There have been relatively few published long-duration, uninterrupted light curves of magnetic cataclysmic variable stars in which the accreting white dwarf’s rotational frequency is slightly desynchronized from the binary orbital frequency. We report Kepler K2 and TESS observations of two such systems. The first, SDSS J084617.11+245344.1, was observed by the Kepler spacecraft for 80 days during Campaign 16 of the K2 mission, and we identify it as a new asynchronous polar with a likely 4.64 hr orbital period. This is significantly longer than any other asynchronous polar, as well as all but several synchronous polars. Its spin and orbital periods beat against each other to produce a conspicuous 6.77-day beat period, across which the system’s accretion geometry gradually changes. The second system in this study, Paloma, was observed by TESS for one sector and was already known to be asynchronous. Until now, there had been an ambiguity in its spin period, but the TESS power spectrum pinpoints a spin period of 2.27 hr. During the resulting 0.7-day spin–orbit beat period, the light curve phased on the spin modulation alternates between being single and double humped. We explore two possible explanations for this behavior: the accretion flow being diverted from one of the poles for part of the beat cycle, or an eclipse of the emitting region responsible for the second hump.
Since its discovery in 1995, V2400 Ophiuchi (V2400 Oph) has stood apart from most known intermediate polar cataclysmic variables due to its proposed magnetic field strength (9–27 MG) and diskless accretion. To date, the exact accretion mechanism of the system is still unknown, and standard accretion models fail to accurately predict the peculiar behavior of its light curve. We present the K2 Campaign 11 light curve of V2400 Oph recording 74.19 days of photometric data cadenced at 1 minute. The light curve is dominated by aperiodic flickering and quasiperiodic oscillations, which make the beat and spin signals inconspicuous on short timescales. Notably, a log–log full power spectrum shows a break frequency at ∼102cycles d−1similar to some disk-fed systems. Through power-spectral analysis, the beat and spin periods are measured as 1003.4 ± 0.2 s and 927.7 ± 0.1 s, respectively. A power spectrum of the entire K2 observation demonstrates beat period dominance. However, time-resolved power spectra reveal a strong dependence between observation length and the dominant frequency of the light curve. For short observations (2–12 hr) the beat, spin, or first beat harmonic can be observed as the dominant periodic signal. Such incoherence and variability indicate a dynamical accretion system more complex than current intermediate polar theories can explain. We propose that a diamagnetic blob accretion model may serve as a plausible explanation for the accretion mechanism.
more » « less- NSF-PAR ID:
- 10362965
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astronomical Journal
- Volume:
- 163
- Issue:
- 1
- ISSN:
- 0004-6256
- Format(s):
- Medium: X Size: Article No. 4
- Size(s):
- ["Article No. 4"]
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract We present the Transiting Exoplanet Surveying Satellite light curve of the intermediate polar YY Draconis (YY Dra, also known as DO Dra). The power spectrum indicates that while there is stream-fed accretion for most of the observational period, there is a day-long, flat-bottomed low state at the beginning of 2020 during which the only periodic signal is ellipsoidal variation and there is no appreciable flickering. We interpret this low state to be a complete cessation of accretion, a phenomenon that has been observed only once before in an intermediate polar. Simultaneous ground-based observations of this faint state establish that when accretion is negligible, YY Dra fades to
g = 17.37 ± 0.12, which we infer to be the magnitude of the combined photospheric contributions of the white dwarf and its red dwarf companion. Using survey photometry, we identify additional low states in 2018–2019 during which YY Dra repeatedly fades to—but never below—this threshold. This implies relatively frequent cessations in accretion. Spectroscopic observations during future episodes of negligible accretion can be used to directly measure the field strength of the white dwarf by Zeeman splitting. Separately, we search newly available catalogs of variable stars in an attempt to resolve the long-standing dispute over the proper identifier of this system. -
more » « less
Abstract We present time-series photometry during the early decline phase of the extremely fast nova V1674 Herculis. The 2021 light curve showed periodic signals at 0.152921(3) days and 501.486(5) s, which we interpret as respectively the orbital and white dwarf spin periods in the underlying binary. We also detected a sideband signal at the difference frequency between these two clocks. During the first 15 days of outburst, the spin period appears to have increased by 0.014(1)%. This increase probably arose from the sudden loss of high-angular-momentum gas (“the nova explosion”) from the rotating, magnetic white dwarf. Both periodic signals appeared remarkably early in the outburst, which we attribute to the extreme speed with which the nova evolved (and became transparent to radiation from the inner binary). After that very fast initial period increase of 71 ms, the period subsequently decreased—at 182(18) ms yr−1in 2021, and 88(18) ms yr−1in 2022. These rates are ∼100× faster than typically seen in intermediate polars. This could be due to high accretion torques from very high mass-transfer rates, which might be common when low-mass donor stars are strongly irradiated by a nova outburst.
