spin and beat phenomena in time-resolved hubble space - mnras

1997MNRAS.288..891S

Mon. Not. R. Astron. Soc. 288, 891-902 (1997)

Spin and beat phenomena in time-resolved Hubble Space Telescope UV spectroscopy of PQ Gem

D. Stavroyiannopoulos, l S. R. Rosen, l M. G. Watson, 1 K. O. Mason2 and S. B. Howen3

1 Department of Physics and Astronomy, University of Leicester, University Road, Leicester LEI 7RH 2 Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT 3 Department of Physics and Astronomy, University of Wyoming, PO Box 3905, University Station, Laramie, WY 82071, USA

Accepted 1997 February 3. Received 1997 January 6; in original form 1996 June 27

1 INTRODUCTION

ABSTRACT Results of the first low-resolution (6 A), time-resolved Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) observation of the prototypical strong-field intermediate polar system, PQ Gem, are presented. The AAl150-2600 continuum light curve is dominated by the 13.9-min rotational signature of the white dwarf at all UV wavelengths covered, with a broadly constant fractional modulation depth. The rotational profile contains a dip which is deepest in the far-UVand which we believe, like its X-ray counterpart, is caused by stream occultation of the white dwarf. The continuum and emission-line fluxes are also modulated on the 14.5-min beat period but, remarkably, vary in antiphase.

This complex behaviour facilitates the identification and partial isolation of two spinmodulated spectral components and a beat component. One spin component has a blue spectral distribution whose temperature is :$ 50000 K if no allowance is made for the absorbing effects of the stream, but may be much hotter (consistent with earlier X-ray estimates) if, as seems likely, the absorber is not completely optically thick. The other spin-modulated component has a red spectral distribution whose temperature (:$ 10000 K) and luminosity probably associate it with the magnetospheric accretion flow itself. The beat continuum component has a temperature in the region of 17 000 K and appears to be radiated by a region whose size is comparable to that of the white dwarf. The beat pulsation in the emission lines may also originate from a region of similar dimensions. We consider one- and two-site hypotheses to explain the antiphased line and continuum beat modulations, but are unable to arrive at a convincing solution.

Key words: binaries: close - stars: individual: PQ Gem (RE 0751+14) - novae, cataclysmic variables - X-rays: stars.

Intennediate polars (IPs), sometimes called DQ Her stars, and polars (or AM Her stars) fonn the two recognized magnetic subclasses of cataclysmic variables. Both contain a magnetic white dwarf accreting material from a low-mass companion. It now appears to be established that many IPs possess weaker fields (BIP~106G) than the polars where BAM ;z: 107 G (e.g. Wickramasinghe, Wu & Ferrario 1991). A consequence of the weaker field in IPs is that the white dwarf rotates asynchronously with respect to the binary, and accretion might arguably involve a partial accretion disc, whereas in polars the white dwarf and companion star are phase locked and accretion proceeds via a stream. Nevertheless, in both cases, the infalling material is at least partially collimated by the magnetic field and focused on to restricted areas around the magnetic poles - the accreting area is

expected to be significantly larger in IPs due to both the diminished funnelling effect of the weaker field and the larger azimuthal range from which material is supplied, especially if a disc is present. The infall energy is released here in the fonn of X-rays and/or EUV emission. In polars, optical IR cyclotron radiation is also important. Extensive reviews of the polars and IPs can be found in Cropper (1990) and Patterson (1994) respectively.

Clearly, low-field IPs cannot be the progenitors of polars. However, until recently it was unclear whether polars and IPs populated a truly bimodal field distribution or whether some IPs might contain a white dwarf whose field strength was sufficient for it to magnetically lock with the companion star and thus become a polar system at some point in its lifetime (Chanmugam & Ray 1984; King, Frank & Ritter 1985). This uncertainty appears to be resolved following the ROSATWide Field Camera (WFC) discovery of the IP binary, PQ Gem (RE 0751 + 14: Mason et al. 1992), which displays the key

