AM Herculis Systems Research Articles

Where are the progenitors of magnetic cataclysmic variables (polars)?

As over a thousand white dwarf plus M dwarf composite spectrum binaries with high quality spectra are now tabulated from the Sloan Digital Sky Survey alone, a remarkable discovery has been made. No pairing of a magnetic white dwarf of two megagauss or greater field strength with an M dwarf has been found. At this field strength, Zeeman splitting of hydrogen or helium lines would be detectable. Thus there are no known progenitors of magnetic cataclysmic variables – AM Herculis systems or polars. The formation of the strong magnetic field by a dynamo mechanism within the common envelope of a binary system has been proposed as the possible explanation.

Photometric and polarimetric observations of the eclipsing polar SDSS J205017.84-053626.8

Photometric and polarimetric observations of the eclipsing polar (AM Herculis system) Sloan Digital Sky Survey 2050-05 are reported, in which both sets of data are found to be modulated on a period of ∼94.2 min. This is the first detection of polarization in this system, unambiguously confirming its identification. Circular polarization variations are seen from ∼− 10 to +4 per cent, consistent with a ‘two-pole’ accretor. Both accreting poles are self-occulted by the white dwarf during parts of the orbit. The photopolarimetry from different epochs shows the system to be in different states. During the epochs showing a higher state the polarimetry suggests accretion occurs preferentially near the pole displaying positive circular polarization, and vice versa during the epoch showing a lower state. From the eclipses we estimate some of the system’s parameters.

Photopolarimetric observations of the new eclipsing polar CTCV J1928 — 5001

We report photopolarimetric observations of a new eclipsing polar (AM Herculis system) discovered in the Calan-Tololo survey. The photometry and polarimetry are modulated on a period of � 101 minutes. Circular polarization variations are seen from � 8 to +12 per cent and from � 0 to 5 per cent in the red and blue parts of the optical spectrum, respectively. Two linearly polarized pulses are detected at orbital phases coinciding with the reversals in the circular polarization. This is consistent with a magnetic field strength of � 20 MG for the white dwarf primary, where accretion takes place at two regions. Both accretion regions are self-occulted by the white dwarf during parts of the orbit. We estimate some of the system's parameters from its eclipses, which we further refine by modelling the polarimetric observations.

XMM-Newton observation of the long-period polar V1309 Orionis: the case for pure blobby accretion

Using XMM-Newton we have obtained the first continuous X-ray observation covering a complete orbit of the longest period polar, V1309 Ori. The X-ray light curve is dominated by a short, bright phase interval with EPIC pn count rates reaching up to 15 cts s-1 per 30 s resolution bin. The bright phase emission is well described by a single blackbody component with eV. The absence of a bremsstrahlung component at photon energies above 1 keV yields a flux ratio . This represents the most extreme case of a soft X-ray excess yet observed in an AM Herculis star. The bright, soft X-ray emission is subdivided into a series of individual flare events supporting the hypothesis that the soft X-ray excess in V1309 Ori is caused by accretion of dense blobs carrying the energy into sub-photospheric layers. On average, the flares have rise and fall times of 10 s. In addition to the bright phase emission, a faint, hard X-ray component is visible throughout the binary orbit with an almost constant count rate of 0.01 cts s-1. Spectral modelling indicates that this emission originates from a complex multi-temperature plasma. At least three components of an optically thin plasma with temperatures , 0.7, and 2.9 keV are required to fit the observed flux distribution. The faint phase emission is occulted during the optical eclipse. Eclipse ingress lasts about 15–20 min and is substantially prolonged beyond nominal ingress of the white dwarf. This and the comparatively low plasma temperature provide strong evidence that the faint-phase emission is not thermal bremsstrahlung from a post-shock accretion column above the white dwarf. A large fraction of the faint-phase emission is ascribed to the spectral component with the lowest temperature and could be explained by scattering of photons from the blackbody component in the infalling material above the accretion region. The remaining hard X-ray flux could be produced in the coupling region, so far unseen in other AM Herculis systems.

Photometric and polarimetric observations of the eclipsing polar SDSS J015543.40+002807.2

We present photometric and polarimetric follow-up observations of the eclipsing Sloan Digital Sky Survey cataclysmic variable SDSS J015543.40+002807.2. Linear and circular polarizations were detected, confirming that it is a new polar (AM Herculis system). An accurate eclipse ephemeris yields an orbital period of 87.1435 ± 0.0002 min. Assuming a range of mass ratios 0.11 > M 2 /M 1 > 0.07, the inclination lies in the range 85° < i < 90°. The circular polarization shows a change in sign, implying that SDSS J015543.40+002807.2 is a two-pole accretor, although the possibility that it could be a single-pole accretor cannot yet be ruled out.

