White Dwarf Spin Period Research Articles

IGR J14257−6117, a magnetic accreting white dwarf with a very strong strong X-ray orbital modulation

IGR J14257-6117 is an unclassified source in the hard X-ray catalogues. Optical follow-ups suggest it could be a Cataclysmic Variable of the magnetic type. We present the first high S/N X-ray observation performed by \XMM\ at 0.3--10 keV, complemented with 10--80 keV coverage by \Swift/BAT, aimed at revealing the source nature. We detected for the first time a fast periodic variability at 509.5\,s and a longer periodic variability at 4.05\,h, ascribed to the white dwarf (WD) spin and binary orbital periods, respectively. These unambiguously identify IGR J14257-6117 as a magnetic CV of the Intermediate Polar (IP) type. The energy resolved light curves at both periods reveal amplitudes decreasing with increasing energy, with the orbital modulation reaching $\sim100\%$ in the softest band. The energy spectrum shows optically thin thermal emission with an excess at the iron complex, absorbed by two dense media (${\rm N_{H}\sim10^{22-23}\,cm^{-2}}$), partially covering the X-ray source. These are likely localised in the magnetically confined accretion flow above the WD surface and at the disc rim, producing the energy dependent spin and orbital variabilities, respectively. IGR J14257-6117, joins the group of strongest orbitally modulated IPs now counting four systems. Drawing similarities with low-mass X-ray binaries displaying orbital dips, these IPs should be seen at large orbital inclinations allowing azimuthally extended absorbing material fixed in the binary frame to intercept the line of sight. For IGR J14257-6117, we estimate ($50^o\,\lesssim\,i\,\lesssim\,70^o$). Whether also the mass accretion rate plays a role in the large orbital modulations in IPs cannot be established with the present data.

Multiband study of RX J0838−2827 and XMM J083850.4−282759: a new asynchronous magnetic cataclysmic variable and a candidate transitional millisecond pulsar

In search for the counterpart to the Fermi-LAT source 3FGL J0838.8-2829, we performed a multi-wavelength campaign, in the X-ray band with Swift and XMM-Newton, performed infrared, optical (with OAGH, ESO-NTT and IAC80) and radio (ATCA) observations, as well as analysed archival hard X-ray data taken by INTEGRAL. We report on three X-ray sources consistent with the position of the Fermi-LAT source. We confirm the identification of the brightest object, RX J0838-2827, as a magnetic cataclysmic variable, that we recognize as an asynchronous system (not associated with the Fermi-LAT source). RX J0838-2827, is extremely variable in the X-ray and optical bands, and timing analysis reveals the presence of several periodicities modulating its X-ray and optical emission. The most evident modulations are interpreted as due to the binary system orbital period of ~1.64h and the white dwarf spin period of ~1.47h. A strong flux modulation at ~15h is observed at all energy bands, consistent with the beat frequency between spin and orbital periods. Optical spectra show prominent Hbeta, HeI and HeII emission lines Doppler-modulated at the orbital period and at the beat period. Therefore, RX J0838-2827, accretes through a diskless configuration and could be either a strongly asynchronous polar or a rare example of a pre-polar system in its way to reach synchronism. Among the other two X-ray sources, XMM J083850.4-282759 showed a variable X-ray emission, with a powerful flare lasting ~600s, similar to what is observed in transitional millisecond pulsars during the sub-luminous disc state: that would possibly associate this source with the Fermi-LAT source.

Resynchronization of the Asynchronous Polar CD Ind

CD Ind is one of only four confirmed asynchronous polars (APs). APs are strongly magnetic cataclysmic variables of the AM Herculis subclass with the characteristic that their white dwarfs rotate a few percent out of synchronism with their binary orbit. Theory suggests that nova eruptions disrupt previously synchronized states. Following the eruption, the system is expected to rapidly resynchronize over a timescale of centuries. The other three asynchronous polars—V1432 Aql, BY Cam, and V1500 Cyg—have resynchronization time estimates ranging from 100 to more than 3500 years, with all but one being less than 1200 years. We report on the analysis of over 46,000 observations of CD Ind taken between 2007 and 2016, combined with previous observations from 1996, and estimate a CD Ind resynchronization time of 6400 ± 800 years. We also estimate an orbital period of 110.820(1) minutes and a current (2016.4) white dwarf spin period of 109.6564(1) minutes.