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properties of both subclasses. Its IP credentials include the very different white dwarf rotational (Pspin = 13.9 min) and binary orbital (Porb = 5.2 h) (e.g. Mason et al. 1992; Rosen, Mittaz & Hakal ; Hilditch & Be111994), the presence of a strong, spinmodulated hard X-ray pUlsation (Mason et a1. 1992; Duck et al. 1994) and an optical photometJjc variation at the synodic (beat) period (Rosen et a1. 1993; Hellier, Ramseyer & Jablonski 1994). At the same time, it exhibits spin-modulated polarization, the signature of polars, and a red rotational fiux variation, both of which point to a luminous cyclotron spectral component and hence a stronger magnetic field than is typical of IPs (Rosen et al. 1993; Piirola, Hakala & Coyne 1993; Vath, Chanmugam & Frank 1996; Potter et al. 1997). Of particular interest was the discovery of a strong soft X-ray/EUV spectral component in PQ Gem, similar to that seen in polar systems (Duck et a1. 1994; see also Mason et a1. 1992). PQ Gem also possesses a relatively narrow, energy-dependent dip in its rotational X-ray light curve, a feature reminiscent of the absorption events, thought to be caused when the magnetically diverted accretion stream occults the white dwarf that is observed in some polars (e.g. EF Eri: Watson et al. 1989). This catalogue of hybrid characteristics suggests that PQ Gem, and similar systems discovered recently (RX J1712.6-2414: Buckley et a1. 1995; RX J0558.0+5353 and 1914.4+2456: Haberl & Motch 1995 and references therein) are almost certainly the first examples of IPs that are destined to become synchronous rotators, i.e. polars. As such, they can offer key insights into the accretion dynamics, field distribution and evolution of magnetic cataclysmic variables.

In this paper we present the results of the first highly

time-resolved UV spectroscopy of PQ Gem conducted with the Hubble Space Telescope (HST).

2 OBSERVATIONS

PQ Gem was observed with the HSTover an interval of about 15 h (- 3 binary cycles) on 1994 May 24125. The total on-source exposure of 5.5 h was divided between 10 contiguous observation slots, the gaps arising from Earth occultations due to the fact that HST is in a low Earth (96-min) orbit. Spectra were recorded using the Faint Object Spectrograph (FOS) (see for example, Keyes 1995) with the blue Digicon detector. The 0.9-arcsec aperture and the G160L grating were employed to measure the 1150-2500 A spectrum at a resolution of about 6.8 A. During each slot, continuous sequences of spectra were obtained using FOS RAPID mode, each exposure being of about lO-s duration with less than Is of dead-time between integrations. A total of 2040 spectra were secured. Data were supplied by STScI pre-calibrated in fiux and wavelength. The wavelength scale is accurate to about 0.3 pixel, i.e. about 2 A or about 400 km S-1 at 1550 A.

3 RESULTS

3.1 The mean spectrum

The mean HST UV spectrum of PQ Gem is shown in Fig. 1. The spectrum is relatively fiat at wavelengths above 1700 A with just a few recognizable lines such as NIV A1719 and AlmA1855. Below 1700 A, the spectrum contains a number of strong emission lines,

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Figure 2. The L-statistic computed in the range 5-20 min for, from top to bottom: the blue, central and red continuum bands, the C N, He II and N v line fluxes and the eN and He II VIR ratios. The dashed and dotted Jines mark the locations of the spin and beat periods respectively.

e.g. Hell Al640, CN A1550, SiN AA1393,1402, CII A1335, a blend of SiIII A1298/Sill A1304/011 A1305, NVA1240, Lye< and C III A1176 - note the intrusion of the flat-field blemish (see HST data handbook) around 1500 A which co~taminates the blue wing of the C IV line. The continuum profile also shows evidence of a tum up shortward of 1700 A. This may, in part, arise from blending of some of the lines, particularly at the shortest wavelengths, due to their relatively close spacing, intrinsic breadth, and the modest resolution of the data. However, the regions between 1270 and 1285 A and 1430 and 1460 A, for example, probably provide a reasonably good indication of the underlying continuum shape, supporting the view that the spectral intensity increases towards far-UV wavelengths. This spectrum is similar, in both spectral shape and absolute flux level, to the International Ultraviolet Explorer (IVE) SWP spectrum obtained by de Martino (private communication: see also Howell et al. 1997).

3.2 Temporal variability

3.2.1 The continuum

To investigate the temporal variations of the UV flux in PQ Gem, we began by accumulating continuum light curves from three separate wavelength ranges that were deemed to be largely unaffected by strong lines. The bluemost band represents the summation of flux

© 1997 RAS, MNRAS 288, 891-902

from the 1270-1285 A, 1320-1325 A, 1355-1360 A and 1430-1460 A wavelength ranges. The central band spans the 1680-1800 A range while the redmost band comprises the 1800-2505 A region (see Fig. 1). Initial examination of the Fourier spectrum of each time series showed the presence of power associated with the 13.9-min rotational period of the white dwarf and aliases. To investigate such time-scales in greater detail, we first removed the gross long-term variations by subtracting a running mean from each light curve. The resulting time series were then subjected to an L-statistic analysis (Davies 1990), attention being focused on the 5 to 20 min range which encompasses the known 13.9-min spin and 14.5-min beat periods and their first harmonics. The results for each wavelength band are shown in Fig. 2. The strongest peaks in the blue data appear at the 6.95-min first harmonic of the white dwarf spin period, an alias (7.49 min) arising from the 96-min satellite sampling pattern, and the fundamental (13.89) of the spin period. There is also power at an unexpected period of 14.98 min and a weak but probably real signal centred at 14.44 min which we believe can be associated with the 14.54-min beat period. The central band periodogram shows the spin period and its first harmonic, together with a more prominent peak at the beat period. It also contains aliases of the spin period (16.24 and 12.1 min), ofits first harmonic (7.49 min) and of the beat signal (17.14min). The red band is dominated by the spin period and its 12.1- and 16.24-min aliases, and the beat period.

