arXiv subjects

Paula Szkody

Publications and source records attributed to Paula Szkody.

At least 55 records · Page 3Linked to original sources

arXiv2020

Short-cadence K2 observations of an accretion-state transition in the polar Tau 4

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.

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arXiv2020

The hydrogen Balmer lines and jump in absorption in accretion disc modeling -- an ultraviolet-optical spectral analysis of the dwarf novae UZ Serpentis and CY Lyrae

The spectra of disc-dominated cataclysmic variables (CVs) often deviate from the spectra of accretion disc models; in particular, the Balmer jump and absorption lines are found to be shallower in the observations than in the models. We carried out a combined ultraviolet-optical spectral analysis of two dwarf novae: UZ Ser in outburst, decline, and quiescence, and CY Lyr on the rise to outburst and in outburst. We fit the Balmer jump and absorption lines, the continuum flux level and slope by adjusting the accretion rate, inclination, and disc outer radius. For both systems we find an accretion rate $\dot{M} \approx 8 \times 10^{-9}M_\odot$/yr in outburst, and $\dot{M} \approx 2-3 \times 10^{-9}M_\odot$/yr for the rise and decline phases. The outer disc radius we derive is smaller than expected ($R_{\rm disc} \approx 0.2a$, where $a$ is the binary separation), except during late rise (for CY Lyr) where $R_{\rm disc}=0.3a$. UZ Ser also reveals a 60,000~K white dwarf. These results show that during a dwarf nova cycle the radius of the disc is the largest just before the peak of the outburst, in qualitative agreement with the disc instability model for dwarf nova outbursts. We suspect that an additional emitting component (e.g. disc wind) is also at work to reduce the slope of the continuum and size of the Balmer jump and absorption lines. We stress that both the outer disc radius and disc wind need to be taken into account for more realistic disc modeling of CVs.

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arXiv2020

Followup ground-based observations of the dwarf nova KZ Gem

We present spectroscopy of stars in the immediate vicinity of the dwarf nova (DN) KZ Gem to confirm its identification, which had been ambiguous in the literature. Analysis of 73 radial velocities spanning from 2014 to 2019 provides a high-precision orbital period of 0.2224628(2)\,d ($\sim5.34$\,hr) and shows KZ\,Gem to be a double-lined DN. Time series photometry taken from 2016 to 2018 shows a variable double-hump modulation with a full amplitude of $\sim0.3$\,mag, along with five Gaussian-like transient events lasting $\sim30$\,min or more. Using the light curve code XRBinary and nonlinear fitting code NMfit, we obtain an optimized binary model of the dwarf nova (DN) KZ Gem, from time series photometry, consisting of a Roche-lobe-filling K type dwarf with a mass transfer rate of $2.7\,-\,7.9\times10^{-10}\,{\rm M}_{\odot}\,{\rm yr}^{-1}$ to a large, cool and thick disk surrounding a white dwarf, in an orbit with an inclination of $51^{\circ}.6(\pm1^{\circ}.4)$. Two hotspots on the disk are demonstrated to cause the observed variations in the ellipsoidal modulations from the secondary star. This physical model is compatible with the Gaia distance of KZ\,Gem.

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arXiv2020

Cataclysmic Variables in the First Year of the Zwicky Transient Facility

Using selection criteria based on amplitude, time and color, we have identified 329 objects as known or candidate cataclysmic variable (CVs) during the first year of testing and operation of the Zwicky Transient Facility (ZTF). Of these, 90 are previously confirmed CVs, 218 are strong candidates based on the shape and color of their light curves obtained during 3-562 days of observations, and the remaining 21 are possible CVs but with too few data points to be listed as good candidates. Almost half the strong candidates are within 10 deg of the galactic plane, in contrast to most other large surveys which have avoided crowded fields. The available Gaia parallaxes are consistent with sampling the low mass transfer CVs, as predicted by population models. Our followup spectra have confirmed Balmer/helium emission lines in 27 objects, with four showing high excitation HeII emission, including candidates for an AM CVn, a polar and an intermediate polar. Our results demonstrate that a complete survey of the galactic plane is needed to accomplish an accurate determination of the number of CVs existing in the Milky Way.

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arXiv2020

The first ultracompact Roche lobe-filling hot subdwarf binary

We report the discovery of the first short period binary in which a hot subdwarf star (sdOB) fills its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic Plane named the Zwicky Transient Facility and exhibits a period of $P_{\rm orb}=39.3401(1)$ min, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of $T_{\rm eff}=42,400\pm300$ K and a surface gravity of $\log(g)=5.77\pm0.05$. A high-signal-to noise GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass $M_{\rm sdOB}=0.337\pm0.015$ M$_\odot$ and a white dwarf accretor with a mass $M_{\rm WD}=0.545\pm0.020$ M$_\odot$. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a $2.5-2.8$ M$_\odot$ star lost its envelope when crossing the Hertzsprung Gap. To match its current $P_{\rm orb}$, $T_{\rm eff}$, $\log(g)$, and masses, we estimate a post-common envelope period of $P_{\rm orb}\approx150$ min, and find the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of $\approx0.1$ M$_\odot$ and will merge with its carbon/oxygen white dwarf companion after $\approx17$ Myr and presumably explode as a thermonuclear supernova or form an R CrB star.

