Search

We analyze 15 years of eclipse timings of the polar V808 Aur. The rapid ingress/egress of the white dwarf and bright accretion region provide timings as precise as a few tenths of a second for rapid cadence photometric data. We find that between 2015 and 2018, the eclipse timings deviated from a linear ephemeris by more than 30 s. The rapid timing change is consistent with the periastron passage of a planet in an eccentric orbit about the polar. The best fit orbital period is 11$\pm 1$ yr and we estimate a projected mass of $Msin(i)=6.8\pm 0.7$ Jupiter masses. We also show that the eclipse timings are correlated with the brightness of the polar with a slope of 1.1 s/mag. This is likely due to the change in the geometry of the accretion curtains as a function of the mass transfer rate in the polar. While an eccentric planet offers an excellent explanation to the available eclipse data for V808 Aur, proposed planetary systems in other eclipsing polars have often struggled to accurately predict future eclipse timings., Comment: 9 pages, 6 figures, tab2.txt is the machine-readable table for Table 2 in the Appendix

Analysis of AR Sco optical light curves spanning nine years show a secular change in the relative amplitudes of the beat pulse pairs generated by the two magnetic poles of its rotating white dwarf. Recent photometry now shows that the primary and secondary beat pulses have similar amplitudes, while in 2015 the primary pulse was approximately twice that of the secondary peak. The equalization in the beat pulse amplitudes is also seen in the linearly polarized flux. This rapid evolution is consistent with precession of the white dwarf spin axis. The observations imply that the pulse amplitudes cycle over a period of $\gtrsim 40$ yrs, but that the upper limit is currently poorly constrained. If precession is the mechanism driving the evolution, then over the next 10 years the ratio of the beat pulse amplitudes will reach a maximum followed by a return to asymmetric beat pulses., Comment: 5 pages, 4 figures, 1 table

We investigate K2BS5, an optical transient that we identified in Campaign 13 of the Kepler/K2 archives by the "K2 Background Survey", and classify it as a new SU UMa-type dwarf nova. Using the light curve generated from Kepler's long-cadence observation mode, we analyze the dwarf nova during quiescence and superoutburst. Following 20 days of quiescence at the start of the observation, the system entered a superoutburst lasting 12 days, after which it experienced at least one rebrightening. K2BS5 clearly meets the criteria for an SU UMa star, but at the peak of the superoutburst, it also shows double-wave oscillations consistent with the spectroscopic orbital period, a phenomenon that closely resembles early superhumps in WZ Sge stars. While we do not classify K2BS5 as a WZ Sge system, we discuss how this phenomenon could complicate efforts to use the suspected detection of early superhumps to distinguish SU UMa-type dwarf novae from the recently recognized class of long-orbital-period WZ Sge systems., Comment: 11 pages, 7 figures. Accepted for publication in the Astronomical Journal

Nebular-phase observations of peculiar Type Ia supernovae (SNe Ia) provide important constraints on progenitor scenarios and explosion dynamics for both these rare SNe and the more common, cosmologically useful SNe Ia. We present observations from an extensive ground-based and space-based follow-up campaign to characterize SN 2022pul, a "super-Chandrasekhar" mass SN Ia (alternatively "03fg-like" SN), from before peak brightness to well into the nebular phase across optical to mid-infrared (MIR) wavelengths. The early rise of the light curve is atypical, exhibiting two distinct components, consistent with SN Ia ejecta interacting with dense carbon-oxygen rich circumstellar material (CSM). In the optical, SN 2022pul is most similar to SN 2012dn, having a low estimated peak luminosity ($M_{B}=-18.9$ mag) and high photospheric velocity relative to other 03fg-like SNe. In the nebular phase, SN 2022pul adds to the increasing diversity of the 03fg-like subclass. From 168 to 336 days after peak $B$-band brightness, SN 2022pul exhibits asymmetric and narrow emission from [O I] $\lambda\lambda 6300,\ 6364$ (${\rm FWHM} \approx 2{,}000$ km s$^{-1}$), strong, broad emission from [Ca II] $\lambda\lambda 7291,\ 7323$ (${\rm FWHM} \approx 7{,}300$ km s$^{-1}$), and a rapid Fe III to Fe II ionization change. Finally, we present the first-ever optical-to-mid-infrared (MIR) nebular spectrum of an 03fg-like SN Ia using data from JWST. In the MIR, strong lines of neon and argon, weak emission from stable nickel, and strong thermal dust emission (with $T \approx 500$ K), combined with prominent [O I] in the optical, suggest that SN 2022pul was produced by a white dwarf merger within carbon/oxygen-rich CSM., Comment: 23 pages, 11 figures, submitted to ApJ

We present an analysis of ground-based and JWST observations of SN~2022pul, a peculiar "03fg-like" (or "super-Chandrasekhar") Type Ia supernova (SN Ia), in the nebular phase at 338d post explosion. Our combined spectrum continuously covers 0.4--14 $\mu$m and includes the first mid-infrared spectrum of an 03fg-like SN Ia. Compared to normal SN Ia 2021aefx, SN 2022pul exhibits a lower mean ionization state, asymmetric emission-line profiles, stronger emission from the intermediate-mass elements (IMEs) argon and calcium, weaker emission from iron-group elements (IGEs), and the first unambiguous detection of neon in a SN Ia. Strong, broad, centrally peaked [Ne II] line at 12.81 $\mu$m was previously predicted as a hallmark of "violent merger'' SN Ia models, where dynamical interaction between two sub-$M_{ch}$ white dwarfs (WDs) causes disruption of the lower mass WD and detonation of the other. The violent merger scenario was already a leading hypothesis for 03fg-like SNe Ia; in SN 2022pul it can explain the large-scale ejecta asymmetries seen between the IMEs and IGEs and the central location of narrow oxygen and broad neon. We modify extant models to add clumping of the ejecta to better reproduce the optical iron emission, and add mass in the innermost region ($< 2000$ km s$^{-1}$) to account for the observed narrow [O I]~$\lambda\lambda6300$, 6364 emission. A violent WD-WD merger explains many of the observations of SN 2022pul, and our results favor this model interpretation for the subclass of 03fg-like SN Ia., Comment: 20 pages, 10 figures, published in ApJ

