arXiv subjects

Edward M. Sion

Publications and source records attributed to Edward M. Sion.

At least 19 recordsLinked to original sources

arXiv2026

The instantaneous mass accretion rate of novae in quiescence: - an archival ultraviolet optical spectral analysis

Based on archival spectra, we derive the quiescent instantaneous mass transfer rates in novae using synthetic disk spectra generated with tlusty, Gaia parallax-derived distances, and updated color excess values. Our results for nine novae, based on ultraviolet spectra and on a number of optical spectra, yield mass accretion rates that are higher than those derived from simple integration of the UV and optical luminosity. HR Del, 20 years after its eruption, has a mass transfer rate of $4\times 10^{-7}M_\odot$/yr, and is likely burning the accreting H-rich material. V842 Cen, thought to be a low mass transfer system, has a comparable mass accretion rate. For both novae, the enhanced mass transfer must be self-sustained by a feedback loop. RR Pic, with $\dot{M}\approx 3\times 10^{-8}M_\odot$/yr, is better fitted with an accretion disk where the outer region is heated up to 12,000 K, in agreement with H and He emission lines coming from the outer disk and a large emission region on the leading side of the disk. Such a heated disk also gives a good fit to the spectra of CP Lac, and DI Lac with $\dot{M}\sim4.5$ and $9\times 10^{-9}M_\odot$/yr. V1974 Cyg and V533 Her, with an accretion rate of $\sim 3\times 10^{-9}M_\odot$/yr, have a rather flat spectrum. V446 Her and BK Lyn, caught in a state of low accretion, have the lowest mass accretion rates with $\dot{M} \sim 10^{-9}$ and $\sim 10^{-10}M_\odot$/yr respectively. The higher mass transfer rate systems, with $\dot{M}\approx \sim 10^{-7}M_\odot$/yr, agree with the standard disk model; the remaining systems are better fitted when the outer disk is heated to $\sim 12,000$~K. We suggest that irradiation from the heated WD, inner disk, together with tidal interaction, the bright spot, and material overflowing the disk edge, can increase the temperature of the outer disk.

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arXiv2026

High Resolution X-ray Spectroscopy of the Nova-Like Cataclysmic Variable BZ Cam using Chandra HETG: Diagnosis of the ADAF-like (Advective) Hot Flow

Nova-likes such as BZ Cam are high state Cataclysmic Variables showing hard X-ray emission that can be characterized with advective hot flows in the inner accretion disk. We explore Chandra High Energy Transmission Grating (HETG) observations of BZ~Cam for detailed line diagnosis and ionization conditions in the X-ray regime. We mostly find H- and He-like emission lines of Mg, Si, S, and Fe. All He-like line components of forbidden, intercombination and resonance lines are present. The R ratios of selected lines indicate plasma densities of a few $\times$10$^{12-14}$ cm$^{-3}$ and G ratios reveal temperatures (3-6)$\times$ 10$^6$ K where the Fe lines yield (1-3)$\times$ 10$^7$ K. The H to He line ratios and the R and G ratios show that the plasma is in a nonequilibrium ionization condition, which is consistent with our previous X-ray results and the accretion flow in the X-ray region being an ADAF-like (advective) hot flow. Simultaneous fits of the HEG and MEG spectra or the broadband joint spectra of ROSAT, Chandra zero order and NuSTAR yield temperatures 3.4-6.3 keV using a VNEI model of plasma emission (in XSPEC) or Bremsstrahlung emission. An additional power law is detected above 98\% Confidence Level in the broadband analysis. The orbital variations and the broadband spectra show dipping/veiling of the X-rays and an additional warm absorber model with an ionization parameter log($ξ$) = 2.7 is required at the 3$σ$ level, along with the VNEI model where the HEG and MEG simultaneous fits yield the log($ξ$) = 3.6 .