Stokes imaging, Doppler mapping and Roche tomography of the AM Herculis system V834 Cen

We report on new simultaneous phase-resolved spectroscopic and polarimetric observations of the polar (AM Herculis star) V834 Cen during a high state of accretion. Strong emission lines and high levels of variable circular and linear polarization are observed over the orbital period. The polarization data are modelled using the Stokes imaging technique of Potter et al. The spectroscopic emission lines are investigated using the Doppler tomography technique of Marsh and Horne and the Roche tomography technique of Dhillon and Watson. Up to now, all three techniques have been used separately to investigate the geometry and accretion dynamics in cataclysmic variables. For the first time, we apply all three techniques to simultaneous data for a single system. This allows us to compare and test each of the techniques against each other and hence to derive a better understanding of the geometry, dynamics and system parameters of V834 Cen. All three techniques are consistent with an interpretation in which a ballistic stream extends to a minimum of ∼40 degrees in azimuth around the white dwarf before becoming threaded by the magnetic field lines. Interestingly, the observed ballistic Doppler velocities do not show a reduced vy component, as found in Doppler imaging of other AM Her systems. Furthermore, the secondary star in V834 Cen shows more He ii (4686 Å) emission on its leading inner face, as opposed to the trailing face like in other AM Her systems. We propose that the accretion shock preferentially illuminates the leading face of the secondary star. In addition, the ballistic stream does not obscure the leading face of the secondary from the accretion shock, and, in fact, our Doppler maps show that the ballistic stream is a strong He ii (4686 Å) emission source in itself and thus adds to the illumination of the leading face of the secondary.

Analysis of new spectropolarimetric data of AR UMa

Optical spectropolarimetric observations of the high field AM Herculis system AR UMa during a low state of activity reveal a strongly structured circular polarization spectrum with a peak polarization of about 6 per cent. The strongest features can be identified with Zeeman split components of Hα and Hβ in an average magnetic field of 140‐170 MG. Attempts at modelling the data using centred and off-centred dipolar fields have only been partially successful. Mildly off-centred dipoles appear to be in better overall agreement with observations, but only marginally. Our favoured models belonging to this class have a dipolar field strength Bd ∼ 200‐235 MG displaced from the centre of the star by ∼5‐15 per cent of the stellar radius. These models also present a reasonable fit to the photospheric Ly απ and σ + components in the ultraviolet spectral region which are less sensitive to field structure. Our failure to obtain detailed agreement with the optical spectropolarimetry suggests that AR UMa, in common with some other well studied magnetic dwarfs, may have a field structure which differs from that of a dipole. Our study supports a previous suggestion that the variable emission feature near 1730 ˚ A and the feature near 2220 ˚ A, which are seen superimposed on the photospheric spectrum, are caused by cyclotron emission from a weakly accreting shock. We identify these features with cyclotron harmonics 4 and 3 respectively, in a field of 154 MG, which is the field strength expected near the weaker pole of our dipole model with Bd = 235 and an offset of 15 per cent of the stellar radius along the dipolar axis. A field of 154 MG can also explain the bump near 4450 ˚ A present in the high state circular polarization data of AR UMa, as harmonic number 2.

3D eclipse mapping in AM Herculis systems –‘genetically modified fireflies’

In order to map the three-dimensional location and shape of the emission originating within the accretion stream in AM Her systems, we have investigated the possibilities of relaxing the hitherto-applied constraint of a predetermined stream trajectory in modelling the eclipse profiles. We use emission points which can be located anywhere in the Roche lobe of the primary, together with a regularization term which favours any curved stream structure, connected at the secondary and white dwarf primary. Our results show that, given suitable regularization constraints, such inversion is feasible. We investigate the effect of removing the regularization term, and also the sensitivity of the fit to input parameters such as inclination.

