arXiv subjects

Paul A. Mason

Publications and source records attributed to Paul A. Mason.

At least 19 recordsLinked to original sources

arXiv2026

TESS light curves of two new magnetic cataclysmic variables: an asynchronous polar at the period minimum, and an eclipsing system with a large spin-to-orbit ratio

A recent development in the study of magnetic cataclysmic variable stars (mCVs) has been the identification of asynchronously spinning mCVs with orbital periods <2 h that have significantly higher white dwarf spin-to-orbital period ratios than their longer-period counterparts. We report the discovery of two additional mCVs in this class. The first, Gaia21akb, is a candidate asynchronous polar at the period minimum. While TESS photometry cannot, in isolation, lead to a conclusive identification of the orbital period, the probable orbital period of 1.29 h would be the second-shortest of any known polar and would result in a spin-to-orbit ratio of 0.9879. The second system in our study, ZTF18aazmehw, is an eclipsing mCV with a 1.50 h orbital period and a spin-to-orbit ratio of 0.867. Contrary to expectations for an asynchronous polar, ZTF18aazmehw does not show discernible evidence of pole switching and might possess a disk-like structure. The increasing number of short-period asynchronous mCVs with large spin-to-orbit ratios lends credence to theoretical predictions that asynchronously rotating mCVs with sufficiently strong white dwarf magnetic fields can achieve synchronization when their orbital separations have shrunk sufficiently.

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arXiv2024

Characterising high and low accretion states in VY Scl CVs using ZTF and TESS data

VY Scl binaries are a sub-class of cataclysmic variable (CV) which show extended low states, but do not show outbursts which are seen in other classes of CV. To better determine how often these systems spend in low states and to resolve the state transitions we have analysed ZTF data on eight systems and TESS data on six systems. Half of the sample spent most of the time in a high state; three show a broad range and one spends roughly half the time transitioning between high and low states. Using the ZTF data we explore the colour variation as a function of brightness. In KR Aur, we identify a series of repeating outburst events whose brightness appears to increase over time. Using TESS data we searched for periods other than the orbital. In LN UMa we find evidence for a peak whose period varies between 3-6 d. We outline the current models which aim to explain the observed properties of VY Scl systems which includes disc irradiation and a white dwarf having a significant magnetic field.

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arXiv2024

An Eccentric Planet Orbiting the Polar V808 Aurigae

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.

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arXiv2023

SDSS J134441.83+204408.3: A highly asynchronous, short-period magnetic cataclysmic variable with a 56 MG field strength

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.

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arXiv2022

A magnetic valve at L1 revealed in TESS photometry of the asynchronous polar BY Cam

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.

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arXiv2022

Locating the flickering source in polars

Flickering is a fast variability observed in all accreting systems. It has been shown that in most cataclysmic variables flickering originates in the accretion disc. However, in polars the strong magnetic field of the white dwarf prevents the formation of an accretion disc. Therefore, the origin of flickering in polars is not clear. We analyzed the changes of flickering amplitude with orbital phase in seven polars in order to reveal its site of origin. We show that at least in some polars there are two separate sources of flickering. Moreover, at least one of the sources is located at a large distance from the main source of light in the system.

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arXiv2022

Short Duration Accretion States of Polars as seen in TESS and ZTF data

Polars are highly magnetic cataclysmic variables which have been long observed to have both high and low brightness states. The duration of these states has been previously seen to vary from a number of days up to years. Despite this; these states and their physical origin has not been explained in a consistent manner. We present observations of the shortest duration states of a number of Polars observed by ZTF and TESS. This has allowed us to determine that short duration states are a relatively common feature across the population of Polars. Furthermore we have been able to generalise the model of star spot migration to explain both short lived high and low states in Polars by incorporating the interaction between the magnetic field of the white dwarf and that of the star spots.

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arXiv2022

Characteristics of the Permanent Superhumps in V533 Herculis

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, $ε$, ranging between $0.055\le ε\le 0.080$. The superhump amplitude is correlated with its frequency such that the amplitude increases as $ε$ decreases. Positive superhumps result from an instability that generates an eccentric accretion disk and $ε$ 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.

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arXiv2022

Kepler K2 and TESS observations of two magnetic cataclysmic variables: The new asynchronous polar SDSS J084617.11+245344.1 and Paloma

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.

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arXiv2022

Hitting a New Low: The Unique 28 h Cessation of Accretion in the TESS Light Curve of YY Dra (DO Dra)

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.

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arXiv2021

Quasi-periodic oscillations in the TESS light curve of TX Col, a diskless intermediate polar on the precipice of forming an accretion disk

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.

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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

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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arXiv2017

A Jansky VLA Survey of Magnetic Cataclysmic Variable Stars: I. The Data

The Jansky Very Large Array was used to observe 121 magnetic cataclysmic variables (MCVs). We report radio detections of 19 stars. Fourteen are new radio sources, increasing the number of MCVs that are radio sources by more than twofold, from 8 to 22. Most detections are at 8.7 GHz (X-band) with a lesser number at 5.4 and 21.1 GHz (C- and K-bands). Most flux density limits are in the range of 47-470 uJy. With the exception of AE Aqr, the maximum flux detected is 818 uJy. Fourteen of the detections show approximately 100% circularly polarized emission, which is characteristic of electron-cyclotron maser emission. The data suggest that MCVs might be divided into two classes of radio emitters: those dominated by weakly polarized gyro-synchrotron emission and those by highly polarized electron-cyclotron maser emission.