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arXiv2022

White Dwarf Photospheric Abundances in Cataclysmic Variables -- II. White Dwarfs With and Without a Mask

Taking advantage of the now available Gaia EDR3 parallaxes, we carry out an archival {\it Hubble Space Telescope} (HST) far ultraviolet spectroscopic analysis of 10 cataclysmic variable systems, including 5 carefully selected eclipsing systems. We obtain accurate white dwarf (WD) masses and temperatures, in excellent agreement with the masses for 4 of the eclipsing systems. For three systems in our sample, BD Pav, HS 2214, and TT Crt, we report the first robust masses for their WDs. We modeled the absorption lines to derive the WD chemical abundances and rotational velocities for each of the ten systems. As expected, for five higher inclination ($i \gtrsim 75^{\circ}$) systems, the model fits are improved with the inclusion of a cold absorbing slab (an iron curtain masking the WD) with $N_{\rm H} \approx 10^{20}-10^{22}$cm$^{-2}$. Modeling of the metal lines in the HST spectra reveals that 7 of the 10 systems have significant subsolar carbon abundance, and six have subsolar silicon abundance, thereby providing further evidence that CV WDs exhibit subsolar abundances of carbon and silicon. We suggest that strong aluminum absorption lines (and iron absorption features) in the spectra of some CV WDs (such as IR Com) may be due to the presence of a {\it thin} iron curtain ($N_{\rm H}\approx 10^{19}$cm$^{-2}$) rather than to suprasolar aluminum and iron abundances in the WD photosphere. The derived WD (projected) rotational velocities all fall in the range $\approx 100-400$~km/s, all sub-Keplerian similar to the values obtained in earlier studies.

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arXiv2021

The Heating and Pulsations of V386 Serpentis after its 2019 Dwarf Nova Outburst

Following the pulsation spectrum of a white dwarf through the heating and cooling involved in a dwarf nova outburst cycle provides a unique view of the changes to convective driving that take place on timescales of months versus millenia for non-accreting white dwarfs. In 2019 January the dwarf nova V386 Ser (one of a small number containing an accreting, pulsating white dwarf), underwent a large amplitude outburst. Hubble Space Telescope ultraviolet spectra were obtained 7 and 13 months after outburst along with optical ground-based photometry during this interval and high-speed photometry at 5.5 and 17 months after outburst. The resulting spectral and pulsational analysis shows a cooling of the white dwarf from 21,020 K to 18,750 K (with a gravity log(g) = 8.1) between the two UV observations, along with the presence of strong pulsations evident in both UV and optical at a much shorter period after outburst than at quiescence. The pulsation periods consistently lengthened during the year following outburst, in agreement with pulsation theory. However, it remains to be seen if the behavior at longer times past outburst will mimic the unusual non-monotonic cooling and long periods evident in the similar system GW Lib.

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arXiv2021

Nova-produced Common Envelope: Source of the Non-solar Abundances and an Additional Frictional Angular Momentum Loss in Cataclysmic Variables

A substantial fraction of Cataclysmic Variables (CVs) reveals non-solar abundances. A comprehensive list of CVs which includes those that have been examined for these abundances is given. Three possible sources of these non-solar abundances on the secondary are accretion during the red giant common envelope phase, an Evolved Main Sequence secondary and nova-processed material. Use of the secondary's cross-section just on the escaping nova material to change the abundances of its convective region has been the killing objection for considering nova-processed material. The key element, ignored in other studies, is that a thermonuclear runaway on a white dwarf causes a strong propagating shock wave which not only ejects material, but also produces a large amount of non-ejected material which forms a common envelope. This nova-produced common envelope contains a large amount of non-solar material. We demonstrate that the secondary has the capacity and time to re-accrete enough of this material to acquire a significant non-solar convective region. This same envelope interacting with the binary will produce a Frictional Angular Momentum Loss which can be the Consequential Angular Momentum Loss needed for the average CV white dwarf mass, WD mass accretion rates, the period minimum, the orbital period distribution, and the space density of CVs problems. This interaction will decrease the orbital period which can cause the recently observed sudden period decreases across nova eruptions. A simple, rapid evolutionary model of the secondary that includes the swept-up nova-produced material and the increasing convective region is developed and applied to individual CVs.