-
more » « less
Abstract Only a tiny fraction ∼1% of stellar tidal disruption events (TDEs) generate powerful relativistic jets evidenced by luminous hard X-ray and radio emissions. We propose that a key property responsible for both this surprisingly low rate and a variety of other observations is the typically large misalignment
ψ between the orbital plane of the star and the spin axis of the supermassive black hole (SMBH). Such misaligned disk/jet systems undergo Lense–Thirring precession together about the SMBH spin axis. We find that TDE disks precess sufficiently rapidly that winds from the accretion disk will encase the system on large scales in a quasi-spherical outflow. We derive the critical jet efficiencyη >η critfor both aligned and misaligned precessing jets to successfully escape from the disk wind ejecta. Asη critis higher for precessing jets, less powerful jets only escape after alignment with the SMBH spin. Alignment can occur through magneto-spin or hydrodynamic mechanisms, which we estimate occur on typical timescales of weeks and years, respectively. The dominant mechanism depends onη and the orbital penetration factorβ . Hence, depending only on the intrinsic parameters of the event {ψ ,η ,β }, we propose that each TDE jet can either escape prior to alignment, thus exhibiting an erratic X-ray light curve and two-component radio afterglow (e.g., Swift J1644+57), or escape after alignment. Relatively rapid magneto-spin alignments produce relativistic jets exhibiting X-ray power-law decay and bright afterglows (e.g., AT2022cmc), while long hydrodynamic alignments give rise to late jet escape and delayed radio flares (e.g., AT2018hyz). -
more » « less
Abstract The eclipsing binary IT Librae is an unusual system of two B-type stars that is situated about 1 kpc above the Galactic plane. The binary was probably ejected from its birthplace in the disk, but the implied time of flight to its current location exceeds the evolutionary lifetime of the primary star. Here we present a study of new high-dispersion spectroscopy and an exquisite light curve from the Kepler K2 mission in order to determine the system properties and resolve the timescale discrepancy. We derive a revised spectroscopic orbit from radial-velocity measurements and determine the component effective temperatures through comparison of reconstructed and model spectra (
T 1= 23.8 ± 1.8 kK,T 2= 13.7 ± 2.5 kK). We use the Eclipsing Light Curve code to model the K2 light curve, and from the inclination of the fit we derive the component masses (M 1= 9.6 ± 0.6M ⊙,M 2= 4.2 ± 0.2M ⊙) and mean radii (R 1= 6.06 ± 0.16R ⊙,R 2= 5.38 ± 0.14R ⊙). The secondary star is overluminous for its mass and appears to fill its Roche lobe. This indicates that IT Librae is a post-mass-transfer system in which the current secondary was the mass donor star. The current primary star was rejuvenated by mass accretion, and its evolutionary age corresponds to the time since the mass transfer stage. Consequently, the true age of the binary is larger than the ejection time of flight, thus resolving the timescale discrepancy.