To compare the X-ray and UV white dwarf rotational light


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curves, the UV time series, detrended of the long-term variations, were folded according to the quadratic ephemeris, T(HJD) = 2448173.95714(5) + 0.009645 8718(10)N + 5.24(4)xIQ-13N2,

of Mason (1997) - errors are in parentheses and refer to the least significant digits of the values. This ephemeris, which suggests that a substantial spin-down torque is operating on the white dwarf, is based on previous X-ray data and has, as its reference epoch (i.e. phase 0.0), the centroid of the X-ray dip. The spin light curves of PQ Gem in the three UV continuum bands defined above are shown in Fig. 3. Each shows a pronounced modulation and a dip near phase 0.1 which is most evident at blue wavelengths. The underlying modulation profile in each UV band is quasi-sinusoidal although the minimum in the cycle near phase 0.65 is narrower than expected for a sinusoid. We estimate semi-amplitudes of 5.5::1:: 1.1,4.4::1::0.8 and 4.1 ::1::0.5 per cent for the blue, central and red bands respectively. There is an additional systematic uncertainty of about 1 per cent,

especially for the blue and central bands, because of our ignorance about the exact morphology of the underlying light curve at the phase of the dip. The modulation profile is also more asymmetric at red wavelengths, displaying a more protracted decline from maximum than during the rise to it.

Based on the above ephemeris, the minimum of the dip in the bluemost UV continuum light curve arrives at phase 0.08::1::0.02, i.e. about 1 min later than the predicted time of the X -ray dip - the error estimate relates only to the measurement of the dip centroid but this dominates over the uncertainty contribution from the ephemeris which amounts to about 0.004 in phase. The UV dip therefore appears to be significantly displaced from the X-ray event. However, in the absence of contemporary X-ray/EUVobservations to facilitate a direct comparison, we are unable to establish whether this reflects a systematic separation of the X-ray and UV dips or a single phenomenon, coincident in both bands, which had undergone a substantial (~300) azimuthal shift at the epoch of the HST run compared to previous and subsequent X-ray observations. In the latter case it is perhaps surprising that none of the six reliable X-ray dip epochs used to constrain the ephemeris deviates by more than 0.02 from the quadratic curve. The issue should be resolved by simultaneous UV and X-ray observations. In the meantime, since it is hard to see why the UV and X-ray dips would arise from separate events, we assume that the UV and X-ray dips have a common origin (occultation by the accretion flow) and that, had the source been observed simultaneously in both bands, the dips would have been coincident.

The dip appears asymmetric in the bluemost band with a steeper ingress than egress, and may persist for as much as a third of the cycle. Whilst significant in each band, the dip is strongest at bluemost wavelengths. A quantitative estimate of its depth is hampered by our uncertainty over the modulation profile at the dip phase, particularly in the blue and central bands. Assuming that the light curves follow a broadly sinusoidal behaviour, the inferred depths of the dip in the blue, central and red bands are 12.9::1::2.7, 10.7::1::3.7 and 4.5::1:: 1.1 per cent respectively. In the bluest band, the depth of the dip is comparable with the peak-to-trough amplitude of the rotational modulation but at the red end of the spectrum, it is certainly ~50 per cent of the modulation depth.

Examination of the periodograms in Fig. 2 showed some evidence of the beat period in the UV flux of PQ Gem. Folding the time series of each continuum band on the beat period (relative to an arbitrary reference epoch of HID 244 9497.353 51) yields a marginal detection of the pulse in the blue band (semi-amplitude of 0.8::1::0.5 per cent) but confirms the prominent beat variation in the central and red bands (see Fig. 4) where we measure semiamplitudes of 2.1 ::1::0.4 and 1.7::1::0.2 per cent respectively. We emphasize that the sampling of the source by the HST observation was such that there is generally uniform coverage of all beat phases for any given spin phase and vice versa. As such, the spin modulation should essentially be averaged out when the data are folded on the beat period. This means that the observed beat modulation is intrinsic to the source and not a result of uneven sampling of the spin pulsation.

3.2.2 The lines

Continuum-subtracted, integrated line fluxes were derived for the two strongest emission lines, C N and He II, from each spectrum. Again the resulting time series were analysed for periodic signals using the L-statistic. The periodograms for both lines, as a function of trial period, are shown in Fig. 2. In contrast to the continuum

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variations, the line fluxes predominantly vary on the 14.5-min beat period. The peaks at about 12.63 and 17.14min are sampling aliases. The flux curves of the two lines, folded on the beat period (referenced to the same epoch used for the continuum data), are shown in Fig. 4. The e rv flux profile is apparently flat-bottomed with the bright interval spanning the 0.93-0.43 phase range. The semi-amplitude of the variation is about 5.5:±: 1.0 per cent in e IV.