When the accreting white dwarf in a magnetic cataclysmic variable star (mCV) has a field strength in excess of 10 MG, it is expected to synchronize its rotational frequency to the binary orbit frequency, particularly at small binary separations, due to the steep radial dependence of the magnetic field. We report the discovery of an mCV (SDSS J134441.83+204408.3; hereafter, J1344) that defies this expectation by displaying asynchronous rotation ($P_{spin}/P_{orb} = 0.893$) in spite of a high surface field strength (B=56 MG) and a short orbital period (114 min). Previously misidentified as a synchronously rotating mCV, J1344 was observed by TESS during sector 50, and the resulting power spectrum shows distinct spin and orbital frequencies, along with various sidebands and harmonics. Although there are several other asynchronous mCVs at short orbital periods, the presence of cyclotron humps in J1344's SDSS spectrum makes it possible to directly measure the field strength in the cyclotron-emitting region; a previously study estimated 65 MG based on its identification of two cyclotron humps, but we revise this to 56$\pm$2 MG based on the detection of a third hump and on our modeling of the cyclotron spectrum. Short-period mCVs with field strengths above 10 MG are normally expected to be synchronous, so the highly asynchronous rotation in J1344 presents an interesting challenge for theoretical studies of spin-period evolution., Comment: Accepted for publication in ApJL

We present JWST near- and mid-infrared spectroscopic observations of the nearby normal Type Ia supernova SN 2021aefx in the nebular phase at $+255$ days past maximum light. Our Near Infrared Spectrograph (NIRSpec) and Mid Infrared Instrument (MIRI) observations, combined with ground-based optical data from the South African Large Telescope (SALT), constitute the first complete optical $+$ NIR $+$ MIR nebular SN Ia spectrum covering 0.3$-$14 $\mu$m. This spectrum unveils the previously unobserved 2.5$-$5 $\mu$m region, revealing strong nebular iron and stable nickel emission, indicative of high-density burning that can constrain the progenitor mass. The data show a significant improvement in sensitivity and resolution compared to previous Spitzer MIR data. We identify numerous NIR and MIR nebular emission lines from iron-group elements and as well as lines from the intermediate-mass element argon. The argon lines extend to higher velocities than the iron-group elements, suggesting stratified ejecta that are a hallmark of delayed-detonation or double-detonation SN Ia models. We present fits to simple geometric line profiles to features beyond 1.2 $\mu$m and find that most lines are consistent with Gaussian or spherical emission distributions, while the [Ar III] 8.99 $\mu$m line has a distinctively flat-topped profile indicating a thick spherical shell of emission. Using our line profile fits, we investigate the emissivity structure of SN 2021aefx and measure kinematic properties. Continued observations of SN 2021aefx and other SNe Ia with JWST will be transformative to the study of SN Ia composition, ionization structure, density, and temperature, and will provide important constraints on SN Ia progenitor and explosion models., Comment: published in ApJ Letters, 17 pages, 12 figures

FO Aqr is a bright intermediate polar that has long displayed large amplitude photometric variations corresponding to the 20.9 min spin period of its white dwarf. Between 2016 and 2020, the system suffered a series of unprecedented low-states, but recent data shows that it has now recovered to its normal optical luminosity. We compare the light curves obtained by K2/Kepler in 2014 with photometry from the TESS mission obtained in 2021. We find that the spin pulse that had been the dominant feature of the light curve in 2014 is now weak over the second half the binary orbit and that a beat pulse is enhanced in the TESS photometry. Variations at approximately twice the spin frequency are now seen over the second half of the orbit. These photometric properties may be the new normal for FO Aqr now that its white dwarf has begun to spin down., Comment: 4 pages, 1 figure (in 4 parts). Accepted for publication in the RNAAS

We present TESS photometry of the asynchronous polar BY Cam, which undergoes a beat-cycle between the 199.384-min white dwarf (WD) spin period and the 201.244-min orbital period. This results in changes in the flow of matter onto the WD. The TESS light curve covers 92% of the beat cycle once and 71% of the beat cycle twice. The strongest photometric signal, at 197.560-min, is ascribed to a side-band period. During times of light-curve stability, the photometry modulates at the spin frequency, supporting our WD spin-period identification. Both one-pole and two-pole accretion configurations repeat from one beat cycle to the next with clear and repeatable beat-phase dependent intensity variations. To explain these, we propose the operation of a magnetic valve at L1. The magnetic valve modulates the mass-transfer rate, as evidenced by a factor of 5 variation in orbital-averaged intensity, over the course of the beat cycle in a repeatable manner. The accretion stream threading distance from the WD is also modulated at the beat-period, because of the variation of the WD magnetic field with respect to the stream and because of changes in the mass transfer rate due to the operation of the magnetic valve. Changes in the threading distance result in significant shifts in the position of accreting spots around the beat cycle. As a consequence, only the faintest photometric minima allow for an accurate ephemeris determination. Three regions on the white dwarf appear to receive most of the accretion flow, suggestive of a complex WD magnetic field.

We analyze two sectors of TESS photometry of the nova-like cataclysmic variable star V533 Her. We detect a periodicity consistent with the binary orbital period and estimate a revised value of 3.53709(2) hr. We also detect a strong signal near a period of 3.8 h that we associate with positive superhumps. The superhump frequency varies over the TESS observations with the fractional difference between the superhump and orbital periods, $\epsilon$, ranging between $0.055\le \epsilon \le 0.080$. The superhump amplitude is correlated with its frequency such that the amplitude increases as $\epsilon$ decreases. Positive superhumps result from an instability that generates an eccentric accretion disk and $\epsilon$ is a measure of the disk precession rate in the binary rest frame. The observed correlation implies that as the disk precession rate slows, the disk eccentricity increases., Comment: 4 pages, 1 figure, accepted for publication in the RNAAS

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 (asynchronous polars). 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 h 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 d 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 h. During the resulting 0.7 d 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., Comment: Submitted to AJ and reviewed favorably. Comments are welcome