CXOGBS J174954.5−294335: a new deeply eclipsing intermediate polar

We present the results of a photometric and spectroscopic analysis of the Galactic Bulge Survey X-ray source CXOGBS J174954.5-294335 (hereafter, referred to as CX19). CX19 is a long period, eclipsing intermediate polar type cataclysmic variable with broad, single-peaked Balmer and Paschen emission lines along with HeII $\lambda4686$ and Bowen blend emission features. With coverage of one full and two partial eclipses and archival photometry, we determine the ephemeris for CX19 to be HJD(eclipse) = 2455691.8581(5) + 0.358704(2)$\times$N. We also recovered the white dwarf spin period of P$_{\rm spin}$ = 503.32(3) seconds which gives a P$_{\rm spin}$/P$_{\rm orb}$ = 0.016(6), comparable to several confirmed, long period intermediate polars. CX19 also shows a clear X-ray eclipse in the 0.3-8.0 keV range observed with Chandra. Two optical outbursts were observed lasting between 6-8 hours (lower limits) reaching $\sim$1.3 mags in amplitude. The outbursts, both in duration and magnitude, the accretion disc dominated spectra and hard X-ray emission are reminiscent of the intermediate polar V1223 Sgr sharing many of the same characteristics. If we assume a main sequence companion, we estimate the donor to be an early G-type star and find a minimum distance of $d \approx$ 2.1 kpc and a 0.5-10.0 keV X-ray luminosity upper limit of 2.0 $\times$ 10$^{33}$ erg s$^{-1}$. Such an X-ray luminosity is consistent with a white dwarf accretor in a magnetic cataclysmic variable system. To date, CX19 is only the second deeply-eclipsing intermediate polar with X-ray eclipses and the first which is optically accessible.

Kepler K2observations of the intermediate polar FO Aquarii

We present photometry of the intermediate polar FO Aquarii obtained as part of the K2 mission using the Kepler space telescope. The amplitude spectrum of the data confirms the orbital period of 4.8508(4) h, and the shape of the light curve is consistent with the outer edge of the accretion disk being eclipsed when folded on this period. The average flux of FO Aquarii changed during the observations, suggesting a change in the mass accretion rate. There is no evidence in the amplitude spectrum of a longer period that would suggest disk precession. The amplitude spectrum also shows the white dwarf spin period of 1254.3401(4) s, the beat period of 1351.329(2) s, and 31 other spin and orbital harmonics. The detected period is longer than the last reported period of 1254.284(16) s, suggesting that FO Aqr is now spinning down, and has a positive $\dot{P}$. There is no detectable variation in the spin period over the course of the K2 observations, but the phase of the spin cycle is correlated with the system brightness. We also find the amplitude of the beat signal is correlated with the system brightness.

Identifying IGR J14091−6108 as a magnetic CV with a massive white dwarf using X-ray and optical observations

IGR J14091-6108 is a Galactic X-ray source known to have an iron emission line, a hard X-ray spectrum, and an optical counterpart. Here, we report on X-ray observations of the source with XMM-Newton and NuSTAR as well as optical spectroscopy with ESO/VLT and NOAO/SOAR. In the X-rays, this provides data with much better statistical quality than the previous observations, and this is the first report of the optical spectrum. Timing analysis of the XMM data shows a very significant detection of 576.3+/-0.6 s period. The signal has a pulsed fraction of 30%+/-3% in the 0.3-12 keV range and shows a strong drop with energy. The optical spectra show strong emission lines with significant variability in the lines and continuum, indicating that they come from an irradiated accretion disk. Based on these measurements, we identify the source as a magnetic Cataclysmic Variable of Intermediate Polar (IP) type where the white dwarf spin period is 576.3 s. The X-ray spectrum is consistent with the continuum emission mechanism being due to thermal Bremsstrahlung, but partial covering absorption and reflection are also required. In addition, we use the IP mass (IPM) model, which suggests that the white dwarf in this system has a high mass, possibly approaching the Chandrasekhar limit.