The He II beat variation modulates in phase with that of e IV but does not appear to be flat-bottomed. Its modulation semi-amplitude is 5.3:±: 1.3 per cent. The most remarkable aspect of the beat period variation of the lines, graphically illustrated in Fig. 4, is that it is in antiphase with that of the continuum modulation.

Perceptible, marginally significant line flux modulations also occur on the spin period (see Fig. 3). The semi-amplitudes of the spin pulsations in erv and Hen are 1.7:±: 1.0 and 4.3:±: 1.6 per cent respectively. The phasing of the variation is consistent with that of the continuum spin profile, reaching maximum near phase 0.15. We do not detect significant counterparts to the continuum dip in either line. Assuming any such dip has the same centroid phase and width as that in the central continuum band, we deduce 3a upper limits of 4 and 11 per cent on its fractional depth in the e rv and He II line light curves respectively.

The low dispersion of the current HST observations prevents a detailed study of the UV emission-line profiles. Nevertheless, we are able to probe possible motion in the lines via measurements of the VIR ratio.! The L-statistic periodograms, included in Fig. 2, betray profile changes on the spin period which presumably reflect rotationally dependent velocity excursions within the lines. The sidebands near 12.13 and 16.24min are aliases. The rotational signature in the velocity motion confirms the weak result gleaned from the optical emission lines (Rosen et al. 1993). The VIR ratio, folded on the rotational ephemeris above, is shown in Fig. 3. If correctly interpreted as radial velocity movements, the data indicate that the lines are maximally redshifted at spin phase 0.8, much earlier (by about 0.27 in phase) than the minimum of the dip in the blue continuum band.

3.3 Spectral components

The presence of colour-dependent rotational and synodic period variability in the UV emission of PQ Gem indicates that multiple spectral components must be combining to produce the mean spectrum. In general, deconvolving any spectrum into its separate components is a difficult process. However, the shape and temporal variability of the UV spectrum highlighted above provides an excellent opportunity to identify and characterize its constituent components in PQ Gem. As a result of the assumptions adopted in this process, we acknowledge from the outset that a secure decomposition of the spectral components (or at least an accurate measure of their spectral distributions) is not possible, not least because of our uncertainty over the effects of absorption in the system. Nevertheless, the analysis yields useful insights.

To examine the spin-modulated spectral components, we begin by noting that the prominent, narrow dip in the X-ray light curves (Duck et al. 1994) is believed to arise from absorption as the magnetically controlled accretion flow shadows the X-ray emitting

1 For each spectrum, masks were applied to the violet (V) and red (R) portions of both the C IV and the He IT lines, the masks butting at the centroid wavelength of the average profile. The ratio formed from the integrated flux in the appropriate Vand R masks was subsequently computed for each line and the resulting time series analysed for periodic behaviour.

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accretion site of the white dwarf. The opacities of absorption processes that would likely affect the UV band either increase towards the red (bound-free, free-free) or are essentially independent of wavelength (electron scattering, partial covering) - note that in reality we do not expect electron scattering or free-free absorption to be important since the column densities through the accretion flow are unlikely to greatly exceed ~1023 cm-2. We discuss the dip and its implications in Section 3.3.1. Thus the observed trend of increasing dip depth towards blue wavelengths in our HST UV data conflicts with what might be expected for absorption of a single modulated component. To explain this, we postulate the presence of two spin-modulated spectral components. We suggest that only one of these is subject to absorption and that it has a distribution that increases towards the blue. In this case, although the absorption of the blue component would likely be greater at red wavelengths, there is simply very little flux emitted there by this component to be absorbed and the modulation at red


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wavelengths is then dominated by the unocculted component which has a redder spectral profile.

To dissect the spectrum, we began by folding the individual exposures into 50 phase bins on the spin cycle and then extracted three phase-resolved spectra representing spin minimum (phase 0.58-0.7), dip minimum (0.05-0.13) and that which would be observed at the phase of the dip in the absence of absorption. The latter spectrum of course needs to be synthesized. To generate it, we constructed a template of the mean spin-modulation profile and replaced data in the dip interval by interpolation of a second-order polynomial that was fitted to data either side of the dip. A function of the form a(}.) + b("'A)T(cf» [where T(cf» is the modified template] was then matched to the light curve of each 6-A wavelength bin along the "'A"'A 1150-2600 spectrum, excluding data in the dip interval from the fit. Although this approach (which is similar to that adopted by Eracleous et al. 1994) does not take proper account of small wavelength-dependent changes in the shape of the underlying light curve [since T(cf» was not made a function of "'A], it proved more robust than fitting a polynomial to each cross-section.