We compare infrared surface brightness fluctuation (IR SBF) distances measured in galaxies that have hosted type Ia supernovae (SNIa) to distances estimated from SNIa light curve fits. We show that the properties of SNIa found in IR SBF hosts are very different from those exploding in Cepheid calibrators, therefore, this is a direct test of systematic uncertainties on estimation of the Hubble constant (Ho) using supernovae. The IR SBF results from Jensen et al. (2021; arXiv:2105.08299) provide a large and uniformly measured sample of IR SBF distances which we directly compare with distances to 25 SNIa host galaxies. We divide the Hubble flow SNIa into sub-samples that best match the divergent supernova properties seen in the IR SBF hosts and Cepheid hosts. We further divide the SNIa into a sample with light curve widths and host masses that are congruent to those found in the SBF-calibrated hosts. We refit the light curve stretch and color correlations with luminosity, and use these revised parameters to calibrate the Hubble flow supernovae with IR SBF calibrators. Relative to the Hubble flow, the average calibrator distance moduli vary by 0.03mag depending on the SNIa subsamples examined and this adds a 1.8% systematic uncertainty to our Hubble constant estimate. Based on the IR SBF calibrators, Ho=74.6$\pm$0.9(stat)$\pm$ 2.7(syst) km/s/Mpc, which is consistent with the Hubble constant derived from supernovae calibrated from Cepheid variables. We conclude that IR SBF provides reliable calibration of SNIa with a precision comparable to Cepheid calibrators, and with a significant saving in telescope time., Comment: 18 pages, 10 figures, 4 tables, accepted for publication in the Astrophysical Journal

Few published ultraviolet (UV) spectra exist for stripped-envelope supernovae, and none to date for broad-lined Type Ic supernovae (SN Ic-bl). These objects have extremely high ejecta velocities and are the only supernova type directly linked to gamma-ray bursts (GRBs). Here we present two epochs of HST/STIS spectra of the SN Ic-bl 2014ad, the first UV spectra for this class. We supplement this with 26 new epochs of ground-based optical spectra, augmenting a rich spectral time series. The UV spectra do not show strong features and are consistent with broadened versions of other SN Ic spectra observed in the UV. We measure Fe II 5169 Angstrom velocities and show that SN 2014ad has even higher ejecta velocities than most SNe Ic both with and without observed GRBs. We construct models of the SN 2014ad UV+optical spectra using TARDIS, a 1D Monte-Carlo radiative-transfer spectral synthesis code. The models fit the data well at multiple epochs in the optical but underestimate the flux in the UV, likely due to simplifying assumptions. We find that high densities at high velocities are needed to reproduce the spectra, with $\sim$3 M$_\odot$ of material at $v >$ 22,000 km s$^{-1}$, assuming spherical symmetry. Our nebular line fits suggest a steep density profile at low velocities. Together, these results imply a higher total ejecta mass than estimated from previous light curve analysis and expected from theory. This may be reconciled by a flattening of the density profile at low velocity and extra emission near the center of the ejecta., Comment: 26 pages, 14 figures, accepted in ApJ

We present the Transiting Exoplanet Surveying Satellite (TESS) 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 $g=17.37\pm0.12$, 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., Comment: 12 pages, 4 figures, submitted to the Astronomical Journal and reviewed favorably

Magnetically gated accretion has emerged as a proposed mechanism for producing extremely short, repetitive bursts of accretion onto magnetized white dwarfs in intermediate polars (IPs), but this phenomenon has not been detected previously in a confirmed IP. We report the 27-day TESS light curve of V1025 Cen, an IP that shows a remarkable series of twelve bursts of accretion, each lasting for less than six hours. The extreme brevity of the bursts and their short recurrence times (~1-3 days) are incompatible with the dwarf-nova instability, but they are natural consequences of the magnetic gating mechanism developed by Spruit & Taam to explain the Type II bursts of the accreting neutron star known as the Rapid Burster. In this model, the accretion flow piles up at the magnetospheric boundary and presses inward until it couples with the star's magnetic field, producing an abrupt burst of accretion. After each burst, the reservoir of matter at the edge of the magnetosphere is replenished, leading to cyclical bursts of accretion. A pair of recent studies applied this instability to the suspected IPs MV Lyr and TW Pic, but the magnetic nature of these two systems has not been independently confirmed. In contrast, previous studies have unambiguously established the white dwarf in V1025 Cen to be significantly magnetized. The detection of magnetically gated bursts in a confirmed IP therefore validates the extension of the Spruit & Taam instability to magnetized white dwarfs., Comment: Accepted for publication in ApJL

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 disk-less 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 lightcurve. 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 quasi-periodic oscillations, which make the beat and spin signals inconspicuous on short timescales. Notably, a log-log full power spectrum shows a break frequency at $\sim10^2$ cycles~d$^{-1}$ similar to some disk-fed systems. Through power spectral analysis, the beat and spin periods are measured as $1003.4\pm0.2$ seconds and $ 927.7\pm 0.1$ seconds respectively. A power spectrum of the entire K2 observation demonstrates beat period dominance. However, time-resolved power spectra reveals a strong dependence between observation length and the dominant frequency of the light curve. For short observations (2-12 hrs) 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., Comment: 10 pages, 11 figures, Published in The Astronomical Journal

We obtained spectra of ASASSN-V J205543.90+240033.5 (J2055), a system that shows photometric variations similar to the white dwarf (WD) pulsar AR Scorpii (Kato et al. arXiv:2109.03979). Our spectra display a continuum rising steeply toward the blue as well as an array of emission lines. Resolved Balmer and Paschen lines are seen with H$\alpha$ and H$\beta$ having central absorption features. The strongest lines are unresolved CII, CIII, and NIII as well as doubly ionized helium. The spectra are similar to that of YY Hya (Kimeswenger et al. arXiv:2110.03935), and suggest that J2055 is a post-common envelope binary consisting of a hot compact star irradiating the face of a secondary of unknown spectral type. Velocity variations detected from the emission lines confirm the binary nature of J2055. The origin of the 10 minute photometric variation remains uncertain., Comment: 3 pages, 1 figure, submitted to RNAAS

