Cataclysmic Variable EX Research Articles

MHD simulation of accretion processes in magnetic cataclysmic variable EX Hya

A hierarchical numerical model for investigation of the accretion processes in the magnetic cataclysmic variable EX Hya has been described. The coarsest level of the hierarchy corresponds to the accretion disk formation due to the mass transfer. The intermediate level describes the structure of the white dwarf magnetosphere. The finest level corresponds to the formation of the accretion curtain at the inner edge of the accretion disk. The model is based on the equations of the modified MHD, where a strong external magnetic field acts as an efficient fluid with which the plasma interacts. Results of simulations show that the accretion in EX Hya proceeds via accretion columns that have curtain-like shapes. These columns are very thin and the accretion regions cover a small fraction of the accretor surface. Numerical results agree with the parameters of the EX Hya system as determined by Semena et al. (2014).

Parameter Determination for the Eclipsing Long-Period Dwarf Nova EX Dra from Photometric Observations during Different Activity States of the System

The results of a long-term photometric observations of the cataclysmic variable EX Dra acquired between 2014 and 2016 at the Crimean Station of the Sternberg Astronomical Institute (24 nights, more than 10 500 measurements) are presented. The observations were performed using CCD photometers mounted on 50-cm and 60-cm telescopes in the visible and red, during both quiescence and the active state. For completeness, photometric observations obtained at the Ondrejov Observatory in 2010 in the V and R Johnson filters are also used in the analysis. The new observations of EX Dra are used to derive the orbital period of the system, which agrees well with earlier determinations. A combined model that takes into account the radiation fluxes from the gaseous stream and a hot spot on the lateral surface of the accretion disk is used to determine the parameters of the system components (white dwarf, red dwarf, accretion disk and hot spot, and gaseous stream). Variations of the parameters when the system changes from one activity state to the other are considered. Six light curves displaying unsatisfactory agreement between the observed and theoretical light curves can be successfully fitted using a version of the combined model that includes hot spots on the secondary’s surface. This model is able to qualitatively reproduce a secondary minima in the light curves that exhibits shifts of this minimum from phase 0.5. The parameters of dark spots on the red-dwarf surface were determined. The data obtained indicate that the outbursts in the EX Dra system are related to instability of the matter outflow from the secondary.

Double cyclic variations in orbital period of the eclipsing cataclysmic variable EX Dra

EX Dra is a long-period eclipsing dwarf nova with $\sim2-3$ mag amplitude outbursts. This star has been monitored photometrically from November, 2009 to March, 2016 and 29 new mid-eclipse times were obtained. By using new data together with the published data, the best fit to the $O-C$ curve indicate that the orbital period of EX Dra have an upward parabolic change while undergoing double-cyclic variations with the periods of 21.4 and 3.99 years, respectively. The upward parabolic change reveals a long-term increase at a rate of $\dot{P}={+7.46}\times10^{-11}{s} {s^{-1}}$. The evolutionary theory of cataclysmic variables (CVs) predicts that, as a CV evolves, the orbital period should be decreasing rather than increasing. Secular increase can be explained as the mass transfer between the secondary and primary or may be just an observed part of a longer cyclic change. Most plausible explanation for the double-cyclic variations is a pair of light travel-time effect via the presence of two companions. Their masses are determined to be $M_{A}sini'_{A}=29.3(\pm0.6) M_{Jup}$ and $M_{B}sini'_{B}=50.8(\pm0.2) M_{Jup}$. When the two companions are coplanar to the orbital plane of the central eclipsing pair, their masses would match to brown dwarfs.

