Intermediate Polar EX Research Articles

High-resolution spectroscopy of the intermediate polar EX Hydrae

EX Hya is one of the best studied, but still enigmatic intermediate polars. We present phase-resolved blue VLT/UVES high-resolution (λ/Δλ ≃ 16.000) spectra of EX Hya taken in January 2004. Our analysis involves a unique decomposition of the Balmer line profiles into the spin-modulated line wings that represent streaming motions in the magnetosphere and the orbital-phase modulated line core that represents the accretion disk. Spectral analysis and tomography show that the division line between the two is solidly located at ∣υrad ∣ ≃ 1200 km s−1, defining the inner edge of the accretion disk at rin ≃ 7 × 109 cm or ∼10R1 (WD radii). This large central hole allows an unimpeded view of the tall accretion curtain at the lower pole with a shock height up to hsh ∼ 1R1 that is required by X-ray and optical observations. Our results contradict models that advocate a small magnetosphere and a small inner disk hole. Equating rin with the magnetospheric radius in the orbital plane allows us to derive a magnetic moment of the WD of μ1 ≃ 1.3 × 1032 G cm3 and a surface field strength B1 ∼ 0.35 MG. Given a polar field strength Bp ≲ 1.0 MG, optical circular polarization is not expected. With an accretion rate Ṁ = 3.9 × 10−11 M⊙ yr−1, the accretion torque is Gacc ≃ 2.2 × 1033 g cm2 s−2. The magnetostatic torque is of similar magnitude, suggesting that EX Hya is not far from being synchronized. We measured the orbital radial-velocity amplitude of the WD, K1 = 58.7 ± 3.9 km s−1, and found a spin-dependent velocity modulation as well. The former is in perfect agreement with the mean velocity amplitude obtained by other researchers, confirming the published component masses M1 ≃ 0.79 M⊙ and M2 ≃ 0.11 M⊙.

Comparison of Dimensionless Parameters in Astrophysical MHD Systems and in Laboratory Experiments

Estimates of typical parameters of accretion flows in the representative intermediate polar EX Hydrae, the polar AM Herculis, and the “hot Jupiter” WASP-12b are presented. Dimensionless parameters of astrophysical systems are compared with those of laboratory experiments on laser ablation in magnetic fields. It is shown that laboratory simulations of astrophysical flows is possible in principle, provided that some adjustment to the magnetic field, plasma density, and plasma velocity are made.

Constraining the Accretion Geometry of the Intermediate Polar EX Hya Using NuSTAR, Swift, and Chandra Observations

Abstract In magnetically accreting white dwarfs, the height above the white dwarf surface where the standing shock is formed is intimately related with the accretion rate and the white dwarf mass. However, it is difficult to measure. We obtained new data with NuSTAR and Swift that, together with archival Chandra data, allow us to constrain the height of the shock in the intermediate polar EX Hya. We conclude that the shock has to form at least at a distance of about one white dwarf radius from the surface in order to explain the weak Fe Kα 6.4 keV line, the absence of a reflection hump in the high-energy continuum, and the energy dependence of the white dwarf spin pulsed fraction. Additionally, the NuSTAR data allowed us to measure the true, uncontaminated hard X-ray (12-40 keV) flux, whose measurement was contaminated by the nearby galaxy cluster Abell 3528 in non-imaging X-ray instruments.