We can express the total spectrum observed in each of our three spectra as follows:

Sunab,(}.) = fBdipB("'A) + fRdip80~) + C("'A) (1)

Sdip("'A) = fBdip[1 - A("'A)]B("'A) + fRdipR("'A) + C("'A) (2)

(3)

where Sunab" Smin and Sdip represent the unabsorbed spectrum during the dip, the spectrum at rotational minimum and that during the dip respectively, Band R are the blue and red components and fBdip and fBmin are the fractions of the blue component visible at the phase of the dip and rotational minimum with corresponding quantities for the red component. We assume that the underlying modulations are in-phase at all wavelengths, a view consistent with the appearance of the light curves. A(}.) is the absorption factor during the dip and C is a spin phase-independent term that includes unmodulated contributions. Note that in these data, beat-modulated spectral components are averaged out, contributing roughly equally at all spin phases, and can thus be incorporated into C("'A).

Subtracting the spectrum at dip minimum (equation 2) from the unabsorbed case (equation 1) then gives the difference spectrum, DB("'A), i.e.

DB("'A) = fBdipA("'A)B("'A). (4)

If, for a moment, we also assume that the blue spectral component is 100 per cent modulated, i.e. that there is no residual DC component (fBdip = I,fBmin = 0), then the fact that the depth of the dip at blue wavelengths is comparable to the modulation depth would suggest that we have near total absorption in the blue. For plausible absorption processes we would then expect total absorption of the blue component at longer wavelengths so A("'A) = 1. In this case, our subtraction will yield the blue spectral component. If, as seems likely, the absorption is not total in the blue but increases towards red wavelengths, the intrinsic spectrum of the occulted source will be steeper than that of the observed spectrum. The difference spectrum actually obtained by this subtraction is displayed in Fig. 5. As expected, the spectrum turns up at blue wavelengths. The difference spectrum contains evidence of residual emission in the SilV and Herr lines whose equivalent widths (18::1::6 and 13::1:: 10 A respectively) are consistent with those in the unabsorbed spectrum. The C IV line, on the other hand, is not significantly detected in the 1535-1568 A range where we measure an equivalent width of -4::1:: 14 A compared with 99::1:: 1 A in the unabsorbed case. We will return to the spectral profile again later.

We can use equations (2) and (3) to obtain a measure of the other spin-modulated component. Subtraction of the spectrum at spin minimurn from that at dip minimum yields

DR("'A) = B("'A)[fBdip(1 - A("'A)) - fBmin] + R(}.)[fRdip - fRmin]. (5)

In the simplest case in which both the red and blue components are 100 per cent modulated (fRdip = I,fRmin = 0, fBdip = I,fBmin = 0) and we have total absorption (A=I), the observed difference spectrum properly represents the red spectral component. The result of this subtraction, depicted in the lower panel of Fig. 5, confirms the presence of the red spectral component. The spectrum possesses residual line emission although the peak line intensities are only about 10 per cent of those in the mean spectrum. Whilst the slope inferred for the red component is dependent on both the level of absorption and the modulation fraction of the blue component, as will be argued later, it seems unlikely that the red component could be much flatter than the observed difference spectrum corresponding to equation (5).

Thus we conclude that the UV spectrum of PQ Gem comprises two oppositely sloped components that cross over near 1800 A and modulate in phase (with a maximum around phase 0.15), giving rise to an approximately constant modulation fraction across the 1150-2600 A range. Both contain weak residual line emission although C [vat least is not significantly detected in the blue component when averaged over the linewidth. This picture of two spin-modulated components is not affected by the likelihood of incomplete absorption during the dip although the inferred slopes of the components would be altered.

We applied the same approach to isolate the spectral ingredient that modulates on the beat period. This was obtained by subtracting the spectrum at minimum from that at the maximum of the beat cycle. Here the spin-modulated spectral components effectively contribute a beat-phase-independent profile which cancels out on subtraction. It should be recalled, however, that the lines are modulated in antiphase with the continuum fluxes and therefore this subtraction will yield a negative line spectrum. The results, presented in Fig. 5, indicate a rather flat continuum distribution.

3.3.1 The magnetospheric accretion flow

Before dealing with the spectral components further, we digress briefly to consider the effect of the accretion flow on the spectrum observed during the dip.