SPIN AND MAGNETISM OF WHITE DWARFS

The magnetism and rotation of white dwarf (WD) stars are investigated in relation to a hydromagnetic dynamo operating in the progenitor during shell burning phases. The downward pumping of angular momentum in the convective envelope, in combination with the absorption of a planet or tidal spin-up from a binary companion, can trigger strong dynamo action near the core-envelope boundary. Several arguments point to the outer core as the source for a magnetic field in the WD remnant: the outer third of a $\sim 0.55\,M_\odot$ WD is processed during the shell burning phase(s) of the progenitor; the escape of magnetic helicity through the envelope mediates the growth of (compensating) helicity in the core, as is needed to maintain a stable magnetic field in the remnant; and the intense radiation flux at the core boundary facilitates magnetic buoyancy within a relatively thick tachocline layer. The helicity flux into the growing core is driven by a dynamical imbalance with a latitude-dependent rotational stress. The magnetic field deposited in an isolated massive WD is concentrated in an outer shell of mass $\lesssim 0.1\,M_\odot$ and can reach $\sim 10\,$MG. A buried toroidal field experiences moderate ohmic decay above an age $\sim 0.3$ Gyr, which may lead to growth or decay of the external magnetic field. The final WD spin period is related to a critical spin rate below which magnetic activity shuts off, and core and envelope decouple; it generally sits in the range of hours to days. WD periods ranging up to a year are possible if the envelope re-expands following a late thermal pulse.

Periodic eclipse variations in asynchronous polar V1432 Aql: evidence of a shifting threading region

We report the results of a 28-month photometric campaign studying V1432 Aql, the only known eclipsing, asynchronous polar. Our data show that both the residual eclipse flux and eclipse O−C timings vary strongly as a function of the spin-orbit beat period. Relying upon a new model of the system, we show that cyclical changes in the location of the threading region along the ballistic trajectory of the accretion stream could produce both effects. This model predicts that the threading radius is variable, in contrast to previous studies which have assumed a constant threading radius. Additionally, we identify a very strong photometric maximum which is only visible for half of the beat cycle. The exact cause of this maximum is unclear, but we consider the possibility that it is the optical counterpart of the third accreting polecap proposed by Rana et al. Finally, the rate of change of the white dwarf's spin period is consistent with it being proportional to the difference between the spin and orbital periods, implying that the spin period is approaching the orbital period asymptotically.

An extensive photometric study of the recently discovered intermediate polar V647 Aur (1RXS J063631.9+353537)

We report the results of photometry of the intermediate polar V647 Aur. Observations were obtained over 42 nights in 2012 and 2013. The total duration of the observations was 246 h. We clearly detected three oscillations with periods of 932.9123+\-0.0011, 1008.30797+\-0.00038 and 1096.955+\-0.004 s, which may be the white dwarf spin period and two orbital sidebands, accordingly. The oscillation with a period of 932.9123 s has a quasi-sinusoidal pulse profile with a slightly changeable semi-amplitude from 10.9 mmag in 2012 to 12.5 mmag in 2013. The oscillation with a period of 1008.30797 s has a slightly asymmetric pulse profile with a remarkable small hump on the ascending part. The semi-amplitude of this oscillation is highly changeable both in a time-scale of days (26-77 mmag) and in a time-scale of years (47 mmag in 2012 and 34 mmag in 2013). The oscillation with a period of 1096.955 s has a highly asymmetric pulse profile with a semi-amplitude of about 6 mmag. The three detected oscillations imply an orbital period of 3.46565+\-0.00006 h. By comparing our data with the data of B. T. Gansicke et al., which were obtained 8 years ago, we discovered that the spin period of the white dwarf in V647 Aur decreases with dP/dt=(-1.36+\-0.08) X 10^-10. This important result should be confirmed by future observations. Our oscillation ephemeredes and times of maxima can be useful for this confirmation.

Remarkable spectral variability on the spin period of the accreting white dwarf in V455 And

We present spin-resolved spectroscopy of the accreting white dwarf binary V455 And. With a suggested spin period of only 67s, it has one of the fastest spinning white dwarfs known. To study the spectral variability on the spin period of the white dwarf, we observed V455 And with 2s integration times, which is significantly shorter than the spin rate of the white dwarf. To achieve this cadence, we used the blue arm of the ISIS spectrograph at the 4.2-m William Herschel Telescope, equipped with an electron multiplying CCD (EMCCD). Strong coherent signals were detected in our time series, which lead to a robust determination of the spin period of the white dwarf (Pspin=67.619 +/- 0.002 s). Folding the spectra on the white dwarf spin period uncovered very complex emission line variations in Hgamma, He I 4472 and He II 4686. We attribute the observed spin phase dependence of the emission line shape to the presence of magnetically controlled accretion onto the white dwarf via accretion curtains, consistent with an intermediate polar type system. We are, however, not aware of any specific model that can quantitatively explain the complex velocity variations we detect in our observations. The orbital variations in the spectral lines indicate that the accretion disc of V455 And is rather structureless, contrary to the disc of the prototype of the intermediate polars, DQ Her. This work demonstrates the potential of electron multiplying CCDs to observe faint targets at high cadence, as readout noise would make such a study impossible with conventional CCDs.