Spectroscopy and polarimetry of the AM Herculis system RX J1313.32−3259

We present polarimetric and spectroscopic observations of the ROSAT source RX J1313.32−3259, recently identified as a polar. Circular polarization is modulated over the orbital period with an amplitude of ∼10 per cent. A brief reversal in circular polarization is detected at phase ∼0.0, when the main accretion region is seen face on to the observer. We deduce that the main cyclotron emission is absorbed by the accretion shock and/or stream at this phase, allowing a second accretion shock, located at the opposite ends of the field lines that feed the main pole, to be seen. Linear polarization is also detected with possible linear pulses at orbital phases where the accretion region is expected to be on the limb of the white dwarf. The general morphology of the photopolarimetry arises as a result of a combination of the absorption of cyclotron radiation by the accretion shock/stream and projection/cyclotron beaming effects of a main cyclotron emission region as the white dwarf rotates. The main cyclotron emission region remains on the visible hemisphere of the white dwarf throughout the orbit. Doppler maps of the emission lines show emission at or near the expected location of the secondary star and an extended arc-like feature coincident with the ballistic and possibly the magnetically confined accretion stream.

Polarized line emission from magnetized accretion flows

We construct models of polarized line emission from magnetized accretion flows and investigate how line circular polarization can be used to infer the dynamics and the physics of the field-channelled flow in AM Herculis systems. We show that the line-polarization characteristics depend strongly on field and velocity gradients in the gas. We identify two regions of the flow which can contribute to the polarized line emission. The first is the funnelled flow out of the orbital plane along undistorted field lines. The second is the flow in the transition region closer to the orbital plane where the field lines are distorted, and the fluid motion changes from ballistic to corotation with the white dwarf. We argue that depending on field orientation and viewing aspect one or the other could dominate. In the high-inclination system AR UMa, the polarized line emission is dominated by field-channelled flow in the transition region. In contrast, in the low-inclination system V884 Her, the polarized line emission appears to be dominated by funnel flow along undistorted field lines.

The short period supersoft source in M31

We show that the recently discovered short period supersoft source in M31 is probably a progenitor of a magnetic cataclysmic variable (CV). The white dwarf spins asynchronously because of the current high accretion rate. However its fieldstrength is typical of an AM Herculis system, which is what it will ultimately become. We discuss the relevance of this system to CV evolution, and its relation to some particular CVs with special characteristics.

UBVRI-polarimetry of the AM Herculis star RX J0203.8+2959

We present results of the rst simultaneous UBVRI-photopolarimetric observations of the long period (4.6 hrs) AM Herculis system RX J0203.8+2959. The observed circular polarization shows both negative and positive polarization, from +7% to 5% in the B-band. A rise of the positive circular polarization is observed near the same orbital phase as a change in HR1 hardness ratio from soft to hard in the earlier ROSAT X-ray observations. The negative circular polarization is observed near orbital phases where a soft X-ray component was seen in ROSAT observations. Linear polarization pulses coincide with the sign reversal of the circular polarization, and coincide also with changes in the X-ray hardness ratio in earlier ROSAT data. The observed polarization variations favour two-pole accretion. Least square ts to the position angle curves during linear pulses suggest high orbital inclination, estimates fromi =6 7 +21 24 to i =7 3 +13 15 .T heUBVRI-lightcurves show smooth variations with a broad maximum and minimum. RX J0203.8+2959 was in intermediate accretion state during observations in November 1998 (V =1 5:7 16:7). The observed flux and circular polarization variations are reproduced by using constant temperature cyclotron emission models having two accretion regions and with parameters: kT =1 0 keV, =1 0 5 and colatitude = 120 140 for the negative pole, and kT =2 0 keV, =1 0 4 and =2 0 30 for

Extreme Ultraviolet Explorer and Optical Observations of the Magnetic Cataclysmic Variables RX J1015.5+0904 and V405 Auriga (RX J0558+5353)11Based on observations with the Apache Point Observatory (APO) 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium (ARC).

ABSTRACTWe present simultaneous Extreme Ultraviolet Explorer (EUVE) and optical observations of one of the shortest orbital period AM Herculis systems (RX J1015.5+0904) and of one of the intermediate polars which shows a soft X‐ray flux (V405 Aur=RX J0558+5353). RX J1015.5+0904 shows a very typical geometry of the accretion area for a one‐pole system. The count rate is consistent with a column at the low end of the ROSAT determinations and, therefore, a relatively high temperature for the heated area. Thus, there is every indication that heating operates efficiently even at low accretion rates onto very cool white dwarfs. V405 Aur has a very low count rate, which does not allow a good determination of the spin variation at EUVE wavelengths, although it is obvious in the optical photometry. Doppler tomography of the optical spectra shows a trend of decreasing disk structure and increasing spot structure with increasing excitation, with a change in the location of the primary emitting zone from previous years.