Spectral analysis of the ROSAT data from PQ Gem (Duck et al. 1994) indicated the presence of an additional absorbing column _1023 cm -2 during the dip interval. Ferrario & Wickramasinghe (1993) have shown that the X-ray and EUV fluxes from the accretion site of the white dwarf heat the infalling material to temperatures in the range 14000-35 OOOK. Assuming the bulk of the gas to be in local thermodynamic equilibrium, we can estimate the likely bound-free continuum opacity in the UV range due to ionization of hydrogen from the n=2 level. We adopt the mean column density implied by the X-ray absorption dip (_1023 cm -2)

and a path-length through the column of7 x 108 cm, which implies a density of 1.4 x 1014 cm-3. Using the Saha-Boltzmann equation, together with the absorption cross-section (e.g. Gray 1992) and adopting a gas temperature of 20000 K, we estimate that the bound-free (Balmer continuum) optical depths at 1400 and 2200 A are 0.05 and 0.2 respectively. These values are sensitive to the adopted temperature, accretion rate and stream size. For example, increasing the stream density by a factor of 5 would yield optical depths of 1.35 and 5.0 at 1400 and 2200 A respectively, making the

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Figure 5. The spectrum of the blue (top panel) and red (second panel) spectral components that modulate on the spin period. Neither is corrected for absorption. The third panel highlights the shape of the beat -modulated continuum component. The bottom panel is a repeat of panel 3, but plotted on a larger flux scale so that the emission lines can be fully seen - the lines appear with negative fluxes because they modulate in antiphase to the continuum variation. The dashed line in each panel shows the blackbody function that best fits the continuum regions of the data. In the case of the blue spin-modulated spectrum, the additional dotted line represents the model white dwarf atmosphere which best matches the continuum regions. This model contains a strong Lye< absorption feature which is not seen in the uncorrected data.

stream marginally optically thick even in the far-UV. Nevertheless, it seems unlikely that the stream is completely opaque in the far-UV. From equation (4), this means that the observed spectrum of the occulted source, DBCA) (i.e. the difference spectrum in the top panel of Fig. 5), must be multiplied by a substantial (but uncertain), wavelength-dependent correction factor to obtain the intrinsic spectrum, BCA).

3.3.2 Spectral fits

We have attempted to parametrize the various spectral components by fitting simple blackbody functions, or white dwarf atmospheres where appropriate, to their continuum profiles. Given our lack of

© 1997 RAS, MNRAS 288, 891-902

accurate knowledge about the absorption properties of the accretion flow, the likely inappropriate use of blackbody models and the complex (probably multi-temperature) nature of the individual components, this is clearly an imprecise exercise. Nevertheless, the results provide crude insights into the temperature of the emitting regions. To gauge the continuum shape in each case, we excluded data around residual emission features (e.g. the Si N, eN

and He II lines). For the blue spin-modulated spectrum, we initially fitted the data

without applying a correction for absorption. The fit suggests a temperature of about 4 eV (46 000 K). If, instead, we employ a white dwarf atmosphere model (Koester 1991), we find that the overall continuum shape is satisfactorily matched if the temperature is near 30 000 K, although such a low-temperature model predicts a


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substantial absorption line due to Lya which is not apparent in the observed spectrum. The data and model fits are displayed in Fig. 5.

We then attempted to make an allowance for the likely wavelength-dependent transmission of the blue spectrum through the absorbing flow. Despite the caveats mentioned above about the accuracy of corrections to the difference spectrum (equation 2), we pursue this exercise here simply to illustrate the potential effects that absorption introduces since they have implications for our interpretations later on. To emphasize the effects of the wavelengthdependent absorption, we used an absorption correction factor to estimate the input spectrum from that observed rather than iteratively finding an input spectrum that yields the observed data. This is possible because in practice, for the parameter ranges we believe are relevant, the absorption function only varies by a few per cent. The correction factor was estimated by employing the HAZY code (Ferland 1993) to generate a transmittance ratio with wavelength as follows. The incident spectrum comprised a combination of a 5.8x105-K (50eV) blackbody (see Duck et al. 1994) with a bo10metric luminosity of 1034 erg S-1 and a 10-keV thermal bremsstrahlung source of similar total output. The luminosity values adopted are probably generous, but would be compatible with the X-ray fluxes inferred from the ROSAT data (Duck et al. 1994) if a correction factor of 1.5 were invoked for projection effects and PQ Gem were located at a distance of about 780pc - a lower limit of 260 pc is obtained using the Bailey (1981) relation and the K-band magnitude of 12.9 (Mason et al. 1992). For the absorbing medium we adopted a stream density and thickness of 1.4 x 1014 cm-3 and 7 x 108 cm respectively and enforced a constant temperature of 20000 K. The occulting part of the stream was assumed to be located at a distance of 35 x 108 cm (-5Rwd ) from the source. The ratio [rCA)] of the transmitted-toincident continua was used to correct the observed UV spectrum, scaling by (1 - r) -1. The effect of this correction, as expected, is to substantially steepen the blue component spectrum. Blackbody fits to the corrected distribution were not usefully bounded at high temperature values but a 90 per cent confidence lower bound of 20 eV was determined. The integrated flux in the blue component ranges from about 1.1 x 10-11 erg S-1 cm-2 for the blackbody model in the uncorrected case to 5.67 x 10-8 erg S-1 cm-2 for a temperature of 50 eV (5.8 x 105 K) in the corrected case. We return to this subject in Section 4.1.1. We reiterate, however, that this exercise is essentially designed to illustrate how the parameters of the blue spectral component are affected by our uncertain knowledge of the absorption properties of the stream.