CC Sculptoris: a superhumping intermediate polar

We present high speed optical, spectroscopic and Swift X-ray observations made during the dwarf nova superoutburst of CC Scl in November 2011. An orbital period of 1.383 h and superhump period of 1.443 h were measured, but the principal new finding is that CC Scl is a previously unrecognised intermediate polar, with a white dwarf spin period of 389.49 s which is seen in both optical and Swift X-ray light curves only during the outburst. In this it closely resembles the old nova GK Per, but unlike the latter has one of the shortest orbital periods among intermediate polars.

XMM–Newton EPIC and OM observation of Nova Centauri 1986 (V842 Cen)

ABSTRACT We report the results from the temporal and spectral analysis of an XMM–Newton observation of Nova Centauri 1986 (V842 Cen). We detect a period at 3.51 ± 0.4 h in the EPIC data and at 4.0 ± 0.8 h in the Optical Monitor (OM) data. The X-ray spectrum is consistent with the emission from an absorbed thin thermal plasma with a temperature distribution given by an isobaric cooling flow. The maximum temperature of the cooling flow model is keV. Such a high temperature can be reached in a shocked region and, given the periodicity detected, most likely arises in a magnetically channelled accretion flow characteristic of intermediate polars. The pulsed fraction of the 3.51-h modulation decreases with energy as observed in the X-ray light curves of magnetic cataclysmic variables, possibly due either to occultation of the accretion column by the white dwarf body or phase-dependent to absorption. We do not find the 57-s white dwarf spin period, with a pulse amplitude of 4 mmag, reported by Woudt et al. in either the OM data, which are sensitive to pulse amplitudes ≳0.03 mag, or the EPIC data, sensitive to pulse fractions p≳ 14 ± 2 per cent.

Characterization of new hard X-ray cataclysmic variables

We aim at characterizing a sample of 9 new hard X-ray selected Cataclysmic Variable (CVs), to unambiguously identify them as magnetic systems of the Intermediate Polar (IP) type. We performed timing and spectral analysis by using X-ray, and simultaneous UV and optical data collected by XMM-Newton, complemented with hard X-ray data provided by INTEGRAL and Swift. The pulse arrival time were used to estimate the orbital periods. The X-ray spectra were fitted using composite models consisting of different absorbing columns and emission components. Strong X-ray pulses at the White Dwarf (WD) spin period are detected and found to decrease with energy. Most sources are spin-dominated systems in the X-rays, though four are beat dominated at optical wavelengths. We estimated the orbital period in all system (except for IGR J16500-3307), providing the first estimate for IGR J08390-4833, IGR J18308-1232, and IGR J18173-2509. All X-ray spectra are multi-temperature. V2069 Cyg and RX J0636+3535 posses a soft X-ray optically thick component at kT 80 eV. An intense K_alpha Fe line at 6.4 keV is detected in all sources. An absorption edge at 0.76 keV from OVII is detected in IGR J08390-4833. The WD masses and lower limits to the accretion rates are estimated. We found all sources to be IPs. IGR J08390-4833, V2069 Cyg, and IGR J16500-3307 are pure disc accretors, while IGR J18308-1232, IGR J1509-6649, IGR J17195-4100, and RX J0636+3535 display a disc-overflow accretion mode. All sources show a temperature gradient in the post-shock regions and a highly absorbed emission from material located in the pre-shock flow which is also responsible for the X-ray pulsations. Reflection at the WD surface is likely the origin of the fluorescent iron line. There is an increasing evidence for the presence of a warm absorber in IPs. The addition of 2 systems to the subgroup of soft X-ray IPs confirms a \sim 30% incidence.