A radial velocity study of the intermediate polar EX Hydrae

A study on the intermediate polar EX Hya is presented, based on simultaneous photometry and high dispersion spectroscopic observations, during four consecutive nights. The strong photometric modulation related to with the 67-min spin period of the primary star is clearly present, as well as the narrow eclipses associated to the orbital modulation. Since our eclipse timings have been obtained almost 91,000 cycles since the last reported observations, we present new linear ephemeris, although we cannot rule out a sinusoidal variation suggested by previous authors. The system mainly shows double-peaked H$\alpha$, H$\beta$ and HeI $\lambda$5876 \AA emission lines. From the profile of the H$\alpha$ line, we find two components; one with a steep rise and velocities not larger than $\sim$1000 km s$^{-1}$ and another broader component extending up to $\sim$2000 km s$^{-1}$, which we interpret as coming mainly from the inner disc. A strong and variable hotspot is found and a stream-like structure is seen at times. We show that the best solution correspond to $K_1 = 58 \pm 5$ km s$^{-1}$ from H$\alpha$, from the two emission components, which are both in phase with the orbital modulation. We remark on a peculiar effect in the radial velocity curve around phase zero, which could be interpreted as a Rositter-MacLaughlin-like effect, which has been taken into account before deriving $K_1$. This value is compatible with the values found in high-resolution both in the ultraviolet and X-ray. We find: $M_{1} = 0.78 \pm 0.03$ M$_{\odot}$, $ M_{2} = 0.10 \pm 0.02$ M$_{\odot}$ and $a = 0.67 \pm 0.01$ R$_{\odot}$. Doppler Tomography has been applied, to construct six Doppler tomograms for single orbital cycles spanning the four days of observations to support our conclusions. Our results indicate that EX Hya has a well formed disc and that the magnetosphere should extend only to about $3.75\,R_{\rm{WD}}$.

Testing the cooling flow model in the intermediate polar EX Hydrae

We use the best available X-ray data from the intermediate polar EX Hydrae to study the cooling-flow model often applied to interpret the X-ray spectra of these accreting magnetic white dwarf binaries. First, we resolve a long-standing discrepancy between the X-ray and optical determinations of the mass of the white dwarf in EX Hya by applying new models of the inner disk truncation radius. Our fits to the X-ray spectrum now agree with the white dwarf mass of 0.79 M$_{\odot}$sun determined using dynamical methods through spectroscopic observations of the secondary. We use a simple isobaric cooling flow model to derive the emission line fluxes, emission measure distribution, and H-like to He-like line ratios for comparison with the 496 ks Chandra High Energy Transmission Grating observation of EX Hydrae. We find that the H/He ratios are not well reproduced by this simple isobaric cooling flow model and show that while H-like line fluxes can be accurately predicted, fluxes of lower-Z He-like lines are significantly underestimated. This discrepancy suggests that some extra heating mechanism plays an important role at the base of the accretion column, where cooler ions form. We thus explored more complex cooling models including the change of gravitational potential with height in the accretion column and a magnetic dipole geometry. None of these modifications to the standard cooling flow model are able to reproduce the observed line ratios. While a cooling flow model with subsolar (0.1 $\odot$) abundances is able to reproduce the line ratios by reducing the cooling rate at temperatures lower than $\sim 10^{7.3}$ K, the predicted line-to-continuum ratios are much lower than observed. We discuss and discard mechanisms such as photoionization, departures from constant pressure, resonant scattering, different electron-ion temperatures, and Compton cooling. [Abridged]

On the origin of the Fe Kα emission line from intermediate polar EX Hyrae

We present spectral analysis of intermediate polar EX Hya observed with Suzaku satellite on 18-07-2007 for 91kiloseconds (ks). We model the spectrum with an absorbed thermal bremsstrahlung and three Gaussian lines for the Fe Kα line complex. We resolved the 6.4, 6.7, and 7.0keV lines, each corresponding to the neutral (or low-ionized), He-like, and H-like iron ions. The fluorescence line is due to irradiation of neutral (or low ionized) material (iron) by hard X-ray sources, as a collisional origin would lead to rapid ionization. The He-like and H-like iron ions are emissions created by collisional excitation and photoionization/excitation in the vicinity of the hot white dwarf (WD). We study the emission of these lines in EX Hya and found that the fluorescence line is likely created by reflection of hard X-rays from the NH absorption columns as our search for the reflection of hard X-rays on the surface of the WD yielded a null result. The 6.7 and 7.0keV lines, however, are largely emitted through collisional excitation in the vicinity of the compact white dwarf shock heated by the shock front. We also discussed Fe Kα emission line as a diagnostic tool for high energy astrophysical sites.

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.