We present the 30-min cadence Kepler/K2 light curve of the Type Ia supernova (SN Ia) SN 2018agk, covering approximately one week before explosion, the full rise phase and the decline until 40 days after peak. We additionally present ground-based observations in multiple bands within the same time range, including the 1-day cadence DECam observations within the first $\sim$5 days after the first light. The Kepler early light curve is fully consistent with a single power-law rise, without evidence of any bump feature. We compare SN 2018agk with a sample of other SNe~Ia without early excess flux from the literature. We find that SNe Ia without excess flux have slowly-evolving early colors in a narrow range ($g-i\approx -0.20\pm0.20$ mag) within the first $\sim 10$ days. On the other hand, among SNe Ia detected with excess, SN 2017cbv and SN 2018oh tend to be bluer, while iPTF16abc's evolution is similar to normal SNe Ia without excess in $g-i$. We further compare the Kepler light curve of SN 2018agk with companion-interaction models, and rule out the existence of a typical non-degenerate companion undergoing Roche-lobe overflow at viewing angles smaller than $45^{\circ}$., Comment: 20 pages, 14 figures, 5 tables. Published in ApJ

We measured high-quality surface brightness fluctuation (SBF) distances for a sample of 63 massive early-type galaxies using the WFC3/IR camera on the Hubble Space Telescope. The median uncertainty on the SBF distance measurements is 0.085 mag, or 3.9% in distance. Achieving this precision at distances of 50 to 100 Mpc required significant improvements to the SBF calibration and data analysis procedures for WFC3/IR data. Forty-two of the galaxies are from the MASSIVE Galaxy Survey, a complete sample of massive galaxies within ~100 Mpc; the SBF distances for these will be used to improve the estimates of the stellar and central supermassive black hole masses in these galaxies. Twenty-four of the galaxies are Type Ia supernova hosts, useful for calibrating SN Ia distances for early-type galaxies and exploring possible systematic trends in the peak luminosities. Our results demonstrate that the SBF method is a powerful and versatile technique for measuring distances to galaxies with evolved stellar populations out to 100 Mpc and constraining the local value of the Hubble constant., Comment: Accepted for publication in Astrophysical Journal Supplement Series; 22 pages, 7 figures, with 61 additional figures to be published as an online figure set

One of the fundamental properties of an intermediate polar is the dynamical nature of the accretion flow as it encounters the white dwarf's magnetosphere. Many works have presumed a dichotomy between disk-fed accretion, in which the WD accretes from a Keplerian disk, and stream-fed accretion, in which the matter stream from the donor star directly impacts the WD's magnetosphere without forming a disk. However, there is also a third, poorly understood regime in which the accretion flow consists of a torus of diamagnetic blobs that encircles the WD. This mode of accretion is expected to exist at mass-transfer rates below those observed during disk-fed accretion, but above those observed during pure stream-fed accretion. We invoke the diamagnetic-blob regime to explain the exceptional TESS light curve of the intermediate polar TX Col, which transitioned into and out of states of enhanced accretion during Cycles 1 and 3. Power-spectral analysis reveals that the accretion was principally stream-fed. However, when the mass-transfer rate spiked, large-amplitude quasi-periodic oscillations (QPOs) abruptly appeared and dominated the light curve for weeks. The QPOs have two striking properties: they appear in a stream-fed geometry at elevated accretion rates, and they occur preferentially within a well-defined range of frequencies (~10-25 cycles per day). We propose that during episodes of enhanced accretion, a torus of diamagnetic blobs forms near the binary's circularization radius and that the QPOs are beats between the white dwarf's spin frequency and unstable blob orbits within the WD's magnetosphere. We discuss how such a torus could be a critical step in producing an accretion disk in a formerly diskless system., Comment: Accepted for publication in AJ

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 (2020) (arXiv:2007.09285), the half-width-at-zero-intensity of the Balmer and HeI lines routinely reaches a maximum of ~3000 km/s, 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, HeI 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 has a steep decrement and it is likely produced in the extended outflow. Most enticingly of all, this outflow produces a narrow P-Cyg absorption component for nearly half of the orbit, and we demonstrate that this scenario closely matches the outflow kinematics predicted by Wynn, King, & Horne (1997). 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., Comment: 14 pages, 11 figures, accepted for publication in the Astrophysical Journal

The compact object in the interacting binary AR Sco has widely been presumed to be a rapidly rotating, magnetized white dwarf (WD), but it has never been detected directly. Isolating its spectrum has proven difficult because the spin-down of the WD generates pulsed synchrotron radiation that far outshines the WD's photosphere. As a result, a previous study of AR Sco was unable to detect the WD in the averaged far-ultraviolet spectrum from a Hubble Space Telescope (HST) observation. In an effort to unveil the WD's spectrum, we reanalyze these HST observations by calculating the average spectrum in the troughs between synchrotron pulses. We identify weak spectral features from the previously unseen WD and estimate its surface temperature to be 11500$\pm$500K. Additionally, during the synchrotron pulses, we detect broad Lyman-$\alpha$ absorption consistent with hot WD spectral models. We infer the presence of a pair of hotspots, with temperatures between 23000K and 28000K, near the magnetic poles of the WD. As the WD is not expected to be accreting from its companion, we describe two possible mechanisms for heating the magnetic poles. The Lyman-$\alpha$ absorption of the hotspots appears relatively undistorted by Zeeman splitting, constraining the WD's field strength to be 100 MG, but the data are insufficient to search for the subtle Zeeman splits expected at lower field strengths., Comment: 10 pages, 7 figures, accepted for publication in the Astrophysical Journal

The dominant radioactive energy source powering Type Ia supernova light curves is expected to switch from the decay of $^{56}$Co to $^{57}$Co at very late epochs. We use archival HST images of SN1992A obtained more than 900 days after explosion to constrain its cobalt isotopic abundance ratio and compare it to the well-studied event SN2011fe. We confirm the $^{57}$Co / $^{56}$Co ratio for SN2011fe of $0.026\pm 0.004$ found by arXiv:1608.01155, consistent with a "double degenerate" progenitor scenario. For SN1992A, we find a ratio of $0.034\pm 0.010$, but the large uncertainty does not allow us to differentiate between progenitor models, Comment: 5 pages, 1 figure. Accepted by RNAAS on 17/11/2020