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arXiv2019

The Intriguing Polar EU Cancri in the Eyes of Kepler K2

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).

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arXiv2019

Key Challenges for AAS Journals in the Next Decade

The American Astronomical Society (AAS) Journals are a vital asset of our professional society. With the push towards open access, page charges are a viable and sustainable option for continuing to effectively fund and publish the AAS Journals. However, the existing page charge model, which requires individual authors to pay page charges out of their grants or even out of pocket, is already challenging to some researchers and could be exacerbated in the Open Access (OA) era if charges increase. A discussion of alternative models for funding page charges and publishing costs should be part of the Astro2020 decadal survey if we wish to continue supporting the sustainable and accessible publication of US research in AAS journals in the rapidly-shifting publication landscape. The AAS Publications Committee recommends that the National Academy of Sciences form a task force to develop solutions and recommendations with respect to the urgent concerns and considerations highlighted in this White Paper.

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arXiv2019

General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system

General relativity predicts that short orbital period binaries emit significant gravitational radiation, and the upcoming Laser Interferometer Space Antenna (LISA) is expected to detect tens of thousands of such systems; however, few have been identified, and only one is eclipsing--the double white dwarf binary SDSS J065133.338+284423.37, which has an orbital period of 12.75 minutes. Here, we report the discovery of an eclipsing double white dwarf binary system with an orbital period of only 6.91 minutes, ZTF J153932.16+502738.8. This system has an orbital period close to half that of SDSS J065133.338+284423.37 and an orbit so compact that the entire binary could fit within the diameter of the planet Saturn. The system exhibits a deep eclipse, and a double-lined spectroscopic nature. We observe rapid orbital decay, consistent with that expected from general relativity. ZTF J153932.16+502738.8 is a significant source of gravitational radiation close to the peak of LISA's sensitivity, and should be detected within the first week of LISA observations.

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arXiv2019

CRTS J035010.7+323230, a new eclipsing polar in the cataclysmic variable period gap

We report the discovery of a new eclipsing polar, CRTS J035010.7+323230 (hereafter CRTS J0350+3232). We identified this cataclysmic variable (CV) candidate as a possible polar from its multi-year Catalina Real-Time Transient Survey (CRTS) optical light curve. Photometric monitoring of 22 eclipses in 2015 and 2017 was performed with the 2.1-m Otto Struve Telescope at McDonald Observatory. We derive an unambiguous high-precision ephemeris. Strong evidence that CRTS J0350+3232 is a polar comes from optical spectroscopy obtained over a complete orbital cycle using the Apache Point Observatory 3.5-m telescope. High velocity Balmer and He II $λ$4686Å emission line equivalent width ratios, structures, and variations are typical of polars and are modulated at the same period, 2.37-hrs (142.3-min), as the eclipse to within uncertainties. The spectral energy distribution and luminosity is found to be comparable to that of AM Herculis. Pre-eclipse dips in the light curve show evidence for stream accretion. We derive the following tentative binary and stellar parameters assuming a helium composition white dwarf and a companion mass of 0.2 M$_{\odot}$: inclination i = 74.68$^{o}$ ${\pm}$ 0.03$^{o}$, semi-major axis a = 0.942 ${\pm}$ 0.024 R$_{\odot}$, and masses and radii of the white dwarf and companion respectively: M$_{1}$ = 0.948 $^{+0.006}_{-0.012}$ M$_{\odot}$, R$_{1}$ = 0.00830 $^{+0.00012}_{-0.00006}$ R$_{\odot}$, R$_{2}$ = 0.249 ${\pm}$ 0.002 R$_{\odot}$. As a relatively bright (V $\sim$ 17-19 mag), eclipsing, period-gap polar, CRTS J0350+3232 will remain an important laboratory for the study of accretion and angular momentum evolution in polars.

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arXiv2019

A new class of pulsating hot subdwarfs

Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot, compact stars. We have found four candidates, exhibiting blue colors ($g-r\leq-0.1$ mag), pulsation amplitudes of $>5\%$, and pulsation periods of $200 - 475$ sec. Fourier transforms of the lightcurves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T$_{\rm eff}$ and log(g) variations over the pulsation cycle, consistent with large amplitude radial oscillations. The mean T$_{\rm eff}$ and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass helium-core pre-white dwarfs and low mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-white dwarf models. Therefore, we suggest that these new pulsators and Blue Large-Amplitude Pulsators (BLAPs) could be members of the same class of pulsators, composed of young $\approx0.25-0.35$ M$_\odot$ helium-core pre-white dwarfs.

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arXiv2019

Fast-Cadence TESS Photometry and Doppler Tomography of the Asynchronous Polar CD Ind: A Revised Accretion Geometry from Newly Proposed Spin and Orbital Periods

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.