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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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arXiv2019

FUSE Spectroscopic Analysis of the Slowest Symbiotic Nova AG Peg During Quiescence

We present a far ultraviolet spectroscopic analysis of the slowest known symbiotic nova AG Peg (M3/4III giant + hot white dwarf; P = 818.4 days) which underwent a nova explosion in 1850 followed by a very slow decline that did not end until 1996, marking the beginning of quiescence. The 19 years of quiescence ended in June 2015, when AG Peg exhibited a Z And-type outburst with an optical amplitude of 1.5 magnitudes. We have carried out accretion disk and WD photosphere synthetic spectral modeling of a Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum obtained on June 5.618, 2003 during the quiescence interval 12 years before the 2015 outburst. The spectrum is heavily affected by ISM absorption as well as strong emission lines. We de-reddened the FUSE fluxes assuming E(B-V) = 0.10, which is the maximum galactic reddening in the direction of AG Peg. We discuss our adoption of the pre-Gaia distance over the Gaia parallax. For a range of white dwarf surface gravities and surface temperatures we find that the best-fitting photosphere is a hot WD with a temperature T = 150,000 K, and a low gravity log(g)~6.0-6.5. For a distance of 800 pc, the scaled WD radius is about 0.06 Rsun, giving log(g) = 6.67 for a 0.65 Msun WD mass. The Luminosity we obtain from this model is 1729 Lsun. The hot photosphere models provide better fits than the accretion disk models which have FUV flux deficits toward the shorter wavelengths of FUSE, down to the Lyman Limit. Given the uncertainty of the nature of a true symbiotic accretion disk, and, while a very hot low gravity degenerate star dominates the FUV flux, the presence of a steady-state (standard) accretion disk cannot be summarily ruled out.

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arXiv2018

HST FUV spectroscopy of the short orbital period recurrent nova CI Aql: Implications for white dwarf mass evolution

An HST COS Far UV spectrum (1170 A to 1800 A) was obtained for the short orbital period recurrent novae (T Pyxidis subclass), CI Aquilae. CI Aql is the only classical CV known to have two eclipses of sensible depth per orbit cycle and also have pre- and post-outburst light curves that are steady enough to allow estimates of mass and orbital period changes. Our FUV spectral analysis with model accretion disks and NLTE high gravity photospheres, together with the Gaia parallax, reveal CI Aql's FUV light is dominated by an optically thick accretion disk with an accretion rate of the order of $4\times 10^{-8}$ $M_{\odot}/yr$. Its database of light curves, radial velocity curves, and eclipse timings is among the best for any CV. Its orbit period ($P$), $dP/dt$, and reference time are re-derived via simultaneous analysis of the three data types, giving a dimensionless post-outburst $dP/dt$ of $-2.49\pm 0.95\times 10^{-10}$. Lack of information on loss of orbital to rotational angular momentum leads to some uncertainty in the translation of $dP/dt$ to white dwarf mass change rate, $dM_1/dt$, but within the modest range of $+4.8\times 10^{-8}$ to $+7.8\times 10^{-8}$ $M_{\odot} /yr$. The estimated white dwarf mass change through outburst for CI Aql, based on simple differencing of its pre- and post outburst orbit period, is unchanged from the previously published $+5.3 \times 10^{-6} M_{\odot}$. At the WD's estimated mass increase rate, it will terminate as a Type Ia supernova within 10 million years.

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arXiv2017

HST/COS Far Ultraviolet Spectroscopic Analysis of U Geminorum Following a Wide Outburst

We have used HST/COS to obtain a series of 4 FUV (915-2148A) spectroscopic observations of the prototypical dwarf nova U Geminorum during its cooling following a two-week outburst. Our FUV spectral analysis of the data indicates that the white dwarf (WD) cools from a temperature of 41,500 K, 15 days after the peak of the outburst, to 36,250 K, 56 days after the peak of the outburst, assuming a massive WD (log(g)=8.8) and a distance of 100.4 pc. These results are self-consistent with a 1.1 solar mass WD with a 5,000 km radius. The spectra show many absorption lines of but no emission features. We find supra-solar abundances of nitrogen confirming the anomalous high N/C ratio. The FUV lightcurve reveals a 5% modulation with the orbital phase, showing dips near phase 0.25 and 0.75, where the spectra exhibit an increase in the depth of some absorption lines and in particular strong absorption lines from Si, Al, and Ar. The phase dependence we observe is consistent with material overflowing the disk rim at the hot spot, reaching a maximum elevation near phase 0.75, falling back at smaller radii near phase 0.5 where it bounces off the disk surface and again rising above the disk near phase 0.25. There is a large scatter in the absorption lines' velocities, especially for the silicon lines, while the carbon lines seem to match more closely the orbital velocity of the WD. This indicates that many absorption lines are affected by- or form in- the overflowing stream material veiling the WD, making the analysis of the WD spectra more difficult.