Portions of the Kepler K2 Short Cadence light curve of the dwarf nova (DN) TW Vir at quiescence are investigated using light curve modeling. The light curve was separated into 24 sections, each with a data length of $\sim\,$0.93\,d, comprising 4 sections before and 20 after a superoutburst (SO). Due to the morphological differences, the quiescent orbital modulation is classified into three types. Using a fixed disk radius and the two component stellar parameters, all 24 synthetic disk models from the sections show a consistent configuration, consisting of a disk and two hotspots: one at the vertical side of the edge of the disk and the other one on the surface of the disk. Before the SO, the disk and a ringlike surface-hotspot are suddenly enhanced, triggering a precursor and then SO. At the end of the quiescent period following the SO and before the first normal outburst, the edge-hotspot becomes hotter, while the surface-hotspot switches into a ``coolspot" with a coverage of nearly one-half of the disk surface. During quiescence, the surface-hotspot is always located at the outer part of the disk with a constant radial width. A flat radial temperature distribution of the disk is found and appears flatter when approaching the outburst. Like many U\,Gem-type DN with orbital periods of 3-5\,hr, the mass transfer rate is significantly lower than the predictions of the standard/revised models of CV evolution., Comment: 23 pages, 13 figures, accepted by AJ

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 this "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 system's 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., Comment: accepted for publication, RNAAS

We analyze 143 Type Ia supernovae (SNeIa) observed in $H$ band (1.6-1.8 $\mu$m) and find SNeIa are intrinsically brighter in $H$-band with increasing host galaxy stellar mass. We find SNeIa in galaxies more massive than $10^{10.43} M_{\odot}$ are $0.13 \pm 0.04$ mag brighter in $H$ than SNeIa in less massive galaxies. The same set of SNeIa observed at optical wavelengths, after width-color-luminosity corrections, exhibit a $0.10 \pm 0.03$ mag offset in the Hubble residuals. We observe an outlier population ($|\Delta H_{\rm max}| > 0.5$ mag) in the $H$ band and show that removing the outlier population moves the mass threshold to $10^{10.65} M_{\odot}$ and reduces the step in $H$ band to $0.08 \pm 0.04$ mag, but the equivalent optical mass step is increased to $0.13 \pm 0.04$ mag. We conclude the outliers do not drive the brightness--host-mass correlation. Less massive galaxies preferentially host more higher-stretch SNeIa, which are intrinsically brighter and bluer. It is only after correction for width-luminosity and color-luminosity relationships that SNeIa have brighter optical Hubble residuals in more massive galaxies. Thus finding SNeIa are intrinsically brighter in $H$ in more massive galaxies is an opposite correlation to the intrinsic (pre-width-luminosity correction) optical brightness. If dust and the treatment of intrinsic color variation were the main driver of the host galaxy mass correlation, we would not expect a correlation of brighter $H$-band SNeIa in more massive galaxies., Comment: 54 pages, 21 Figures, 15 Tables. Accepted to the Astrophysical Journal

We analyze rapid-cadence, multiwavelength photometry of AR Scorpii from three observatories, covering five observing seasons. We measure the arrival times of the system's beat pulses and use them to compute an updated ephemeris. The white dwarf spin-down rate is estimated with an uncertainty of only 4%. These results confirm, beyond any doubt, that the white dwarf's spin period is increasing at the rate consistent with by that of Stiller et al. (2018). We study the evolution of the beat pulse's color index across the orbit. The color of the primary pulse maxima varies significantly across the orbit, with the peaks being bluer after superior conjunction than in the first half of the orbit. Specifically, at orbital phase 0.5, the color index of the primary pulse shows a very sharp discontinuity towards bluer indices. This supports the Potter & Buckley (2018b) synchrotron emission model where the two emitting poles differ significantly in color. However, no corresponding jump in the color of the secondary pulses is seen. Furthermore, our analysis reveals that the arrival times of the pulses can differ by as much as 6s in simultaneous $u$ and $r$ photometry, depending on the binary orbital phase. If left uncorrected, this wavelength-dependent timing offset could lead to erroneous measurements of the spin-period derivative, particularly with heterogeneous datasets., Comment: 9 pages, 8 figures, 3 tables. Accepted for publication in MNRAS

We develop a model of the white dwarf (WD) - red dwarf (RD) binaries AR Sco and AE Aqr as systems in a transient propeller stage of highly asynchronous intermediate polars. The WDs are relatively weakly magnetized with magnetic field of $\sim 10^6$ G. We explain the salient observed features of the systems due to the magnetospheric interaction of two stars. Currently, the WD's spin-down is determined by the mass loading of the WD's magnetosphere from the RD's at a mild rate of $\dot{M}_{WD} \sim 10^{-11} M_\odot $/yr. Typical loading distance is determined by the ionization of the RD's wind by the WD's UV flux. The WD was previously spun up by a period of high accretion rate from the RD via Roch lobe overflow with $\dot{M} \sim 10^{-9} M_\odot $/yr, acting for as short a period as tens of thousands of years. The non-thermal X-ray and optical synchrotron emitting particles originate in reconnection events in the magnetosphere of the WD due to the interaction with the flow from the RD. In the case of AR Sco, the reconnection events produce signals at the WD's rotation and beat periods - this modulation is due to the changing relative orientation of the companions' magnetic moments and resulting variable reconnection conditions. Radio emission is produced in the magnetosphere of the RD, we hypothesize, in a way that it is physically similar to the Io-induced Jovian decametric radiation.

The Kepler spacecraft observed a total of only four AM Herculis cataclysmic variable stars during its lifetime. We analyze the short-cadence K2 light curve of one of those systems, Tau 4 (RX J0502.8+1624), which underwent a serendipitous jump from a low-accretion state into a high state during the final days of the observation. Apart from one brief flare, there was no evidence of accretion during the 70 d of observations of the low state. As Tau 4 transitioned into a high state, the resumption of accretion was very gradual, taking approximately six days (~90 binary orbits). We supplement Tau 4's K2 light curve with time-resolved spectroscopy obtained in both high and low states of accretion. High-excitation lines, such as He II 468.6 nm, were extraordinarily weak, even when the system was actively accreting. This strongly suggests the absence of an accretion shock, placing Tau 4 in the bombardment regime predicted for AM Herculis systems with low accretion rates. In both the high-state and low-state spectra, Zeeman absorption features from the white dwarf's photosphere are present and reveal a surface-averaged field strength of $15\pm2$ MG. Remarkably, the high-state spectra also show Zeeman-split emission lines produced in a region with a field strength of $12\pm1$ MG. Zeeman emission has not been previously reported in an AM Herculis system, and we propose that the phenomenon is caused by a temperature inversion in the WD's atmosphere near the accretion region., Comment: Submitted to AAS journals. Comments welcome