High-resolution spectroscopy of the intermediate polar EX Hydrae

High-resolution spectroscopy of the intermediate polar EX Hydrae

Orbital and spin phase-resolved spectroscopy of the intermediate polar EX Hya using XMM-Newton data

We present the orbital phase-resolved spectra of an intermediate polar, EX Hya, together with the spin phase-resolved spectra during two different epochs using the X-ray Multi-Mirror Mission (XMM–Newton), European Photon Imaging Camera (pn instrument). We find that the source at the two epochs has the same X-ray luminosity of ∼6.5 × 10 31 erg s −1 . We detect spectral variations between the 2000 and 2003 observations of the source. We fitted the spectrum using a neutral hydrogen absorption model with or without covering fraction together with Gaussians for emission lines, two collisional equilibrium plasma emission models (MEKAL) and a cooling-flow plasma emission model (VMCFLOW). We find that two of the three emission components (kT = 0.6–0.8 and 1.3–1.7 keV) fitted by the MEKAL models are almost constant over the spin and orbital phases and also over the two different epochs with the normalization varying directly proportional to the flux when the data are folded according to the orbital and spin phase indicating that the slight variation may be due to occultation. The emission modelled by the VMCFLOW changes over the spin and orbital phases and the 2000 and 2003 observations reveal two different ranges of temperatures (3–33 and 8–61 keV, respectively) that model the shock zone in the accretion column(s). The ratios of the spin maximum to minimum and the orbital maximum to minimum spectra along with the increase in the plasma temperatures indicate that the spectrum gets harder in the minimum phases of both orbital and spin periods. In the 2003 observation, a 6.4 keV fluorescent Fe emission line is present at the orbital minima in a range of phases from 0.9 to 1.3 and it is absent otherwise. This indicates that there is reflection from the disc most likely from a large bulge at the accretion impact zone.

PHOTOIONIZED FEATURES IN THE X-RAY SPECTRUM OF EX HYDRAE

We present the first results from a long (496 ks) Chandra High Energy Transmission Grating observation of the intermediate polar EX Hydrae. In addition to the narrow emission lines from the cooling post-shock gas, for the first time we have detected a broad component in some of the X-ray emission lines, namely O VIII 18.97, Mg XII 8.42, Si XIV 6.18, and Fe XVII 16.78. The broad and narrow components have widths of ~ 1600 km s^-1 and ~ 150 km s^-1, respectively. We propose a scenario where the broad component is formed in the pre-shock accretion flow, photoionized by radiation from the post-shock flow. Because the photoionized region has to be close to the radiation source in order to produce strong photoionized emission lines from ions like O VIII, Fe XVII, Mg XII, and Si XIV, our photoionization model constrains the height of the standing shock above the white dwarf surface. Thus, the X-ray spectrum from EX Hya manifests features of both magnetic and non-magnetic cataclysmic variables.

High-resolution spectroscopy of the intermediate polar EX Hydrae

High-resolution spectroscopy of the intermediate polar EX Hydrae

Spectroscopic observations of the intermediate polar EX Hydrae in quiescence

Results from spectroscopic observations of the intermediate polar (IP) EX Hya in quiescence during 1991 and 2001 are presented. Spin-modulated radial velocities consistent with an outer disc origin were detected for the first time in an IP. The spin pulsation was modulated with velocities near ∼500–600 km s−1. These velocities are consistent with those of the material circulating at the outer edge of the accretion disc, suggesting co-rotation of the accretion curtain with the material near the Roche lobe radius. Furthermore, spin Doppler tomograms have revealed evidence of the accretion curtain emission extending from velocities of ∼500 to ∼1000 km s−1. These findings have confirmed the theoretical model predictions for EX Hya, which predict large accretion curtains that extend to a distance close to the Roche lobe radius in this system. Evidence for overflow stream of material falling on to the magnetosphere was observed, confirming the result of Belle et al. that disc overflow in EX Hya is present during quiescence as well as outburst. It appears that the Hβ and Hγ spin radial velocities originated from the rotation of the funnel at the outer disc edge, while those of Hα were produced due to the flow of the material along the field lines far from the white dwarf (narrow component) and close to the white dwarf (broad-base component), in agreement with the accretion curtain model.

The Fe XXII I(11.92 Å)/I(11.77 Å) Density Diagnostic

AbstractUsing the Livermore X-ray Spectral Synthesizer, which calculates spectral models of highly charged ions based on HULLAC atomic data, we investigate the temperature, density, and photoexcitation dependence of the I(11.92 Å)/I(11.77 Å) line ratio of Fe XXII. Applied to the Chandra HETG spectrum of the intermediate polar EX Hya, we find that the electron density of its Te ≈ 12 MK plasma is orders of magnitude greater than that observed in the Sun or other late-type stars.