A blackbody representation of the spin-modulated red component indicates a temperature in the range 8700-10 000 K. The inferred bolometric flux is 1.52 X 10-11 erg S-1 cm-2. Again, we stress that since the method of deducing the shape of this component was inextricably coupled to that of the blue component via equation (5), the fits only provide a crude constraint on the temperature of the emission region that gives rise to this spectral constituent. We return to this issue in Section 4.1.2.

For the beat-modulated continuum spectral component, we find a temperature around 17 000 K and a bolometric flux of 5 x 10-12 erg s-1 cm-2. The data and model are compared in the lower panel of Fig. 5. Although the blackbody fit does not accurately match the data, the constraint (11 000 :5 T :5 24000 K) probably applies since the continuum appears to peak in the observed wavelength range. The beat-modulated spectrum is not dependent on our knowledge of the absorption level. Nor does it appear to be obviously contaminated by the presence of a second beat-modulated spectral component although given the limited

wavelength range of the data, such contamination cannot be completely ruled out.

4 DISCUSSION

Our HST spectra of PQ Gem have uncovered a number of properties of the system that provide important information on the nature and locations of the emitting regions within the binary.

To place the observations in context, we outline the current basic picture of this particular system. The dominance of the spin signal in the X-ray band (Mason et al. 1992; Duck et al. 1994) in PQ Gem suggests that material feeds the magnetosphere in a fairly azimuthally uniform manner, favouring the presence of a disc or ring in this binary. This circulating flow is presumably disrupted by the magnetic field of the white dwarf at some point and the material latches on to the field lines, becoming entrained in the rotating magnetosphere. The presence of the relatively narrow X-ray dip suggests that this magnetospheric flow is azimuthally confined or concentrated.

This simple picture can accommodate a number of likely sources of UV emission. We might expect some broadly constant (or orbitally modulated) UV emission to arise from the gas in the disc even though the hottest, most UV-intense annuli in the disc may well be absent due to the magnetic disruption. Similarly, the unheated surface of the white dwarf will contribute a fraction of the constant flux. The heated atmosphere of the white dwarf surrounding the polar accretion site(s) can potentially generate a spin-modulated continuum component, as can the magnetically constrained gas flow which may be a source of line and continuum emission. Finally, the spin-pulsed X-ray/EUV flux can give rise to beat-modulated emission when it is incident on, and reprocessed by, material at locations fixed in the binary frame (e.g. a bulge on the accretion disc or the secondary star). In principle, a beat modulation can also arise if material from an orbitally fixed location such as the accretion stream (perhaps overflowing the disc) preferentially couples to field lines that occupy a restricted azimuthal range within the magnetosphere. This latter scenario modulates (gates) the mass flux entering the magnetosphere on the beat period. In the following discussion, we consider the spectral components we identify in our HST spectra in terms of the likely emitting sites outlined above.

4.1 The spin components

4.1.1 The blue component

Our HST spectra contain a spin-modulated spectral component which, irrespective of whether absorption during the dip is total or not, rises in the far-UV. A simple blackbody characterization of this component, uncorrected for absorption, suggested a temperature -4eV (46000K). If the component is fully modulated, the total luminosity is -6.2 x 1030 Dioo erg s -1 which implies a fractional white dwarf radiating area of 0.004Dtoo, where D100 is the distance in units of 100 pc and we have assumed a white dwarf radius of 7 x 108 cm. An additional unknown correction factor (2!: 1) allowing for foreshortening effects of a spot-like emission site has been ignored. The flux and implied radiating area are further increased if the modulation is not total.

If, as seems likely, a correction for absorption needs to be applied, the steeper intrinsic spectrum would be characterized by a higher blackbody temperature which could be compatible with that determined from the soft X -ray component observed by ROSAT