We present the light curve of EU Cnc, the first published analysis of a Kepler light curve of a polar. Although EU Cnc was extremely faint during campaign 16 and 18 its light curve showed a large-amplitude pulse that lasted for half of each orbit, which we interpret to be light from an accretion region that rotates behind the limb of the white dwarf for half of the orbit. Remarkably, the pulse profile showed no appreciable variability in either campaign. Additionally, we note weak dips of unknown origin with a depth of ~10% at the beginning and end of the orbital hump. The K2 waveform is consistent with all previously reported variations on the orbital timescale, suggesting that EU Cnc has possibly been at the same low state for its entire observational history. We then explore the possibility that EU Cnc's unusual combination of a low mass-transfer rate and consistent light curve indicates the possibility that EU Cnc might be a low-accretion-rate polar (LARP).

AR Scorpii (AR Sco) is a binary star system containing the only known white dwarf (WD) pulsar. Previously reported photometric datasets only provide coverage back to 2005, but we extend the observational baseline for AR Sco back to the beginning of the 20th century by analyzing observations from the Digital Access to a Sky Century at Harvard project (DASCH). We find that the orbital waveform of AR Sco remained constant across that baseline with no significant deviations from its average brightness. This result strongly suggests that the absence of accretion in modern observations is a long-term feature of AR Sco. Additionally, the DASCH light curve provides an opportunity to test an earlier prediction that an obliquity of the WD would result in a precessional period observable in long-term studies of the orbital light curve. The DASCH observations do not indicate the presence of such a period, and we show that previous, inconclusive tests of this hypothesis were insensitive to the existence of a precessional period. Furthermore, the long DASCH baseline enables us to constrain the rate of change of the orbital frequency to $\dot{\nu} \lesssim 3.8\times10^{-20}$ Hz s$^{-1}$, constraining the efficacy of magnetic braking as a mechanism of angular-momentum loss in this system. Finally, we discuss how the combination of the orbital waveform's stability, high amplitude, and short period should make it possible to identify additional WD pulsars in all-sky survey data., Comment: Published in AJ on September 2, 2019; Accepted for publication on June 17, 2019, 10 pages, 7 figures

The intermediate polar FO Aquarii (FO Aqr) experienced its first-reported low-accretion states in 2016, 2017, and 2018, and using newly available photographic plates, we identify pre-discovery low states in 1965, 1966, and 1974. The primary focus of our analysis, however, is an extensive set of time-series photometry obtained between 2002 and 2018, with particularly intensive coverage of the 2016-2018 low states. After computing an updated spin ephemeris for the white dwarf (WD), we show that its spin period began to increase in 2014 after having spent 27 years decreasing; no other intermediate polar has experienced a sign change of its period derivative, but FO Aqr has now done so twice. Our central finding is that the recent low states all occurred shortly after the WD began to spin down, even though no low states were reported in the preceding quarter-century, when it was spinning up. Additionally, the system's mode of accretion is extremely sensitive to the mass-transfer rate, with accretion being almost exclusively disk-fed when FO Aqr is brighter than V~14 and substantially stream-fed when it is not. Even in the low states, a grazing eclipse remains detectable, confirming the presence of a disk-like structure (but not necessarily a Keplerian accretion disk). We relate these various observations to theoretical predictions that during the low state, the system's accretion disk dissipates into a non-Keplerian ring of diamagnetic blobs. Finally, a new XMM-Newton observation from a high state in 2017 reveals an anomalously soft X-ray spectrum and diminished X-ray luminosity compared to pre-2016 observations., Comment: Accepted for publication in ApJ

The TESS spacecraft observed the asynchronous polar CD Ind at a two-minute cadence almost continuously for 28 days in 2018, covering parts of 5 consecutive cycles of the system's 7.3-day beat period. These observations provide the first uninterrupted photometry of a full spin-orbit beat cycle of an asynchronous polar. Twice per beat cycle, the accretion flow switched between magnetic poles on the white dwarf, causing the spin pulse of the white dwarf (WD) to alternate between two waveforms after each pole-switch. An analysis of the waveforms suggests that one accretion region is continuously visible when it is active, while the other region experiences lengthy self-eclipses by the white dwarf. We argue that the previously accepted periods for both the binary orbit and the WD spin have been misidentified, and while the cause of this misidentification is a subtle and easily overlooked effect, it has profound consequences for the interpretation of the system's accretion geometry and doubles the estimated time to resynchronization. Moreover, our timings of the photometric maxima do not agree with the quadratic ephemeris from Myers et al. (2017), and it is possible that the optical spin pulse might be an unreliable indicator of the white dwarf's rotation. Finally, we use Doppler tomography of archival time-resolved spectra from 2006 to study the accretion flow. While the accretion flow showed a wider azimuthal extent than is typical for synchronous polars, it was significantly less extended than in the three other asynchronous polars for which Doppler tomography has been reported., Comment: Accepted for publication in ApJ. For an independent analysis of the TESS light curve of CD Ind, see Hakala et al. 2019, MNRAS, 486, 2549

We obtained high temporal resolution spectroscopy of the unusual binary system AR Sco covering nearly an orbit. The H$\alpha$ emission shows a complex line structure similar to that seen in some polars during quiescence. Such emission is thought to be due to long-lived prominences originating on the red dwarf. A difference between AR Sco and these other systems is that the white dwarf in AR Sco is rapidly spinning relative to the orbital period. "Slingshot" prominences stable at 3 to 5 stellar radii require surface magnetic fields between 100 and 500 G. This is comparable to the estimated WD magnetic field strength near the surface of the secondary. Our time-resolved spectra also show emission fluxes, line equivalent widths, and continuum color varying over the orbit and the beat/spin periods of the system. During much of the orbit, the optical spectral variations are consistent with synchrotron emission with the highest energy electrons cooling between pulses. On the time-scale of the beat/spin period we detect red and blue-shifted H$\alpha$ emission flashes that reach velocities of 700 km/s. Red-shifted Balmer emission flashes are correlated with the bright phases of the continuum beat pulses while blue-shifted flashes appear to prefer the time of minimum in the beat light curve. We propose that much of the energy generated in AR Sco comes from fast magnetic reconnection events occurring near the inward face of the secondary and we show that the energy generated by magnetic reconnection can account for the observed excess luminosity from the system., Comment: Accepted for publication in ApJ, December 27, 2018, 15 page, 14 figures

