VV Pup Research Articles

Fast quasi-periodic oscillations in the eclipsing polar VV Puppis from VLT and XMM-Newton observations

We present high time resolution optical photometric data of the polar VV Puppis obtained simultaneously in three filters (u′, HeII λ4686, r′) with the ULTRACAM camera mounted at the ESO-VLT telescope. An analysis of a long 50 ks XMM-Newton observation of the source, retrieved from the database, is also provided. Quasi-periodic oscillations (QPOs) are clearly detected in the optical during the source bright phase intervals when the accreting pole is visible, confirming the association of the QPOs with the basis of the accretion column. QPOs are detected in the three filters at a mean frequency of ∼0.7 Hz with a similar amplitude ∼1%. Mean orbitally-averaged power spectra during the bright phase show a rather broad excess with a quality factor Q = ν/Δν = 5−7 but smaller data segments commonly show a much higher coherency with Q up to 30. The X-ray Multi-mirror Mission XMM (0.5–10 keV) observation provides the first accurate estimation of the hard X-ray component with a high kT ∼ 40 keV temperature and confirms the high extreme ultraviolet (EUV)-soft/hard ratio in the range of 4−15 for VV Pup. The detailed X-ray orbital light curve displays a short Δϕ ≃ 0.05 ingress into self-eclipse of the active pole, indicative of an accretion shock height of ∼75 km. No significant X-ray QPOs are detected with an amplitude upper limit of ∼30% in the range 0.1–5 Hz. Detailed hydrodynamical numerical simulations of the post-shock accretion region with parameters consistent with VV Pup demonstrate that the expected frequencies from radiative instability are identical for X-rays and optical regime at values ν ∼ 40–70 Hz, more than one order magnitude higher than observed. This confirms previous statements suggesting that present instability models are unable to explain the full QPO characteristics within the parameters commonly known for polars.

ROSAT x-ray analysis of the AM Her cataclysmic variable VV Pup

VV Pup is typical system of AM Her stars, where the main accreting pole rotates in and out of view during the orbital cycle. In the present paper we present ROSAT data analysis for the magnetic cataclysmic variable VV Pup. We obtained the X-ray light curves of VV Pup in high state, the PSPC count rate 0.1-2.0 keV is plotted as a function of time with bins of 10 sec and the count rate is folded over the orbital period of 100.4 min with bin size of 100 sec for individual observations. We calculate the mean best-fit PSPC spectrum, with a three-component spectral fit including a soft X-ray blackbody, hard X-ray bremsstrahlung, and Gaussian line covers the phase intervals, for a bright phase (phi_orb=0.9-1.1), the dip data (phi_orb=0.18-0.7), the egress from the dip (phi_orb=0.7-0.8), the phase interval phi = 0.1-0.18 and the mean best-fit spectrum for all data (phi_orb=0.0-1.0). We calculate spectral parameters, the hardness ratios, count rate and total integrated black body flux.

VV Puppis in a low state: secondary-star irradiation or stellar activity?

Aims. Emission lines in polars show complex profiles with multiple components that are typically ascribed to the accretion stream, threading region, accretion spot, and the irradiated secondary-star. In low-state polars the fractional contribution by the accretion stream, and the accretion spot is greatly reduced offering an opportunity to study the effect of the secondary-star irradiation or stellar activity. We observed VV Pup during an exceptional low-state to study and constrain the properties of the line-forming regions and to search for evidence of chromospheric activity and/or irradiation.