An extensive photometric study of the recently discovered intermediate polar V515 And (XSS J00564+4548)

We report results of photometry of the intermediate polar V515 And. The observations were obtained over 33 nights in 2008 and 2009. The total duration of the observations was 233 h. We clearly detected two oscillations with periods of 465.48493\pm0.00007$ and 488.61822\pm0.00009 s, which may be the white dwarf spin period and the orbital sideband. The semi-amplitudes of the oscillations are 25 and 20 mmag, accordingly. The oscillation with a period of 465.48493 s has a stable smooth asymmetric pulse profile whereas the pulse profile of the oscillation with a period of 488.61822 s reveals significant changes from a quasi-sinusoidal shape to a shape somewhat resembling a light-curve of an eclipsing binary. Two detected oscillations imply an orbital period of 2.73 h. V515 And is one of the most rapidly spinning intermediate polars with orbital periods less than 3 h and may be not in spin equilibrium. This can be proved by future observations. For this purpose we obtained oscillation ephemerises with a formal shelf life of about 100 yr. (a 1 sigma confidence level).

X-ray characteristics and the spectral energy distribution of AE Aquarii

Using data from contemporaneous observations with Chandra and Swift, it is shown that the X-ray emission below 10 keV is predominantly thermal, characterized by flares and emission lines and dominated by the soft component. The Chandra and Swift X-ray spectra (EX ≤ 10 keV) can be reproduced by multicomponent thermal emission models with a time-averaged X-ray luminosity of LX ∼ 10 31 erg s −1 . The pulsed 33-s soft X-ray emission below 10 keV is confirmed in both the Chandra and Swift data sets. The epoch of pulse maximum of the 33-s white dwarf spin period is consistent with the recently derived ephemeris based upon Suzaku measurements. The recently detected Suzaku hard X-ray component above 10 keV shows a non-thermal power-law nature, with a photon index of � ∼ 1.2, possibly the result of synchrotron emission of high-energy electrons in the white dwarf magnetosphere. The hard X-ray luminosity of LX,hard ≤ 5 × 10 30 erg s −1 also constitutes κ ∼ 0.1 per cent of the total spin-down luminosity of the white dwarf. This places AE Aquarii in the same category as young spin-powered pulsars between 2 and 20 keV. Additionally, it is shown that electrons can be accelerated to energies in excess of 10 TeV outside the light cylinder radius, providing interesting possibilities for VHE-TeV follow-up observations. The X-ray emission below EX ≤ 10 keV, on the other hand, is explained in terms of plasma heating at the magnetospheric radius, the result of the dissipation of gravitational potential energy. It is found that a conversion efficiency of α ∼ 0.01 is sufficient to heat the plasma at the magnetospheric boundary to temperatures kT ≤ 10 keV, sufficient to drive the X-ray emission below 10 keV. Only a small fraction (β ∼ 0.3 per cent) of the mass flow at the magnetospheric radius eventually accretes on to the surface of the white dwarf, emphasizing the very effective magnetospheric propeller process in the system.

Confirmation of 1RXS J165443.5–191620 as an intermediate polar and its orbital and spin periods

Aims: We investigate the physical nature of the X-ray emitting source 1RXS J165443.5-191620 through optical photometry and time-resolved spectroscopy. Methods: Optical photometry is obtained from a variety of telescopes all over the world spanning about 27 days. Additionally, time-resolved spectroscopy is obtained from the MDM observatory. Results: The optical photometry clearly displays modulations consistent with those observed in magnetic cataclysmic variables: a low-frequency signal interpreted as the orbital period, a high-frequency signal interpreted as the white dwarf spin period, and an orbital sideband modulation. Our findings and interpretations are further confirmed through optical, time-resolved, spectroscopy that displays H-alpha radial velocity shifts modulated on the binary orbital period. Conclusion: We confirm the true nature of 1RXS J165443.5-191620 as an intermediate polar with a spin period of 546 seconds and an orbital period of 3.7 hours. In particular, 1RXS J165443.5-191620 is part of a growing subset of intermediate polars, all displaying hard X-ray emission above 15keV, white dwarf spin periods below 30 minutes, and spin-to-orbital ratios below 0.1.

DISCOVERY OF POLARIZED EMISSION FROM TWO SOFT X-RAY-EMITTING INTERMEDIATE POLARS: UU Col AND NY Lup

We aim to investigate the magnetic field strengths and cyclotron emission of the two soft X-ray-emitting intermediate polars (IPs) UU Col and NY Lup. We study the connection between polars and soft X-ray-emitting IPs by searching for evidence of circularly polarized light in these two systems, which may be examples of progenitors of polars. We carried out photopolarimetric observations of our targets using the Very Large Telescope (UT2) and FORS1 at Paranal. Imaging polarimetry with good signal-to-noise and relatively high time resolution is possible for these targets using such a large telescope. Detection of circular polarization, modulated according to a white dwarf (WD) spin period, is clear evidence of cyclotron emission processes near the WD surface. The color dependence of the polarization allows us to make estimates of the magnetic field strength. We found that both UU Col and NY Lup emit circularly polarized light in the B and I bands, modulated at the spin period of the WD in each case. We add further confirmation to the idea that soft X-ray-emitting IPs emit circularly polarized light and that cyclotron emission plays an important role in these systems. This also suggests that some soft X-ray-emitting IPs might be progenitors of polars.