GY Cnc is a deeply eclipsing cataclysmic variable star with an orbital period of 4.21 hours that has shown several dwarf nova outbursts. The variable was continuously observed by the K2/Kepler satellite with a short cadence in Campaign 5 (C05) for 75 days during 2015. The star was again observed in 2017/2018 for 80 consecutive days during Campaign 16 (C16). 419 well-observed eclipses were measured over C5 and 446 timings were determined in C16. A new ephemeris was calculated combining the K2 data, previously published timings, new light curves from the KELT Follow-Up Network, and additional observations in the AAVSO database. We have refined the orbital period of GY Cnc and improved its ephemeris which had accumulated an error of about 300s over 18 years. A quadratic term was not found to be significant indicating that there is currently no detectable orbital period derivative. We observed a correlation between the quiescent system brightness and the eclipse timing residuals that are as large as +/-15s in the K2 data., Comment: Accepted for publication in the RNAAS

We used the light curve code XRBinary to model the quiescent K2 light curves of three low-inclination cataclysmic variables (CVs): 1RXS\,J0632+2536 (J0632+2536), RZ\,Leo, TW\,Vir and the pre-CV WD\,1144+011. Optimized light curve models were obtained using a nonlinear fitting code NMfit and visualized by Phoebe 2.0. The disk model of J0632+2536 shows that one hotspot at the edge of the disk is enough to describe its light curve, while the other two dwarf nova (DN): RZ\,Leo and TW\,Vir require two hotspots. A typical pre-CV model with a weak irradiation effect for WD\,1144+011 can explain its single-hump modulation, and the newly observed spectrum confirms its previous classification. The synthetic analyses for the DN clearly indicate that phase zero of the double-hump modulations occurs around the secondary minimum and the primary hump is mainly caused by the hotspot at the edge of the disk. The quiescent disk has a flat temperature distribution with a power index of $\sim0.11$. The disk model of RZ\,Leo implies a truncated disk, supporting its previously speculated classification as an intermediate polar (IP). Except for the IP model of RZ\,Leo, which lacks a component related to the inferred accretion curtain, the models of J0632+2536, TW\,Vir and WD\,1144+011 are consistent with results from the Gaia mission. The derived masses and radii of the secondaries of the three DN are consistent with the semi-empirical relations for CV donor stars, while their effective temperatures are higher than the predictions. Irradiation of the donor stars is investigated to explain this discrepancy., Comment: 32 pages, 8 figures, accepted by AJ

Observations of type Ia supernovae include information about the characteristic nucleosynthesis associated with these thermonuclear explosions. We consider observational constraints from iron-group elemental and isotopic ratios, to compare with various models obtained with the most-realistic recent treatment of electron captures. The nucleosynthesis is sensitive to the highest white-dwarf central densities. Hence, nucleosynthesis yields can distinguish high-density Chandrasekhar-mass models from lower-density burning models such as white-dwarf mergers. We discuss new results of post-processing nucleosynthesis for two spherical models (deflagration and/or delayed detonation models) based upon new electron capture rates. We also consider cylindrical and 3D explosion models (including deflagration, delayed-detonation, or a violent merger model). Although there are uncertainties in the observational constraints, we identify some trends in observations and the models. We make a new comparison of the models with elemental and isotopic ratios from five observed supernovae and three supernova remnants. We find that the models and data tend to fall into two groups. In one group low-density cores such as in a 3D merger model are slightly more consistent with the nucleosynthesis data, while the other group is slightly better identified with higher-density cores such as in single-degenerate 1D-3D deflagration models. Hence, we postulate that both types of environments appear to contribute nearly equally to observed SNIa. We also note that observational constraints on the yields of $^{54}$Cr and $^{54}$Fe, if available, might be used as a means to clarify the degree of geometrical symmetry of SNIa explosions., Comment: Accepted for publication in ApJ

CRTS J035905.9+175034 is the first eclipsing SU UMa system for which a superoutburst has been observed by Kepler in the short-cadence mode. The light curve contains one superoutburst, eight normal outbursts (including a precursor to the superoutburst), and several minioutbursts that are present before -- but not after -- the superoutburst. The superoutburst began with a precursor normal outburst, and shortly after the peak of the precursor, the system developed large-amplitude superhumps that achieved their maximum amplitude after just three superhump cycles. The period excess of the initial superhump period relative to the orbital period implies a mass ratio of 0.281 +/- 0.015, placing it marginally above most theoretical predictions of the highest-possible mass ratio for superhump formation. In addition, our analysis of the variations in eclipse width and depth, as well as the hot spot amplitudes, generally provides substantiation of the thermal-tidal instability model. The K2 data, in conjunction with our ground-based time-resolved spectroscopy and photometry from 2014-2016, allows us to determine many of the fundamental parameters of this system., Comment: accepted for publication, AJ

The unique binary AR Scorpii consists of an asynchronously rotating, magnetized white dwarf (WD) that interacts with its red-dwarf companion to produce a large-amplitude, highly coherent pulsation every 1.97 minutes. Over the course of two years, we obtained thirty-nine hours of time-resolved, optical photometry of AR Sco at a typical cadence of 5 seconds to study this pulsation. We find that it undergoes significant changes across the binary orbital period and that its amplitude, phase, and waveform all vary as a function of orbital phase. We show that these variations can be explained by constructive and destructive interference between two periodic, double-peaked signals: the spin-orbit beat pulse, and a weaker WD spin pulse. Modelling of the light curve indicates that in the optical, the amplitude of the primary spin pulse is 50% of the primary beat amplitude, while the secondary maxima of the beat and spin pulses have similar amplitudes. Finally, we use our timings of the beat pulses to confirm the presence of the disputed spin-down of the WD. We measure a beat-frequency derivative of -5.14(32) x 10^-17 Hz/s and show that this is attributable to the spin-down of the WD. This value is approximately twice as large as the estimate from Marsh et al. (2016) but is nevertheless consistent with the constraints established in Potter & Buckley (2018). Our precise measurement of the spin-down rate confirms that the decaying rotational energy of the magnetized white dwarf is sufficient to power the excess electromagnetic radiation emitted by the binary., Comment: Accepted for publication in AJ on July 10, 2018