Cyclotron Modeling Phase‐Resolved Infrared Spectroscopy of Polars. II. EQ Ceti, AN Ursa Majoris, and VV Puppis

We present phase-resolved low-resolution infrared spectra of the polars EQ Cet, AN UMa, and VV Pup obtained with SPEX on the IRTF. In addition, we supplement our IRTF observations for VV Pup with optical data sets. The infrared spectra cover the wavelength range 0.8 μ m ⩽ λ ⩽ 2.4 μm and show cyclotron emission at all orbital phases for the three objects. We use a constant-lambda prescription to attempt to model the changing cyclotron features seen in the spectra. We model the EQ Cet data using two different approaches, yielding final parameters of B≃ 34.0 MG, kT≃ 3.5 keV, and log Λ ≃ 4.1. For AN UMa we find orbitally averaged models of B = 32.1 MG, kT = 5.0 keV, and log Λ ≃ 4.3. We modeled phase-resolved spectra for VV Pup at four different epochs. We find the following range in parameters: 30.0 MG ⩽ B⩽ 31.9 MG, 4.0 keV ⩽ kT ⩽ 15.0 keV, and 3.7 ⩽ log Λ ⩽ 6.5. Finally, we find that in B-kT space, the four epochs are well separated based on the visual brightness of the system at the time of observation.

Spitzer Space TelescopeObservations of Magnetic Cataclysmic Variables: Possibilities for the Presence of Dust in Polars

We present Spitzer Space Telescope photometry of six short-period polars, EF Eri, V347 Pav, VV Pup, V834 Cen, GG Leo, and MR Ser. We have combined the Spitzer Infrared Array Camera (3.6 -8.0 microns) data with the 2MASS J, H, K_s photometry to construct the spectral energy distributions of these systems from the near- to mid-IR (1.235 - 8 microns). We find that five out of the six polars have flux densities in the mid-IR that are substantially in excess of the values expected from the stellar components alone. We have modeled the observed SEDs with a combination of contributions from the white dwarf, secondary star, and either cyclotron emission or a cool, circumbinary dust disk to fill in the long-wavelength excess. We find that a circumbinary dust disk is the most likely cause of the 8 micron excess in all cases, but we have been unable to rule out the specific (but unlikely) case of completely optically thin cyclotron emission as the source of the observed 8 micron flux density. While both model components can generate enough flux at 8 microns, neither dust nor cyclotron emission alone can match the excess above the stellar components at all wavelengths. A model combining both cyclotron and dust contributions, possibly with some accretion-generated flux in the near-IR, is probably required, but our observed SEDs are not sufficiently well-sampled to constrain such a complicated model. If the 8 micron flux density is caused by the presence of a circumbinary dust disk, then our estimates of the masses of these disks are many orders of magnitude below the mass required to affect CV evolution.

First detection of Zeeman absorption lines in the polar VV Puppis

Aims. We investigated the low state of the polar VV Pup by collecting high S/N time series spectra. Methods. We monitored VV Pup with VLT+FORS1 and analyzed the evolution of its spectroscopic features across two orbits. Results. We report the first detection of photospheric Zeeman lines in VV Puppis. We argue that the photospheric field structure is inconsistent with the assumption that the accretion shocks are located close to the foot points of a closed field line in a dipolar field distribution. A more complex field structure and coupling process is implied making VV Puppis similar to other well studied AM Herculis type systems.

Mass Determination and Detection of the Onset of Chromospheric Activity for the Substellar Object in EF Eridani

EF Eri is a magnetic cataclysmic variable that has been in a low accretion state for the past 9 yr. Low-state optical spectra reveal the underlying Zeeman-split white dwarf absorption lines. These features are used to determine a value of 13-14 MG as the white dwarf field strength. Recently, 5-7 yr into the low state, Balmer and other emission lines have appeared in the optical. An analysis of the Hα emission line yields the first radial velocity solution for EF Eri, leading to a spectroscopic ephemeris for the binary and, using the best available white dwarf mass of 0.6 M☉, a mass estimate for the secondary of 0.055 M☉. For a white dwarf mass of 0.95 M☉, the average for magnetic white dwarfs, the secondary mass increases to 0.087 M☉. At EF Eri's orbital period of 81 minutes, this higher mass secondary could not be a normal star and still fit within the Roche lobe. The source of the Balmer and other emission lines is confirmed to be from the substellar secondary, and we argue that it is due to stellar activity. We compare EF Eri's emission-line spectrum and activity behavior to that recently observed in AM Her and VV Pup and attributed to stellar activity. We explore observations and models originally developed for V471 Tau, for the RS CVn binaries, and for extrasolar planets. We conclude that irradiation of the secondary in EF Eri and similar systems is unlikely and, in polars, the magnetic field interaction between the two stars (with a possible tidal component) is a probable mechanism that would concentrate chromospheric activity on the secondary near the substellar point of the white dwarf.