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arXiv2016

Constraining the Radiation and Plasma Environment of the Kepler Circumbinary Habitable Zone Planets

The discovery of many planets using the Kepler telescope includes ten planets orbiting eight binary stars. Three binaries, Kepler-16, Kepler-47, and Kepler-453, have at least one planet in the circumbinary habitable-zone (BHZ). We constrain the level of high-energy radiation and the plasma environment in the BHZ of these systems. With this aim, BHZ limits in these Kepler binaries are calculated as a function of time, and the habitability lifetimes are estimated for hypothetical terrestrial planets and/or moons within the BHZ. With the time-dependent BHZ limits established, a self-consistent model is developed describing the evolution of stellar activity and radiation properties as proxies for stellar aggression toward planetary atmospheres. Modeling binary stellar rotation evolution, including the effect of tidal interaction between stars in binaries is key to establishing the environment around these systems. We find that Kepler-16 and its binary analogs provide a plasma environment favorable for the survival of atmospheres of putative Mars-sized planets and exomoons. Tides have modified the rotation of the stars in Kepler-47 making its radiation environment less harsh in comparison to the solar system. This is a good example of the mechanism first proposed by Mason et al. Kepler-453 has an environment similar to that of the solar system with slightly better than Earth radiation conditions at the inner edge of the BHZ. These results can be reproduced and even reparametrized as stellar evolution and binary tidal models progress, using our online tool http://bhmcalc.net.

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arXiv2015

The Case for a Low Mass Black Hole in the Low Mass X-ray Binary V1408 Aquilae (= 4U 1957+115)

There are very few confirmed black holes with a mass that could be $\sim\! 4\, M_\odot$ and no neutron stars with masses greater than $\sim\! 2\, M_\odot$, creating a gap in the observed distribution of compact star masses. Some black holes with masses between 2 and $4\, M_\odot$ might be hiding among other X-ray sources, whose masses are difficult to measure. We present new high-speed optical photometry of the low-mass X-ray binary V1408 Aql (= 4U 1957+115), which is a persistent X-ray source thought to contain a black hole. The optical light curve of V1408~Aql shows a nearly sinusoidal modulation at the orbital period of the system superimposed on large night-to-night variations in mean intensity. We combined the new photometry with previously-published photometry to derive a more precise orbital period, $P = 0.388893(3)$\ d, and to better define the orbital light curve and night-to-night variations. The orbital light curve agrees well with a model in which the modulation is caused entirely by the changing aspect of the heated face of the secondary star. The lack of eclipses rules out orbital inclinations greater than $65^{\circ}$. Our best models for the orbital light curve favor inclinations near $13^{\circ}$ and black hole masses near $3\, M_\odot$ with a 90\% upper bound of $6.2\, M_\odot$, and a lower bound of $2.0\, M_\odot$ imposed solely by the maximum mass of neutron stars. We favor a black hole primary over a neutron star primary based on evidence from the X-ray spectra, the high spin of the compact object, and the fact that a type I X-ray burst has not been observed for this system. Although uncertainties in the data and the models allow higher masses, possibly much higher masses, the compact star in V1408~Aql is a viable candidate for a black hole lying in the mass gap.

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arXiv2014

Circumbinary Habitability Niches

Binaries could provide the best niches for life in the galaxy. Though counterintuitive, this assertion follows directly from stellar tidal interaction theory and the evolution of lower mass stars. There is strong evidence that chromospheric activity of rapidly rotating young stars may be high enough to cause mass loss from atmospheres of potentially habitable planets. The removal of atmospheric water is most critical. Tidal breaking in binaries could help reduce magnetic dynamo action and thereby chromospheric activity in favor of life. We call this the Binary Habitability Mechanism (BHM), that we suggest allows for water retention at levels comparable to or better than Earth. We discuss novel advantages that life may exploit, in these cases, and suggest that life may even thrive on some circumbinary planets. We find that while many binaries do not benefit from BHM, high quality niches do exist for various combinations of stars between 0.55 and 1.0 solar masses. For a given pair of stellar masses, BHM operates only for certain combinations of period and eccentricity. Binaries having a solar-type primary seem to be quite well suited niches having wide and distant habitable zones with plentiful water and sufficient light for photosynthetic life. We speculate that, as a direct result of BHM, conditions may be suitable for life on several planets and possibly even moons of giant planets orbiting some binaries. Lower mass combinations, while more restrictive in parameter space, provide niches lasting many billions of years and are rich suppliers of photosynthetic photons. We provide a publicly available web-site (\href{http://bit.ly/BHM-calculator}{http://bit.ly/BHM-calculator}, \href{http://bit.ly/BHM-calculator-mirror}{http://bit.ly/BHM-calculator-mirror}), which calculates the BHM effects presented in this paper.

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arXiv2014

Radius constraints from high-speed photometry of 20 low-mass white dwarf binaries

We carry out high-speed photometry on 20 of the shortest-period, detached white dwarf binaries known and discover systems with eclipses, ellipsoidal variations (due to tidal deformations of the visible white dwarf), and Doppler beaming. All of the binaries contain low-mass white dwarfs with orbital periods less than 4 hr. Our observations identify the first eight tidally distorted white dwarfs, four of which are reported for the first time here, which we use to put empirical constraints on the mass-radius relationship for extremely low-mass (<0.30 Msun) white dwarfs. We also detect Doppler beaming in several of these binaries, which confirms the high-amplitude radial-velocity variability. All of these systems are strong sources of gravitational radiation, and long-term monitoring of those that display ellipsoidal variations can be used to detect spin-up of the tidal bulge due to orbital decay.

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