cease and the low-amplitude flickering is suppressed. Weinterpret this as an eclipse of the flare-production site. single flare is present during the dip, making this the only part of the orbit to lackthem; in contrast, the CRTS light curve of AE Aqr (Fig. 6 in ˇSimon 2020) showsflares at all orbital phases. Based on these arguments, we interpret the dip as aneclipse by the secondary. Although the ∼ Littlefield & Garnavich
Of the observational data analyzed by Thorstensen (2020), only the CRTS lightcurve has a sufficiently long baseline to be significantly impacted by our revisedephemeris. The CRTS data were obtained between 2005 and 2013, whereas the epochof inferior conjunction was measured in 2019.96. In contrast, the Thorstensen (2020)time-series photometry and spectroscopy (his Figs. 3 and 4) were obtained within ∼ λ α line (particularly its high-velocity wings). This behavior can be easily ex-plained by an eclipse of the corresponding line-forming regions. Likewise, Fig. 4 inThorstensen (2020) shows that in several different photometric time series, a shallowdip occurs at inferior conjunction. The morphology of that dip experiences significantorbit-to-orbit variation, suggesting that the eclipsed structure changes appreciably onorbital timescales.If we assume, for the sake of argument, that LAMOST J024048.51+195226.9 is amember of the AE Aqr club, the eclipses will provide new insight into the magneticpropeller phenomenon. For example, the eclipse of the flare-production region im-plies that it is relatively close to the WD, likely favoring models in which flares occurwhen blobs of infalling matter are shocked as they encounter the magnetosphere (Er-acleous & Horne 1996), as opposed to scenarios in which collisions between expelledblobs at large distances from the WD are the culprit (Welsh et al. 1998). Althoughthese prospects are enticing, follow-up observations are still necessary to confirm thatLAMOST J024048.51+195226.9 is an AE Aqr object, and the detection of a veryshort spin period will be a particularly critical test of that hypothesis (Thorstensen2020). ACKNOWLEDGMENTSWe thank John Thorstensen and Paula Szkody for helpful discussions.REFERENCES de Jager, O. C., Meintjes, P. J.,O’Donoghue, D., & Robinson, E. L.1994, MNRAS, 267, 577,doi: 10.1093/mnras/267.3.577Drake, A. J., Djorgovski, S. G., Mahabal,A., et al. 2009, ApJ, 696, 870,doi: 10.1088/0004-637X/696/1/870 Drake, A. J., Graham, M. J., Djorgovski,S. G., et al. 2014, ApJS, 213, 9,doi: 10.1088/0067-0049/213/1/9Eastman, J., Siverd, R., & Gaudi, B. S.2010, PASP, 122, 935,doi: 10.1086/655938 clipses in a candidate AE Aqr object5