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arXiv2017

Modifying the Standard Disk Model for the Ultraviolet Spectral Analysis of Disk-dominated Cataclysmic Variables. I. The Novalikes MV Lyrae, BZ Camelopardalis, and V592 Cassiopeiae

The standard disk is often inadequate to model disk-dominated cataclysmic variables (CVs) and generates a spectrum that is bluer than the observed UV spectra [Puebla et al 2007]. X-ray observations of these systems reveal an optically thin boundary layer (BL) expected to appear as an inner hole in the disk. Consequently, we truncate the inner disk. However, instead of removing the inner disk, we impose the no-shear boundary condition at the truncation radius, thereby lowering the disk temperature and generating a spectrum that better fits the UV data. With our modified disk, we analyze the archival UV spectra of three novalikes that cannot be fitted with standard disks. For the VY Scl systems MV Lyr and BZ Cam, we fit a hot inflated white dwarf WD with a cold modified disk ($\dot{M} \sim $ a few $10^{-9}M_{\odot}$/yr). For V592 Cas, the slightly modified disk ($\dot{M} \sim 6 \times 10^{-9}M_{\odot}$/yr) completely dominates the UV. These results are consistent with Swift X-ray observations of these systems [Balman et al 2014], revealing BLs merged with ADAF-like flows and/or hot coronae, where the advection of energy is likely launching an outflow and heating the WD, thereby explaining the high WD temperature in VY Scl systems. This is further supported by the fact that the X-ray hardness ratio increases with the shallowness of the UV slope in a small CV sample we examine. Furthermore, for 105 disk-dominated systems, the International Ultraviolet Explorer (IUE) spectra UV slope decreases in the same order as the ratio of the X-ray flux to optical/UV flux: from SU UMa's, to U Gem's, Z Cam's, UX UMa's, and VY Scl's.

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arXiv2017

Probing the Accreting Hot Components in Six S-Type Symbiotic Variables

We have carried out a spectroscopic analysis of the far ultraviolet spectra of six symbiotic variables. Two systems, LT Del, which has had one recorded outburst, and BD-21 3873 (= IV Vir) which has had no recorded outburst, are yellow symbiotic systems. Two other systems, V443 Her and RW Hya, have also never had a recorded outburst. Two other symbiotics, StHa190 and CQ Dra, are more strongly interacting with an outburst history. We have studied these systems during their quiescence in order to shed light on the nature of their hot components by fitting their archival far ultraviolet spectra with optically thick accretion disk models and NLTE model white dwarf photospheres. Using the critical advantage offered by extending wavelength coverage down to the Lyman Limit with FUSE spectra, we find that the hot component in RW Hya is a low mass white dwarf with a surface temperature of 160,000K while the symbiotic system CQ Dra is a triple system with a red giant transferring matter to a hot component made up of a cataclysmic variable whose white dwarf has a surface temperature of $\sim$50,000K. Implications are discussed.

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arXiv2017

Hubble Space Telescope Ultraviolet Light Curves Reveal Interesting Properties of CC Sculptoris and RZ Leonis

Time-tag ultraviolet data obtained on the Hubble Space Telescope in 2013 reveal interesting variability related to the white dwarf spin in the two cataclysmic variables RZ Leo and CC Scl. RZ Leo shows a period at 220s and its harmonic at 110s, thus identifying it as a likely Intermediate Polar (IP). The spin signal is not visible in a short single night of ground based data in 2016, but the shorter exposures in that dataset indicate a possible partial eclipse. The much larger UV amplitude of the spin signal in the known IP CC Scl allows the spin of 389s, previously only seen at outburst, to be visible at quiescence. Spectra created from the peaks and troughs of the spin times indicate a hotter temperature of several thousand degrees during the peak phases, with multiple components contributing to the UV light.