Accretion funnels in AM Herculis systems -- I. Model characteristics

ABSTRA C T We present calculations of the thermal structure of accretion funnels heated by hard X-ray emission from the accretion shock, soft X-ray reprocessed radiation from the white dwarf surface and magnetic heating in the matter‐field interaction region close to the orbital plane. The calculations determine self-consistently the thermal structure of the funnel, allowing for radiative transfer, electron scattering and the trapping of radiation within the funnel, and represent a substantial improvement on previous purely kinematical models. We show that, while models that allow only for X-ray heating can explain the observed intensities of the Balmer lines, they cannot, at the same time, explain the intensities of the He i and He ii lines in the optical spectra. These lines appear to be formed mainly in the magnetically heated transition region near the orbital plane, with this region playing a role similar to the hotspot in accretion discs. We show that, with the inclusion of this region, models can be constructed that are in close agreement with the optical line and continuum emission observed in AM Herculis systems: that is, they exhibit a flat or inverted Balmer decrement, He i lines, a strong He ii l 4686 line and complex emission-line profiles which vary dramatically in velocity and shape over the orbital period of the white dwarf. We also show that the continuum emission from the accretion funnel provides an important source of unpolarized background radiation, which reduces the degree of polarization of the cyclotron radiation from the accretion shocks, and produces the polarization standstills that are a well-known characteristic of these systems.

X‐Ray and Optical Observations of BL Hydri

We acquired hard X-ray and unfiltered optical photometric data of the AM Herculis system BL Hydri. The X-ray data were obtained using the Proportional Counter Array detector of the Rossi X-Ray Timing Explorer satellite on 1997 October 17. The optical data were obtained using the 1.5 m telescope of the Cerro Tololo Inter-American Observatory on 1997 September 24. At the time of our observations, BL Hyi was in an unusual high-luminosity state with average visual magnitude mV ≈ 14.4 and orbital phase-averaged 2-10 keV X-ray flux ≈1.5 × 10-11 ergs cm-2 s-1, roughly 50% larger than had been seen previously. The X-ray light curves suggested that two accretion poles were active. The primary X-ray emission region was extended, covering ~45° in longitude on the white dwarf, while the secondary X-ray emission region was consistent with a point source with flux ~33% that of the primary region. The X-ray spectra were well fitted by absorbed bremsstrahlung with Gaussian emission-line models; and absorbed power law with Gaussian emission-line models. For the thermal models, the X-ray spectra were consistent with kTX ≈ 11 keV and an emission line at E ≈ 6.7-6.8 keV with equivalent width EW = 0.86-1.2 keV. The absorbed power-law models had slope α ≈ 2.1 and an emission line at E = 6.7-6.8 keV with EW = 1.1-1.5 keV. The optical data showed, 2.7% ± 0.2% rms quasi-periodic oscillations (QPOs) over the frequency range 0.2-0.8 Hz modulated strongly on the orbital period suggesting that the QPOs were connected with the dominant accretion hot spot. We did not find corresponding QPOs in the X-ray emission for an upper limit of 20% rms. There were no detections of other short-period QPOs or coherent features in either the optical or the X-ray data. The X-ray and optical data were consistent with a radiative shock model. Based primarily on the X-ray continuum and line spectrum, we infer that the mass of the white dwarf in the BL Hyi system is 0.3-0.7 M☉, depending on the relative cyclotron and hard X-ray luminosities.

Accretion Processes in Magnetic Binaries

AbstractIn this paper, we give a brief summary of the talks on accretion processes in AM Herculis systems which were presented at the ANU Astrophysical Theory Centre workshop on ‘Magnetic Fields and Accretion’. One of the topics to be discussed was the mechanism that leads to the formation of magnetically funnelled accretion flows in close interacting magnetic binaries. New solutions to the Bernoulli integral indicate that the field lines must be twisted and have a strong toroidal component at the base of the funnel in order for channelled flow to be possible. The magnetic field pressure of these toroidal fields first lifts the material out of the orbital plane allowing it to ‘levitate’ before freely falling along magnetic field lines towards the stellar surface. Results of recent calculations of the thermal structure and radiation properties of accretion funnels were also presented. These new 3D calculations allow for heating by the soft X-rays originating from the accretion shock, and by magnetic heating at the base of the funnel, and determine self-consistently the thermal structure, and the continuum and line emissions, allowing for both transfer of the external radiation field and the trapping of radiation within the funnel. Calculations were also presented of the expected properties of H- and He-like Fe lines originating from the accretion shock itself at the stellar surface. These lines are predicted to be rather strong and can be used as diagnostics of the accretion flow. Finally, the stability of the accretion shock was also addressed. In particular, it was shown that radiative cooling may cause thermal instability and an oscillatory behaviour, with two competing processes coming into play: bremsstrahlung cooling, which promotes instability, and cyclotron cooling, which tends to dampen the oscillations.