Discovery of heavily-obscured AGN among seven INTEGRAL hard X-ray sources observed by Chandra

We observed 7 INTEGRAL sources with the Chandra X-ray Observatory to refine their localization to ~2 arcsec and to study their X-ray spectra. Two sources are inferred to have a Galactic origin: IGR J08390-4833 is most likely a magnetic cataclysmic variable with a white dwarf spin period ~1,450 s; and IGR J21343+4738 is a high-mass X-ray binary. Five sources (IGR J02466-4222, IGR J09522-6231, IGR J14493-5534, IGR J14561-3738, and IGR J23523+5844) prove to be AGN with significant intrinsic X-ray absorption along the line of sight. Their redshifts and hard X-ray (17-60 keV) luminosities range from 0.025 to 0.25 and from ~2x10^43 to ~2x10^45 erg/s, respectively, with the distance to IGR J14493-5534 remaining unknown. The sources IGR J02466-4222 and IGR J14561-3738 are likely Compton-thick AGN with absorption column densities NH>10^24 cm^-2, and the former further appears to be one of the nearest X-ray bright, optically-normal galaxies. With the newly-identified sources, the number of heavily-obscured (NH>10^24 cm^-2) AGN detected by INTEGRAL has increased to ~10. Therefore, such objects constitute 10-15% of hard X-ray bright, non-blazar AGN in the local Universe. The small ratio (<<1%) of soft (0.5-8.0 keV) to hard (17-60 keV) band fluxes (Chandra to INTEGRAL) and the non-detection of optical narrow-line emission in some of the Compton-thick AGN in our sample suggests that there is a new class of objects in which the central massive black hole may be surrounded by a geometrically-thick dusty torus with a narrow ionization cone.

SDSS J233325.92+152222.1 and the evolution of intermediate polars

Intermediate polars (IPs) are cataclysmic variables which contain magnetic white dwarfs with a rotational period shorter than the binary orbital period. Evolutionary theory predicts that IPs with long orbital periods evolve through the 2-3 h period gap, but it is very uncertain what the properties of the resulting objects are. Whilst a relatively large number of long-period IPs are known, very few of these have short orbital periods. We present phase-resolved spectroscopy and photometry of SDSS J233325.92+152222.1 (SDSS J2333) and classify it as the IP with the shortest-known orbital period (83.12 +/- 0.09 min), which contains a white dwarf with a relatively long spin period (41.66 +/- 0.13 min). We estimate the white dwarf's magnetic moment to be mu(WD) approximate to 2 x 10(33) G cm(3), which is not only similar to three of the other four confirmed short-period IPs but also to those of many of the long-period IPs. We suggest that long-period IPs conserve their magnetic moment as they evolve towards shorter orbital periods. Therefore, the dominant population of long-period IPs, which have white dwarf spin periods roughly 10 times shorter than their orbital periods, will likely end up as short-period IPs like SDSS J2333, with spin periods a large fraction of their orbital periods.

Accretion Column Structure of Magnetic Cataclysmic Variables from X-Ray Spectroscopy

Using Chandra HETG data, we present light curves for individual spectral lines of Mg XI and Mg XII for EX Hydrae, an intermediate polar-type cataclysmic variable. The Mg XI light curve, folded on the white dwarf spin period, shows two spikes that are not seen in the Mg XII or broadband light curves. Occultation of the accretion column by the body of the white dwarf would produce such spikes for an angle between the rotation axis and the accretion columns of α = 18° and a height of the Mg XI emission above the white dwarf surface of 0.0004 white dwarf radii, or 4 km. The absence of spikes in the Mg XII and broadband light curves could then be explained if the bulk of its emission forms at much larger height, more than 0.004 white dwarf radii or over 40 km, above the white dwarf surface, although this is not consistent with the predictions of the standard Aizu model of the accretion column.