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arXiv2016

GW Librae: Still Hot Eight Years Post-Outburst

We report continued Hubble Space Telescope (HST) ultraviolet spectra and ground-based optical photometry and spectroscopy of GW Librae eight years after its largest known dwarf nova outburst in 2007. This represents the longest cooling timescale measured for any dwarf nova. The spectra reveal that the white dwarf still remains about 3000 K hotter than its quiescent value. Both ultraviolet and optical light curves show a short period of 364-373 s, similar to one of the non-radial pulsation periods present for years prior to the outburst, and with a similar large UV/optical amplitude ratio. A large modulation at a period of 2 h (also similar to that observed prior to outburst) is present in the optical data preceding and during the HST observations, but the satellite observation intervals did not cover the peaks of the optical modulation so it is not possible to determine its corresponding UV amplitude. The similarity of the short and long periods to quiescent values implies the pulsating, fast spinning white dwarf in GW Lib may finally be nearing its quiescent configuration.

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arXiv2015

Far Ultraviolet Spectroscopy of Old Novae I. V603 Aquila

We present the results of a synthetic spectral analysis of the far ultraviolet archival IUE, HST and FUSE observations of the fast old nova V603 Aql, obtained some 90 years after its 1918 nova outburst. Our analysis utilizes the new Hubble FGS parallax distance for this nearly face-on old nova, a high white dwarf mass and a low reddening. Our analysis includes non-truncated optically thick accretion disks since V603 Aql is neither a polar nor an intermediate polar. Our synthetic spectral modeling of the FUSE and HST spectra analyzed separately indicate a mass transfer rate of 1.5-2.2xe-9 solar mass per year for the FUSE and HST spectra respectively, assuming a WD mass of 1.2Msun. The mass accretion rate also depends on the assumed WD mass, and increases by a factor of two for a WD mass of 0.8Msun. Combining the FUSE and HST spectra together lead to the same results. Potential implications are discussed.

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arXiv2015

HST Images Flash Ionization of Old Ejecta by the 2011 Eruption of Recurrent Nova T Pyxidis

T Pyxidis is the only recurrent nova surrounded by knots of material ejected in previous outbursts. Following the eruption that began on 2011 April 14.29, we obtained seven epochs (from 4 to 383 days after eruption) of Hubble Space Telescope narrowband Ha images of T Pyx . The flash of radiation from the nova event had no effect on the ejecta until at least 55 days after the eruption began. Photoionization of hydrogen located north and south of the central star was seen 132 days after the beginning of the eruption. That hydrogen recombined in the following 51 days, allowing us to determine a hydrogen atom density of at least 7e5 cm^-3 - at least an order of magnitude denser than the previously detected, unresolved [NII] knots surrounding T Pyx. Material to the northwest and southeast was photoionized between 132 and 183 days after the eruption began. 99 days later that hydrogen had recombined. Both then (282 days after outburst) and 101 days later, we detected almost no trace of hydrogen emission around T Pyx. There is a large reservoir of previously unseen, cold diffuse hydrogen overlapping the previously detected, [NII] - emitting knots of T Pyx ejecta. The mass of this newly detected hydrogen is probably an order of magnitude larger than that of the [NII] knots. We also determine that there is no significant reservoir of undetected ejecta from the outer boundaries of the previously detected ejecta out to about twice that distance, near the plane of the sky. The lack of distant ejecta is consistent with the Schaefer et al (2010) scenario for T Pyx, in which the star underwent its first eruption within five years of 1866 after many millennia of quiescence, followed by the six observed recurrent nova eruptions since 1890. This lack of distant ejecta is not consistent with scenarios in which T Pyx has been erupting continuously as a recurrent nova for many centuries or millennia.