The Fe xxii I (11.92 )/ I (11.77 ) Density Diagnostic Applied to the Chandra High Energy Transmission Grating Spectrum of EX Hydrae

Using the Livermore X-ray Spectral Synthesizer, which calculates spectral models of highly charged ions based primarily on HULLAC atomic data, we investigate the temperature, density, and photoexcitation dependence of the I(11.92 A)/I(11.77 A) line ratio of Fe XXII. We find that this line ratio has a critical density nc ≈ 5 × 1013 cm-3, is approximately 0.3 at low densities and 1.5 at high densities, and is very insensitive to temperature and photoexcitation; therefore, it is a useful density diagnostic for sources like magnetic cataclysmic variables, in which the plasma densities are high and in which the efficacy of the He-like ion density diagnostic is compromised by the presence of a bright ultraviolet continuum. Applying this diagnostic to the Chandra High Energy Transmission Grating spectrum of the intermediate polar EX Hya, we find that the electron density of its Te ≈ 12 MK plasma is ne = 1.0 × 1014 cm-3, which is orders of magnitude greater than that typically observed in the Sun or other late-type stars.

An Extreme‐Ultraviolet Study of the Intermediate Polar EX Hydrae

On 2000 May 5, we began a large, multiwavelength campaign to study the intermediate polar (IP) EX Hydrae. The simultaneous observations from six satellites and four telescopes were centered around a 1 million second observation with Extreme Ultraviolet Explorer (EUVE). Although EX Hya has been studied previously with EUVE, our higher signal-to-noise ratio observations present new results and challenge the current IP models. Previously unseen dips in the light curve are reminiscent of the stream dips seen in polar light curves. Also of interest is the temporal extent of the bulge dip, approximately 0.5 in phase, implying that the bulge extends over half of the accretion disk. We propose that the magnetic field in EX Hya is strong enough (a few MG) to begin pulling material directly from the outer edge of the disk, thereby forming a large accretion curtain, which would produce a very broad bulge dip. This would also result in magnetically controlled accretion streams originating from the outer edge of the disk. We also present a period analysis of the photometric data, which shows numerous beat frequencies with strong power and also intermittent and wandering frequencies, an indication that physical conditions within EX Hya changed over the course of the observation. Iron spectral line ratios give a temperature of log T = 6.5-6.9 K for all spin phases and a poorly constrained density of ne = 1010-1011 cm-3 for the emitting plasma. This paper is the first in a series detailing our results from this multiwavelength observational campaign.

Multi-wavelength spectrophotometry of EX Hydrae

We present phase-resolved infrared and optical spectrophotometry of the intermediate polar EX Hya supplemented by archival ultraviolet data. The spin-modulated emission from the accretion funnel and the emission from the accretion disk or ring contain substantial optically thin components. The white dwarf dominates the unmodulated flux in the ultraviolet and is identied by numerous absorption lines. Metal absorption in the accretion curtain may add to the observed spectral features. The secondary star is of spectral type M4 1 and is detected by its ellipsoidal modulation. We derive a distance of 65 11 pc which makes EX Hydrae one of the closest cataclysmic variables with a known distance. The luminosity derived from the integrated overall spectral energy distribution is 3 10 32 erg s 1 . The accretion rate of 3 10 15 gs 1 (for an 0.6M white dwarf) is in reasonable agreement with the rates expected from angular momentum loss by gravitational radiation and from the observed spin-up of the white dwarf.

First Application of the FexviiI(17.10 A)/I(17.05 A) Line Ratio to Constrain the Plasma Density of a Cosmic X‐Ray Source

We show that the Fe XVII I(17.10 u/I(17.05 u line ratio observed in the Chandra HETG spectrum of the intermediate polar EX Hydrae is significantly smaller than that observed in the Sun or other late-type stars. Using the Livermore X-ray Spectral Synthesizer, which calculates spectral models of highly charged ions based on HULLAC atomic data, we find that the observed I(17.10 u/I(17.05 u line ratio can be explained if the plasma density ne > 3 × 10 14 cm −3 . However, if photoexcitation is included in the level population kinetics, the line ratio can be explained for any density if the photoexcitation temperature Tbb > 55 kK. For photoexcitation to dominate the Fe XVII level population kinetics, the relative size of the hotspot on the white dwarf surface must be f 2 × 10 14 cm −3 for the post-shock flow. Either way, then, the Chandra HETG spectrum of EX Hya requires a plasma density which is orders of magnitude greater than that observed in the Sun or other late-type stars.