On the area of accretion curtains from fast aperiodic time variability of the intermediate polar EX Hya

We present results of a study of the fast timing variability of the magnetic cataclysmic variable (mCV) EX Hya. It was previously shown that one may expect the rapid flux variability of mCVs to be smeared out at timescales shorter than the cooling time of hot plasma in the post shock region of the accretion curtain near the WD surface. Estimates of the cooling time and the mass accretion rate, thus provide us with a tool to measure the density of the post-shock plasma and the cross-sectional area of the accretion funnel at the WD surface. We have probed the high frequencies in the aperiodic noise of one of the brightest mCV EX Hya with the help of optical telescopes, namely SALT and the SAAO 1.9m telescope. We place upper limits on the plasma cooling timescale $\tau<$0.3 sec, on the fractional area of the accretion curtain footprint $f<1.6\times10^{-4}$, and a lower limit on the specific mass accretion rate $\dot{M}/A \gtrsim $3 g/sec/cm$^{-2}$. We show that measurements of accretion column footprints via eclipse mapping highly overestimate their areas. We deduce a value of $\Delta r/r \lesssim 10^{-3}$ as an upper limit to the penetration depth of the accretion disc plasma at the boundary of the magnetosphere.

Period changes of the long-period cataclysmic binary EX Draconis

The cataclysmic variable star EX Dra is a relatively faint but frequently investigated eclipsing dwarf nova. In total 35 new eclipses were measured photometrically as part of our long-term monitoring of interesting eclipsing systems. Using published and new mid-eclipse times obtained between 2004 and 2011 we constructed the observed-minus-calculated diagram. The current data present 21 years of period modulation with a semi-amplitude of 2.5 min. The eclipse timings show significant deviations from the best sinusoidal fit, which indicates that this ephemeris is not a complete description of the data. The fractional period change is roughly ΔP/P = 3 × 10 −6 .

X‐Ray Light Curves and Accretion Disk Structure of EX Hydrae

We present X-ray light curves for the cataclysmic variable EX Hydrae obtained with the Chandra High Energy Transmission Grating Spectrometer and the Extreme Ultraviolet Explorer Deep Survey photometer. We confirm earlier results on the shape and amplitude of the binary light curve and discuss a new feature: the phase of the minimum in the binary light curve, associated with absorption by the bulge on the accretion disk, increases with wavelength. We discuss several scenarios that could account for this trend and conclude that, most likely, the ionization state of the bulge gas is not constant, but rather decreases with binary phase. We also conclude that photoionization of the bulge by radiation originating from the white dwarf is not the main source of ionization, but that it is heated by shocks originating from the interaction between the in-flowing material from the companion and the accretion disk. The findings in this paper provide a strong test for accretion disk models in close binary systems.

The Radial Velocity and Mass of the White Dwarf of EX Hydrae Measured withChandra

We present the first detection of orbital motion in the cataclysmic variable EX Hydrae based on X-ray data from Chandra. The large collecting area of the telescope and the high resolution of the HETG spectrometers allow for an unprecedented velocity accuracy of ~15 km s-1 in the X-ray wavelength regime. We find an emission line velocity amplitude of 58.2 ± 3.7 km s-1 and infer a white dwarf mass of 0.49 ± 0.13 M☉, in good agreement with previous studies using optical, ultraviolet, and far-ultraviolet data.

A precise HST parallax of the cataclysmic variable EX Hydrae, its system parameters, and accretion rate

Using the HST Fine Guidance Sensor, we have measured a high precision astrometric parallax of the cataclysmic variable EX Hydrae, $\pi=15.50\,\pm\,0.29$ mas. From the wavelength-integrated accretion-induced energy flux, we derive a quiescent accretion luminosity for EX Hya of $L_{\rm acc} = (2.6\pm 0.6)\times 10^{32}\,$ erg s -1 . The quiescent accretion rate then is $\dot M_{\rm av}=(6.2\pm 1.5)\times 10^{-11}(M_1/0.5\,{M}_\odot)^{-1.61}\, {M}_{\odot}\,{\rm yr}^{-1}$. The time-averaged accretion rate, which includes a small correction for the rare outbursts, is 6% higher. We discuss the system parameters of EX Hya and deduce $M_1 = 0.4-0.7$ $M_{\odot}$, $M_2 = 0.07-0.10$ $M_{\odot}$, and $i = 76.0^\circ-77.6^\circ$, using recent radial velocity measurements of both components and restrictions imposed by other observational and theoretical constraints. We conclude that the secondary is undermassive, overluminous, and expanded over a ZAMS star of the same mass. Near the upper limit to M 1 , the accretion rate of the white dwarf coincides with that due to near-equilibrium angular momentum loss by gravitational radiation and angular momentum transfer from the orbit into the spin-up of the white dwarf. Near the lower mass limit, the correspondingly higher accretion rate requires that either an additional angular momentum loss process is acting besides gravitational radiation or that accretion occurs on a near-adiabatic time scale. The latter possibility would imply that EX Hya is in a transient phase of high mass transfer and the associated spin-up of the white dwarf.