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arXiv2014

SWIFT XRT Observations of the Nova-like Cataclysmic Variables MV Lyr, BZ Cam and V592 Cas

We present a total of ~ 45 ksec (3$\times$15 ksec) of SWIFT XRT observations for three non-magnetic nova-like (NL) Cataclysmic Variables (CVs) (MV Lyr, BZ Cam, V592 Cas) in order to study characteristics of Boundary Layers (BL) in CVs. The nonmagnetic NLs are found mostly in a state of high mass accretion rate ($\ge$1$\times$10$^{-9}$ Msun yr$^{-1}$) and some show occasional low states. Using the XRT data, we find optically thin multiple-temperature cooling flow type emission spectra with X-ray temperatures (kT$_{max}$) of 21-50 keV. These hard X-ray emitting boundary layers diverge from simple isobaric cooling flows indicating X-ray temperatures that are of virial values in the disk. In addition, we detect power law emission components from MV Lyr and BZ Cam and plausibly from V592 Cas which may be a result of the Compton scattering of the optically thin emission from the fast wind outflows in these systems and/or Compton up-scattering of the soft disk photons. The X-ray luminosities of the (multi-temperature) thermal plasma emission in the 0.1-50.0 keV range are (0.9-5.0)$\times$10$^{32}$ erg/sec. The ratio of the X-ray and disk luminosities (calculated from the UV-optical wavelengths) yield an efficiency (L$_{x}$/L$_{disk}$) ~ 0.01-0.001. Given this non-radiative ratio for the X-ray emitting boundary layers with no significant optically thick blackbody emission in the soft X-rays (consistent with ROSAT observations) together with the high/virial X-ray temperatures, we suggest that high state NL systems may have optically thin BLs merged with ADAF-like flows and/or X-ray coronae. In addition, we note that the axisymmetric bipolar and/or rotation dominated fast wind outflows detected in these three NLs (particularly BZ Cam and V592 Cas) or some other NL may also be explained in the context of ADAF-like BL regions.

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arXiv2014

On the Effect of Explosive Thermonuclear Burning on the Accreted Envelopes of White Dwarfs in Cataclysmic Variables

The detection of heavy elements at suprasolar abundances in the atmospheres of some accreting white dwarfs in cataclysmic variables, coupled with the high temperatures needed to produce these elements requires explosive thermonuclear burning. The central temperatures of any formerly more massive secondary stars in CVs undergoing hydrostatic CNO burning are far too low to produce these elements. Evidence is presented that at least some cataclysmic variables contain donor secondaries that have been contaminated by repeated novae ejecta and are transferring this material back to the white dwarf. This scenario does not exclude the channel in which formerly more massive donor stars underwent CNO processing in ystems that underwent thermal timescale mass transfer. Implications for the progenitors of CVs are discussed.

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arXiv2013

Hubble Space Telescope and Ground-Based Observations of V455 Andromedae Post-Outburst

Hubble Space Telescope spectra obtained in 2010 and 2011, three and four years after the large amplitude dwarf nova outburst of V455 And, were combined with optical photometry and spectra to study the cooling of the white dwarf, its spin, and possible pulsation periods after the outburst. The modeling of the ultraviolet (UV) spectra show that the white dwarf temperature remains ~600 K hotter than its quiescent value at three years post outburst, and still a few hundred degrees hotter at four years post outburst. The white dwarf spin at 67.6 s and its second harmonic at 33.8 s are visible in the optical within a month of outburst and are obvious in the later UV observations in the shortest wavelength continuum and the UV emission lines, indicating an origin in high temperature regions near the accretion curtains. The UV light curves folded on the spin period show a double-humped modulation consistent with two-pole accretion. The optical photometry two years after outburst shows a group of frequencies present at shorter periods (250-263 s) than the periods ascribed to pulsation at quiescence, and these gradually shift toward the quiescent frequencies (300-360 s) as time progresses past outburst. The most surprising result is that the frequencies near this period in the UV data are only prominent in the emission lines, not the UV continuum, implying an origin away from the white dwarf photosphere. Thus, the connection of this group of periods with non-radial pulsations of the white dwarf remains elusive.

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