Low-State Phase-Resolved Infrared Spectroscopy of VV Puppis

We present phase-resolved low-resolution JHK and higher resolution K-band spectroscopy of the polar VV Pup. All observations were obtained when VV Pup was in a low-accretion state having a K magnitude near 15. The low-resolution observations reveal cyclotron emission in the J band during some phases, consistent with an origin near the active 30.5 MG pole on the white dwarf. The secondary in VV Pup appears to be a normal M7 V star, and we find that the H- and K-band fluxes are entirely due to this star at all orbital phases during the low-accretion state. We use our higher resolution Keck spectroscopy to produce the first K-band radial velocity curve for VV Pup. Our orbital solution yields K2 = 414 ? 27 km s-1 and leads to mass estimates of M1 = 0.73 ? 0.05 M? and M2 = 0.10 ? 0.02 M?. We find that the mass accretion rates during the normal low states of the polars VV Pup, EF Eri, and EQ Cet are near 10-13 M? yr-1. The fact that is not zero in low-state polars indicates active secondary stars in these binary systems, including the substellar donor star present in EF Eri.

Far‐Ultraviolet Spectroscopy of Magnetic Cataclysmic Variables

We have obtained Hubble Space Telescope (HST) STIS data for a total of 11 polars as part of a program aimed at compiling a homogeneous database of high-quality far-ultraviolet (FUV) spectra for a large number of cataclysmic variables (CVs). Of the 11 polars, eight were found in a state of low accretion activity (V347 Pav, VV Pup, V834 Cen, BL Hyi, MR Ser, ST LMi, RX J1554.2+2721, and V895 Cen) and three in a state of high activity ( CD Ind, AN UMa, and UW Pic). The STIS spectra of the low-state polars unambiguously reveal the photospheric emission of their white dwarf (WD) primaries. We have used pure hydrogen WD models to fit the FUV spectra of the low-state systems (except RX J1554.2+2721, which is a high-field polar) in order to measure the WD effective temperatures. In all cases, the fits could be improved by adding a second component, which is presumably due to residual accretion onto the magnetic pole of the WD. The WD temperatures obtained range from 10,800 to 14,300 K for log g = 8.0. Our analysis more than doubles the number of polars with accurate WD effective temperatures. Comparing the WD temperatures of polars with those of nonmagnetic CVs, we find that at any given orbital period the WDs in polars are colder than those in nonmagnetic CVs. The temperatures of polars below the period gap are consistent with gravitational radiation as the only active angular momentum loss mechanism. The differences in WD effective temperatures between polars and nonmagnetic CVs are significantly larger above the period gap, suggesting that magnetic braking in polars might be reduced by the strong field of the primary. We derive distance estimates to the low-state systems from the flux scaling factors of our WD model fits. Combining these distance measurements with those from the literature, we establish a lower limit on the space density of polars of 1.3 x 10(-6) pc(-3).

Irregular Mass Transfer in the Polars VV Puppis and V393 Pavonis during the Low State

The polars VV Pup and V393 Pav were observed with XMM-Newton during states of low accretion rate with peak X-ray luminosities of ~1 x 10^30 and ~1 x 10^31 erg/s, respectively. In both polars, accretion onto the white dwarf was extremely irregular, and the accretion rate varied by more than 1 order of magnitude on timescales of ~1 hr. Our observations suggest that this type of irregular accretion is a common phenomenon in polars during the low state. The likely cause of the accretion rate fluctuations are coronal mass ejections or solar flares on the companion star that intermittently increase the mass transfer into the accretion stream. Our findings demonstrate that the companion stars in cataclysmic variables possess highly active atmospheres.