We present time-resolved photometry and spectroscopy of the recently classified polar MASTER OT J132104.04+560957.8. The spectrum shows a smooth, non-thermal continuum at the time of maximum light, without any individually discernible cyclotron harmonics. Using homogeneous cyclotron modeling, we interpret this as cyclotron radiation whose individual harmonics have blended together, and on this basis, we loosely constrain the magnetic field strength to be less than ~30 MG. In addition, for about one-tenth of the orbital period, the Balmer and He I emission lines transition into absorption features, with He II developing an absorption core. We use our observations of this phenomenon to test theoretical models of the accretion curtain and conclude that the H and He I lines are produced throughout the curtain, in contravention of theoretical predictions of separate H and He I line-forming regions. Moreover, a significant amount of He II emission originates within the accretion curtain, implying that the curtain is significantly hotter than expected from theory. Finally, we comment on the object's long-term photometry, including evidence that it recently transitioned into a prolonged, exceptionally stable high state following a potentially decades-long low state., Comment: accepted for publication, AJ

We present an analysis of 15 Type Ia supernovae (SNe Ia) at redshift z > 1 (9 at 1.5 < z < 2.3) recently discovered in the CANDELS and CLASH Multi-Cycle Treasury programs using WFC3 on the Hubble Space Telescope. We combine these SNe Ia with a new compilation of 1050 SNe Ia, jointly calibrated and corrected for simulated survey biases to produce accurate distance measurements. We present unbiased constraints on the expansion rate at six redshifts in the range 0.07 < z < 1.5 based only on this combined SN Ia sample. The added leverage of our new sample at z > 1.5 leads to a factor of ~3 improvement in the determination of the expansion rate at z = 1.5, reducing its uncertainty to ~20%, a measurement of H(z=1.5)/H0=2.67 (+0.83,-0.52). We then demonstrate that these six measurements alone provide a nearly identical characterization of dark energy as the full SN sample, making them an efficient compression of the SN Ia data. The new sample of SNe Ia at z > 1 usefully distinguishes between alternative cosmological models and unmodeled evolution of the SN Ia distance indicators, placing empirical limits on the latter. Finally, employing a realistic simulation of a potential WFIRST SN survey observing strategy, we forecast optimistic future constraints on the expansion rate from SNe Ia., Comment: 14 pages, 5 figures, 7 tables; submitted to ApJ

The quiescent periodic photometric modulations of two low-inclination cataclysmic variables observed in Kepler K2 Campaigns 0 and 1, KZ Gem and TW Vir, are investigated. A phase-correcting method was successfully used to detect the orbital modulations of KZ Gem and TW Vir and improve their orbital periods. The light curve morphologies of both CVs were further analyzed by defining flux ratios and creating colormaps. KZ Gem shows ellipsoidal modulations with an orbital period of 0.22242(1) day, twice the period listed in the updated RK catalogue (Edition 7.24). With this newly determined period, KZ Gem is no longer a CV in the period gap, but a long-period CV. A part of the quiescent light curve of TW Vir that had the highest stability was used to deduce its improved orbital period of 0.182682(3) day. The flat patterns shown in the colormaps of the flux ratios for KZ Gem demonstrate the stability of their orbital modulations, while TW Vir show variable orbital modulations during the K2 datasets. In TW Vir, the single versus double-peaked nature of the quiescent orbital variations before and after superoutburst may be related to the effect of the superoutburst on the accretion disk., Comment: 10 pages, 12 figures, accepted by A&A

We analyze long-cadence Kepler K2 observations of AR Sco from 2014, along with survey photometry obtained between 2005 and 2016 by the Catalina Real-Time Sky Survey and the All-Sky Automated Survey for Supernovae. The K2 data show the orbital modulation to have been fairly stable during the 78 days of observations, but we detect aperiodic deviations from the average waveform with an amplitude of ~2% on a timescale of a few days. A comparison of the K2 data with the survey photometry reveals that the orbital waveform gradually changed between 2005 and 2010, with the orbital maximum shifting to earlier phases. We compare these photometric variations with proposed models of this unusual system., Comment: Accepted for publication, ApJL

SweetSpot is a three-year National Optical Astronomy Observatory (NOAO) Survey program to observe Type Ia supernovae (SNe Ia) in the smooth Hubble flow with the WIYN High-resolution Infrared Camera (WHIRC) on the WIYN 3.5-m telescope. We here present data from the first half of this survey, covering the 2011B-2013B NOAO semesters, and consisting of 493 calibrated images of 74 SNe Ia observed in the rest-frame near-infrared (NIR) from $0.02 < z < 0.09$. Because many observed supernovae require host galaxy subtraction from templates taken in later semesters, this release contains only the 186 NIR ($JHK_s$) data points for the 33 SNe Ia that do not require host-galaxy subtraction. The sample includes 4 objects with coverage beginning before the epoch of B-band maximum and 27 beginning within 20 days of B-band maximum. We also provide photometric calibration between the WIYN+WHIRC and Two-Micron All Sky Survey (2MASS) systems along with light curves for 786 2MASS stars observed alongside the SNe Ia. This work is the first in a planned series of three SweetSpot Data Releases. Future releases will include the full set of images from all 3 years of the survey, including host-galaxy reference images and updated data processing and host-galaxy reference subtraction. SweetSpot will provide a well-calibrated sample that will help improve our ability to standardize distance measurements to SNe Ia, examine the intrinsic optical-NIR colors of SNe Ia at different epochs, explore nature of dust in other galaxies, and act as a stepping stone for more distant, potentially space-based surveys., Comment: Published in AJ. 10 tables. 11 figures. Lightcurve plots included as a figureset and available in source tarball. Data online at http://www.phyast.pitt.edu/~wmwv/SweetSpot/DR1_data/