ORFEUS IIandIUESpectroscopy of EX Hydrae

Using ORFEUS-SPAS II FUV spectra, IUE UV spectra, and archival EUVE deep survey photometry, we present a detailed picture of the behavior of the magnetic cataclysmic variable EX Hydrae. Like HUT spectra of this source, the FUV and UV spectra reveal broad emission lines of He II, C II-IV, N III and V, O VI, Si III-IV, and Al III superposed on a continuum which is blue in the UV and nearly flat in the FUV. Like ORFEUS spectra of AM Her, the O VI doublet is resolved into broad and narrow emission components. Consistent with its behavior in the optical, the FUV and UV continuum flux densities, the FUV and UV broad emission line fluxes, and the radial velocity of the O VI broad emission component all vary on the spin phase of the white dwarf, with the maximum of the FUV and UV continuum and broad emission line flux light curves coincident with maximum blueshift of the broad O VI emission component. On the binary phase, the broad dip in the EUV light curve is accompanied by strong eclipses of the UV emission lines and by variations in both the flux and radial velocity of the O VI narrow emission component. The available data are consistent with the accretion funnel being the source of the FUV and UV continuum and the O VI broad emission component, and the white dwarf being the source of the O VI narrow emission component.

Time-resolved spectrophotometry of the cataclysmic variable EX Hydrae

Resultats d'une etude basee sur 264 spectres optiques resolus dans le temps, pour cette nova naine a eclipses inhabituelles

EX Hydrae timing data

The 98 min eclipsing cataclysmic variable EX Hya possesses a strong 67 min modulation in its light. This has led to a discussion of EX Hya in the context of intermediate polar variables. The observed P/Ṗ of the 67 min modulation provides a useful constraint on EX Hya models. Here we report additional timing data obtained over the interval 1982 to 1985 which bears on this matter.

Variability of Soft X-Ray Emission of Ex Hydrae Observed with Einstein Observatory

The cataclysmic variable star EX Hydrae has been observed with the High Resolution Imager (HRI) and the Imaging Proportional Counter (IPC) onboard the Einstein Observatory. The X-ray position is coincident within 3 arcsec of the optical position as measured on Schmidt survey plates. During a 15 1/2 hour observation with IPC we have searched for a modulation of the X-ray flux. Strong evidence for a 67 min period (one of two known optical periods) has been found in the energy range 0.1-3.5 keV with the IPC. The time dependence of modulations is used to discuss a model and evolutionary status of this close binary system.

A model for the X-ray emitting cataclysmic variable EX Hydrae

view Abstract Citations (37) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A model for the X-ray emitting cataclysmic variable EX Hye. Cowley, A. P. ; Hutchings, J. B. ; Crampton, D. Abstract A high-time-resolution spectroscopic study of EX Hya indicates that the broad emission lines originate in a large accretion disk surrounding a white dwarf of approximately 0.7 solar masses. An unseen companion of 0.16 solar masses briefly eclipses part of the disk during the 98-minute orbital period. No clear evidence for the 67-minute photometric period is found spectroscopically. Publication: The Astrophysical Journal Pub Date: June 1981 DOI: 10.1086/158948 Bibcode: 1981ApJ...246..489C Keywords: Eclipsing Binary Stars; Novae; Stellar Spectra; Variable Stars; White Dwarf Stars; X Ray Sources; Emission Spectra; Line Spectra; Orbital Mechanics; Spectral Line Width; Stellar Mass Accretion; Stellar Motions; Astrophysics full text sources ADS | data products SIMBAD (6)