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arXiv2019

Astro2020 Science White Paper: Understanding the evolution of close white dwarf binaries

Interacting binaries containing white dwarfs can lead to a variety of outcomes that range from powerful thermonuclear explosions, which are important in the chemical evolution of galaxies and as cosmological distance estimators, to strong sources of low frequency gravitational wave radiation, which makes them ideal calibrators for the gravitational low-frequency wave detector LISA mission. However, current theoretical evolution models still fail to explain the observed properties of the known populations of white dwarfs in both interacting and detached binaries. Major limitations are that the existing population models have generally been developed to explain the properties of sub-samples of these systems, occupying small volumes of the vast parameter space, and that the observed samples are severely biased. The overarching goal for the next decade is to assemble a large and homogeneous sample of white dwarf binaries that spans the entire range of evolutionary states, to obtain precise measurements of their physical properties, and to further develop the theory to satisfactorily reproduce the properties of the entire population. While ongoing and future all-sky high- and low-resolution optical spectroscopic surveys allow us to enlarge the sample of these systems, high-resolution ultraviolet spectroscopy is absolutely essential for the characterization of the white dwarfs in these binaries. The Hubble Space Telescope is currently the only facility that provides ultraviolet spectroscopy, and with its foreseeable demise, planning the next ultraviolet mission is of utmost urgency.

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arXiv2019

Machine Learning for the Zwicky Transient Facility

The Zwicky Transient Facility is a large optical survey in multiple filters producing hundreds of thousands of transient alerts per night. We describe here various machine learning (ML) implementations and plans to make the maximal use of the large data set by taking advantage of the temporal nature of the data, and further combining it with other data sets. We start with the initial steps of separating bogus candidates from real ones, separating stars and galaxies, and go on to the classification of real objects into various classes. Besides the usual methods (e.g., based on features extracted from light curves) we also describe early plans for alternate methods including the use of domain adaptation, and deep learning. In a similar fashion we describe efforts to detect fast moving asteroids. We also describe the use of the Zooniverse platform for helping with classifications through the creation of training samples, and active learning. Finally we mention the synergistic aspects of ZTF and LSST from the ML perspective.

astro-ph.IM↗

arXiv2018

Mapping the Periphery and Variability of the Magellanic Clouds

We propose a combination of a modified Wide-Fast-Deep survey, a mini-survey of the South Celestial Pole, and a Deep Drilling-style survey to produce a 3-D map of the Magellanic System and to provide a detailed census of the transient and variable populations in the Clouds. We support modifying the Wide-Fast-Deep survey to cover the declination range $-72.25°<{\rm Dec}<12.4°$ and the Galactic latitude range $|b|>15°$, as proposed in a separate white paper. We additionally propose a mini-survey covering the 950$°^2$ with ${\rm Dec} < -72.25$ in $ugriz$ to the standard LSST single-exposure depth and with 40 visits per filter per field. Finally, we propose a mini-survey covering $\sim100 °^2$ of the main bodies of the Clouds with twelve total pointings, 2000 total visits per field, and shorter exposure time.

astro-ph.IM↗

arXiv2018

A Phenomenological Model for the Light Curve of three Quiescent Low-inclination Dwarf Novae and one Pre-Cataclysmic Variable

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.

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arXiv2018

K2 study of the Magnetic Precataclysmic Variable V1082 Sagittarius

We present a long-term light curve of the precataclysmic variable (CV) V1082 Sgr obtained by the K2-mission over the course of 81 days. We analyze the entire complex light curve as well as explore several sections in detail with a sliding periodogram. The long dataset allows the first detection of the orbital period in the light curve, as well as the confirmation of cyclical variability on a longer timescale of about a month. A portion of the light curve in deep minimum reveals a clean, near-sinusoidal variability attributed to the rotation of the spotted surface of the donor star. We model that portion of the light curve assuming that the donor star grossly under-fills its Roche lobe, has cool spots similar to a chromospherically active, slightly evolved early K-star, and might be irradiated by the X-ray beam from the magnetically accreting white dwarf. The fast variability of the object in the active phases resembles the light curves of magnetic CVs (polars).

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arXiv2018

A Comprehensive K2 and Ground-Based Study of CRTS J035905.9+175034, an Eclipsing SU UMa System with a Large Mass Ratio

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.

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arXiv2017

Followup Observations of SDSS and CRTS Candidate Cataclysmic Variables II

Spectra of 38 candidate or known cataclysmic variables are presented. Most are candidate dwarf novae or systems containing possible highly magnetic white dwarfs, while a few (KR Aur, LS Peg, V380 Oph and V694 Mon) are previously known objects caught in unusual states. Individual spectra are used to confirm a dwarf nova nature or other classification while radial velocities of 15 systems provide orbital periods and velocity amplitudes that aid in determining the nature of the objects. Our results substantiate a polar nature for four objects, find an eclipsing SW Sex star below the period gap, another as a likely intermediate polar, as well as two dwarf novae with periods in the middle of the gap.

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