FUSE Results on Magnetic CVs: VV Pup, YY Dra, LS Peg and DW UMa

AbstractIn Cycles 2 and 3, we obtained FUSE observations of the Polar VV Pup, and the IP/SW Sex stars YY Dra, LS Peg and DW UMa. The spectra show a wide range of line intensities and structure. The phase-resolved spectra in each case reveal interesting properties about the hot accretion zones.

A spectroscopic search for faint secondaries in cataclysmic variables

The secondary in cataclysmic variables (CVs) is usually detected by cross-correlation of the CV spectrum with that of a K or M dwarf template, to produce a radial velocity curve. Although this method has demonstrated itspower, it has its limits in the case of noisy spectra, such as are found when the secondary is faint. A method of coadding spectra, called skew mapping, has been proposed in the past. Gradually, examples of its application are being published; none the less, so far no journal article has described the technique in detail. To answer this need, this paper explores in detail the capabilities of skew mapping when determining the amplitude of the radial velocity for faint secondaries. It demonstrates the power of the method over techniques that are more conventional, when the signal-to-noise ratio is poor. The paper suggests an approach to assessing the quality of results. This leads in the case of the investigated objects to a first tier of results, where we find K 2 = 127 ′ 23 km s - 1 for SY Cnc, K 2 = 144 ′ 18 km s - 1 for RW Sex and K 2 = 262 ′ 14 km s - 1 fon UX UMa. These we believe to be the first direct determinations of K 2 for these objects. Furthermore, we also obtain K 2 = 263 ′ 30 km s - 1 for RW Tri, close to a skew mapping result obtained elsewhere. In the first three cases, we use these results to derive the mass of the white dwarf companion. A second tier of results includes UU Aqr, EX Hya and LX Ser, for which we propose more tentative values of K 2 . Clear failures of the method are also discussed (EF Eri, VV Pup and SW Sex).

Observations of the Magnetic Cataclysmic Variable VV Puppis with the [ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL

We present the first far-ultraviolet (FUV) observations of the magnetic cataclysmic variable VV Puppis, obtained with the Far Ultraviolet Spectroscopic Explorer satellite. In addition, we have obtained simultaneous ground-based optical photometric observations of VV Pup during part of the FUV observation. The shapes of the FUV and optical light curves are consistent with each other and with those of past observations at optical, extreme-ultraviolet, and X-ray wavelengths. Time-resolved FUV spectra during the portion of VV Pup's orbit when the accreting magnetic pole of the white dwarf can be seen show an increasing continuum level as the accretion spot becomes more directly visible. The most prominent features in the spectrum are the O VI λλ1031.9, 1037.6 emission lines. We interpret the shape and velocity shift of these lines in the context of an origin in the accretion funnel near the white dwarf surface. A blackbody function with Tbb 90,000 K provides an adequate fit to the FUV spectral energy distribution of VV Pup.

Coordinated Optical and X‐Ray Observations of the AM Herculis Objects VV Puppis, V834 Centauri, and EF Eridani