ASCAX-ray observations of EX Hya: spin-resolved spectroscopy

A B STR A CT We analyse the spectral changes over the spin modulation in the intermediate polar EX Hya using archival ASCA data. We find that the modulation can be modelled as either (1) the effect of occultation of the accretion poles by the limb of the white dwarf, or (2) the effect of phase-dependent photoelectric absorption. We argue, on the basis of the partial X-ray eclipse, that the accretion columns in the system are tall, with shock height 1R wd, and hence that the spin modulation is caused mainly by occultation. We find that the temperature distribution along the accretion shocks is incompatible with the calculations of Aizu, except for a restricted parameter regime with a high M wd. Hence the material in the shock must cool faster than predicted by theory.

The Far‐Ultraviolet Spectrum of the Intermediate Polar EX Hydrae

The intermediate polar EX Hya was observed in 1995 March with the Hopkins Ultraviolet Telescope on the Astro-2 space shuttle mission. The time-resolved, 820-1860 A data that were obtained are the first spectra of an intermediate polar to extend to the Lyman limit. They cover phases 67 = 0.05-0.50 of the 67 minute white dwarf rotational period. Fitted to a blackbody, the continuum has an effective temperature of about 25,000 K. At 67 = 0.0, the spectrum shows strong, broad (2500 km s-1 FWHM) lines of C IV λ1549, C III λ977, λ1176, N V λ1240, Si IV λ1394, and a blend of lines including Lyβ and O VI λ1034. The line ratios suggest a photoionized emission region with densities 1013 cm-3. As the phase increases to 0.5, the line fluxes drop by more than a factor of 4, but the continuum flux declines by only ~20%. There are ±30% fluctuations in the continuum flux, but not the lines, on timescales of less than a minute during the second half of the observation, which suggests that the lines and continuum arise in physically distinct regions of the accretion flow. These data are compatible with models of EX Hya in which the UV lines arise mainly in an optically thick accretion curtain and the UV continuum arises in the innermost part of the accretion curtain and/or the heated face of the white dwarf. The profile of the C IV λ1549 line changes considerably over the course of the observation, with a strong absorption appearing on the blue wing of the line from 67 = 0.36 to the end of the observation.

EUV Observations of EX Hydrae: 107K Gas near a White Dwarf Surface

We have observed the intermediate polar EX Hydrae for 180,000 s with the Extreme Ultraviolet Explorer. The EUV flux is strongly modulated at both the binary period (98 minutes) and the white dwarf spin period (67 minutes). The secondary star eclipse is total, with a duration of 41 ± 2 s. The ingress and egress of the secondary eclipse have a duration <3 s (68% confidence). The centroid of the secondary eclipse is independent of the white dwarf spin phase to within 6 s. The eclipse attributed to the disk bulge has an energy dependence consistent with photoelectric absorption through a neutral column of about 1.3 × 1020 cm-2. Folded on the white dwarf spin period, the EUV flux is modulated by a factor of 3.7. The shape of the light curve can be accurately matched with a simple geometrical absorption model, although the variation in modulation depth with energy (e.g., EUV vs. X-rays) requires a more sophisticated model. The EUV spectrum reveals many narrow emission lines characteristic of a plasma around 107 K, and possibly features indicating temperatures as low as 106 K. Assuming optically thin emission, we find a volume emission measure around 3 × 1054 cm-3 for the 107 K plasma, and an accretion rate around 3 × 1016 g s-1. By direct geometrical arguments we constrain the EUV emission region to a region extending over less than 4% of the white dwarf surface area located within 0.5 white dwarf radii (in the orbital plane) and 4.5 white dwarf radii (perpendicular to the plane) relative to the center of the white dwarf. Two independent arguments lead to the conclusion that the electron density is >1013 cm-3, probably >1015 cm-3, in the EUV-emitting region.