Magnetic braking in magnetic binary stars

The role of an external magnetic field in the magnetic braking of a star with a dipolar field is investigated. In a magnetic cataclysmic variable system (i.e. the primary compact star has a strong magnetic field), the field external to the braking star (a late-type main-sequence star with a dynamo-generated field) originates from the compact star. A closed field region — the system dead zone — is formed within the binary system, and it does not take part in magnetic braking. The overall braking rate depends on the extent of this region and of the open flux, and is dependent on centrifugal effects. In the case of two interacting dipoles, the dipole orientations relative to the spin axes and to each other are found to be important, leading to different amounts of open flux and therefore of magnetic braking, owing to different centrifugal effects on closed field regions. However, in circumstances consistent with observations and dynamo theory, the white dwarf’s field reduces the magnetic braking of the secondary significantly, a finding qualitatively similar to the results previously obtained for two anti-aligned dipoles perpendicular to the orbital plane. In the cases where the two dipole axes are not perpendicular to the orbital plane, but are inclined in the plane that links them, the ‘cut-off’ in magnetic braking is less abrupt, and this effect is more obvious as the inclinations increase. Only in the extreme cases when the two dipole axes are aligned in the orbital plane does the braking increase with white dwarf field strength. We conclude that detailed evolutionary modelling of AM Herculis systems needs to take account of the inclination effect.

Polarimetry of the eclipsing polar RX J0929.1 -- 2404

We report polarimetric, spectropolarimetric and photometric observations of the eclipsing ROSAT cataclysmic variable RX J0929.1−2404, which confirm that the system is a new polar (AM Herculis system). This brings the number of eclipsing polars to nine, with RX J0929.1−2404 being only the third such system above the period gap. Circular polarization variations from ∼−20 to 10 per cent are seen over the 3.39-h orbital period, with a minimum around the time of eclipse. The photopolarimetric data were modelled using arc-shaped cyclotron emission regions in a centred dipole geometry. Results imply that RX J0929.1−2404 is a ‘two-pole’ system, with one emission region partially visible at all orbital phases. Spectropolarimetry observations show some evidence for the presence of cyclotron humps in the continuum, with spacings consistent with a magnetic field strength of ∼20 MG. Photometry of the eclipses provides information on the size of the emission region, which is consistent with a hotspot on the surface of the white dwarf. The eclipse duration implies an inclination in the range 70°≲i≲78°.

A Ginga hard X-ray search for 1-3 s quasi-periodic oscillations in AM Herculis systems

We describe a search for the expected X -ray counterpart to the 1-3 s QPOs seen in the optical emission from a number of polars. The Ginga hard X-ray data of the three systems AM Her, EF Eri and V834 Cen have been examined for such QPOs. However, none has been detected, and 95 per cent upper limits to the rms amplitude of the QPOs of 4, 5 and 18 per cent, respectively, have been obtained. The broad-band optical rms values for the rapid QPOs are typically 1-3 per cent for EF Eri and V834 Cen, while no such QPO has been seen in the optical data of AM Her (with an upper limit < 0.25 per cent). The only viable models for the optical QPOs are the driven oscillating shock models. They predict approximately equal X-ray and optical modulation amplitudes, and are thus consistent with our results.

The magnetic fields of EF Eridani and BL Hydri

We present near-infrared spectroscopic observations of the AM Herculis systems EF Eridani and BL Hydri over the wavelength range 0.9 to 2.5 μm. During these observations, broad and resolvable cyclotron emission harmonics were visible near 1.05, 1.30, 1.70 and 2.20 μm in EF Eridani, and near 1.25, 1.60 and 2.20 μm in BL Hydri. We interpret these features as arising from cyclotron emission regions located near the polar caps of the white dwarf of magnetic field strength B = 23 MG in BL Hydri, and from two separate cyclotron emission regions of field strengths B=16.5 and 21 MG in EF Eridani.