ABSTRACTWe obtained coordinated broadband optical photometry and hard X‐ray data on the AM Her objects VV Pup, V834 Cen, and EF Eri. The X‐ray data were obtained using the Proportional Counter Array detectors of the Rossi X‐Ray Timing Explorer, and the optical data were obtained using the 1 m telescope of the Cerro Tololo Inter‐American Observatory. During our observations, VV Pup and V834 Cen were in their high optical luminosity states with average mV∼17 and 15.1, respectively. EF Eri was in a very low state at the time of our observation: mV>17.7. VV Pup and V834 Cen had average 2–10 keV X‐ray fluxes of 1.7 × 10-12 and 2.13 × 10-11 ergs cm−2 s−1, respectively. EF Eri was not detected in X‐rays. The X‐ray spectrum of V834 Cen was adequately modeled using an absorbed bremsstrahlung continuum with a Gaussian emission line. The best‐fit model had continuum temperature kTX = 11.8 ± 1.0 keV and absorption column nH = (1.03 ± 0.39) × 1022 cm−2. The Gaussian emission line had line center energy E = 6.7 ± 0.1 keV and equivalent width EW = 0.64 keV. VV Pup was too faint to produce reliable spectral fits. The 2–10 keV flux for VV Pup quoted above was based on the best‐fit spectral model. Holding kTX fixed in the fitting process yielded roughly the same flux for kTX = 0.5–30 keV.We detected ∼1 Hz optical quasi‐periodic oscillations (QPOs) with amplitude ∼2.2% from VV Pup and ∼1% from V834 Cen. We did not detect optical QPOs over the range 0.2–1 Hz from EF Eri at a 90% confidence upper limit of 4.5%. The QPO properties are consistent with those found in the literature. None of the targets showed X‐ray QPO activity. However, only V834 Cen yielded a meaningful upper limit: ∼14% at the 90% confidence level over 0.2–1.2 Hz.

Extreme Ultraviolet Photometry and Spectroscopy of BL Hydri

We have analyzed Extreme Ultraviolet Explorer (EUVE) observations of BL Hyi obtained during a high-accretion state in 1995 October. The light curve obtained with the Deep Survey instrument reveals a broad hump covering the phases in which both poles have been known to be active in X-ray emission. In contrast to the X-ray, both poles contribute a comparable extreme ultraviolet (EUV) flux. The short-wavelength spectrum can be fitted with either a relatively cool (5 eV), pure-hydrogen white dwarf or with a 17 eV blackbody; both fits imply a neutral hyrogen column density in the local interstellar medium of ~3 × 1019 cm-2. However, nearly simultaneous IUE far-ultraviolet (FUV) spectroscopic measurements rule out pure-hydrogen white dwarf models with temperatures of less than 8 eV. BL Hyi has a cooler accretion area than AM Her and VV Pup, but it is similar in temperature to QS Tel. The total EUV energy is 2-4 × 1032 ergs s-1, which is only slightly greater than the hard X-ray and cyclotron energy. This suggests that BL Hyi has a conventional accretion column and that the polar cap is primarily heated from above by radiation from the accretion shock, rather than from below by dense blobs that penetrate the photosphere.

Cyclotron spectroscopy of VV Puppis

AbstractWe present phase‐resolved spectrophotometric observations of VV Puppis obtained during two different states of accretion. We confirm the detection of cyclotron lines from emission regions at both poles (Wickramasinghe et al. 1989) having significantly different field strength of B1 ≃ 31 MG and B2 ≃ 54 MG. Our phase‐resolved data allowed the detection of phase‐dependent wavelength shifts of the cyclotron lines from the main accretion pole which is due to the varying aspect of the observer. A corresponding motion of the cyclotron lines from the secondary pole appears likely.Compared to 1984, the cyclotron lines from the main pole appeared redshifted in 1989, during an episode of higher system brightness. This shift can be explained assuming either different locations of the accretion spot and, hence, variations of the magnetic field, or variations of the plasma temperature both triggered by variations of the mass accretion rate.The cyclotron lines from the second pole do not show such changes suggesting that the coupling region for the weakly accreting pole is more or less stationary whereas the coupling region of the main pole varies in space, most likely depending on the mass accretion rate.

High Speed UV Photometry of the AM Her Systems AM Her, VV Pup And V834 Cen, with the Hubble Space Telescope

We have used the High Speed Photometer (HSP) aboard the Hubble Space Telescope to observe the magnetic cataclysmic variables VV Pup, AM Her, and V834 Cen in the UV (1400…3300 Å) with 0.01 s time resolution. We detected low frequency flickering in all three systems, and compare the time-scales with the predictions of King (1989). At higher frequencies we searched for shock oscillations from the accretion column(s) in these systems. The data were analyzed using the Gabor transform wavelet-like technique (Heil & Walnut 1989) to search for frequency evolution throughout each observation. Preliminary analysis suggests the detection of rapid UV quasi-periodic oscillations (QPOs) in VV Pup at 0.74 Hz, and at 4.4 Hz in V834 Cen. As in ground based observations, our observations failed to yield any rapid QPOs in AM Her itself.

A search for spectral variations in ROSAT observations of AM Her, VV Pup, BL Hyi and UZ For

We continue our series of papers examining ROSAT observations of AM Her stars which seek to improve our understanding of the physical processes occurring in the accretion region in these systems. In this paper, we present the results of a search for spectral variability in AM Her, VV Pup, BL Hyi and UZ For using ROSAT data. We find a significant variation in the soft X-ray component of AM Her, W Pup and BL Hyi, and also a significant variation in the ratio of the soft to hard X-ray flux in AM Her and BL Hyi. We attribute these results to dense blobs of material impacting into the photosphere of the white dwarf. Further, the strength of the magnetic field of the accreting pole influences the proportion of dense blobs in the accretion stream.

The Three-Dimensional Structure of EUV Accretion Regions of AM Her Stars: Analysis of EUVE Light Curves

We present EUV light curves for a number of AM Her systems observed either as guest observer targets or with the EUVE Right Angle Program. We have formed light curves for eight AM Her stars and show in our presentation the similarities and differences present. We draw some conclusions by grouping the systems by inclination, magnetic field strength, and accretion region geometry. In order to understand the physical structures responsible for the EUV emissions, we have developed a software model to generate synthetic light curves. We find that the EUV accretion regions in the systems UZ For, VV Pup, and AM Her cannot be fit with a flat spot confined to the white dwarf surface, regardless of its shape or brightness profile. Rather, a small, symmetric, raised spot is the only shape consistent with the data. The light curves for systems EF Eri, RE1149+28, AN UMa, and V834 Cen show evidence for additional structure that precedes the primary accretion region in phase. Our model indicates that a large portion of the light curve in each system is seen in absorption. Finally, in three systems, we detect a very gradual rise and fall in the EUV flux (<5% of the peak flux) at phases when the spot is completely obscured behind the white dwarf (WD) limb. We attribute this detection to emission from the accretion column that decreases exponentially with distance from the WD and detect emission at heights up to 15% of the WD radius.

Extreme ultraviolet spectroscopy and photometry of VV Puppis during a high accretion state

We determine the physical properties of the accretion region of the AM Her-type binary VV Puppis using extreme ultraviolet (EUV) medium-resolution spectroscopy and photometry obtained with the Extreme Ultraviolet Explorer (EUVE) observatory. The EUV continuum from VV Pup was detected in the wavelength range from 75 to 135 A and was simultaneously recorded with the Deep Survey/Spectrometer (DS/S) imaging telescope, allowing for the extraction of an accurate light curve. VV Pup appeared to have entered a high-accretion state just prior to the pointed EUVE observations. We use the EUV light curve to infer the diameter of the accretion region (d = 220 km) assuming a hemispherical geometry and a radius of 9000 km for the white dwarf. We perform a model atmosphere analysis and, based on the light curve properties and assuming a distance of 145 pc, we derive an effective temperature of the accretion region in the range 270,000 is less than T(sub eff) is less than 360,000 K and a neutral hydrogen column density in the local interstellar medium of n(sub H) = 1.9 - 3.7 x 10(exp 19)/sq cm. The total EUV/soft X-ray energy radiated by the accretion region is approximately 3.5 x 10(exp 32) ergs/s. Our results provide a first verification of past suggestions that deep heating of the white dwarf surface produces the soft X-ray flux from the polars. We present a possible detection of O VI absortion features, and we suggest that extensive EUVE observations targeting high-accretion events may result in oxygen and heavier element abundance determination in the accretion region.