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(- 50eV). Based on our example corrected spectrum from Section 3.3.2, for a temperature of 50 eV (near the lower bound), the inferred bolometric flux is -1.5 x 1034 Dioo erg s -I, which would require a fractional emitting area of o.oOlDioo. If the blueUV component is associated with the Rayleigh-Jeans tail of the soft X-ray spectrum, the flux implied from the UV spectrum is a factor - 140 times larger than that deduced from the X-ray data. This apparent discrepancy, which is made worse if the UV component is not fully modulated, may, however, simply reflect a combination of the problems associated with the correction of the spectrum for absorption and the use of blackbody models as a substitute for selfconsistent white dwarf atmosphere models (see, for example, Heise, van Teeseling & Kahabka 1994). Nevertheless, the overall properties of this component, including its sympathetic phasing with the X-ray modulation, suggest that it is associated with a hot region (T ;<:: 25 000 K) which is presumably located close to, or on the surface of, the white dwarf. The apparent lack of a strong Lya absorption line might be taken as evidence to support an emission region temperature well above this lower limit. However, it should be noted that in attempting to correct for the absorption, we assumed a constant temperature absorber. If, instead, the accretion flow that occults the white dwarf is not of uniform temperature but, for example, has cooler (T - 104 K) strata on the side facing away from the white dwarf, absorption within the flow could imprint a significant Lya line on the transmitted spectrum. If not corrected for (and it is not in our treatment), this would translate into an apparent emission feature in the difference spectrum, at least partially filling in any intrinsic Lya absorption in the source spectrum.

Finally, it should be borne in mind that a single temperature emission region is unrealistic. One would naturally expect a temperature gradient, as inferred from the IUE data of AM Her (Glinsicke, Beuermann & de Martino 1995). The low-temperature domain of the parameter space that characterizes the blue component, with its correspondingly lower inferred luminosity, might then reflect the integrated radiation from a stratified surface emission region. It should be remembered, though, that the low-temperature fits arise from the cases in which only modest corrections for absorption are applied to the difference spectrum.

4.1.2 The red component

The positive slope of the red component is probably not substantially affected by the uncertainty over the level of absorption or any unmodulated fraction of the blue component. From equation (5), it is evident that the red component can only be flatter than the observed difference spectrum if {fBdip[1 - A(~)] - IBmin} is smaller (or more negative) at the blue end of the spectrum than at the red end. In either case, since IBdip and IBmin should be constant, this requires A(red) < A(blue), contrary to the wavelength dependence of likely absorption processes.

The low-temperature (T -10 000 K), rotationally modulated red spectral component is perhaps most naturally associated with the accretion flow itself. The flux variation of this component appears to be in phase with that of the blue component (and the X-ray variation), suggesting that it arises from optically thick material on the outer surface of the accretion flow which faces us when the accreting region is inclined towards the observer (at X-ray flux maximum). The blackbody fits infer a total flux -8 x 10-12 erg S-I cm-2 which could be radiated by a surface of area _1019 Dioo cm2 - these values will be higher if the red component is not completely modulated. This is comparable

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(within a factor of 2) to the likely radiating area of a tube-like accretion flow of radius RWd and length 1 ORwd'

4.2 The beat components

4.2.1 The continuum flux

The continuum component that varies on the beat period can be roughly characterized by a temperature of 17000 K and a correspondingbolometric flux of2.2 x 10-12 erg s-I cm-2, which would be increased if the modulation of this component were < lOOper cent. Again treating the emission as a blackbody, this can be radiated by an area of2.6 x 1017 DIoo cm2 • If we assume that the beat-modulated component arises from reprocessing and that the outbound flux equates to the (isotropically emitted) soft X-ray radiation received when the X-ray beam passes across the site, then for the inferred soft X-ray flux of 1.9 x 10-10 erg S-1 cm-2 (Duck et al. 1994), the reprocessing site subtends a solid angle, n, where 01411'= 0.01. Combining these constraints on the emitting area and solid angle suggests that the site is located at a distance ;<:: 2RwdD100 cm from the irradiating source. In a 5.2-h binary, the secondary star has a mean radius Rsec - 4 X lOla cm and is separated from the white dwarf by a distance of about 1.2 x 1011 cm (the star would be of late K to MO spectral type). Treating the secondary star as spherical (for simplicity), the surface area facing the white dwarf is _1022 cm2 so if the beat-modulated continuum flux originates there, only a tiny fraction of the surface area of the secondary is involved unless either D100 ~ 10 and/or the modulation fraction of the component is very small (:s a few per cent). Thus for plausible distances (e.g. 500 pc), the secondary star is unlikely to be the continuum reprocessing site. One might also consider a stream-disc impact region as a potential site. The sizes of such sites are poorly known. However, if we depict it as a cylindrical bar, bent around the edge of the disc (Rdisc - 3.7 X 1010 cm -50Rwd or about 0.6 of the Roche lobe of the primary - Harrop-Allin & Warner (1996», with a cylinder radius of O.IRdisc and an azimuthal extent similar to that (100°) inferred from the soft X-ray dip in EX Hya (Cordova, Mason & Kahn 1985), its surface area facing the white dwarf would be about 8 x 1020 cm -2,

still far larger than needed to radiate the observed continuum flux. So a stream impact bulge also seems an unlikely contender for the reprocessing site although it becomes a more realistic possibility if PQ Gem lies at a distance in excess of 500 pc.

A further potential constraint on the location of the source is of interest here. The deep dip in the rotational X-ray signal ofPQ Gem is strong evidence that the X-ray source is occulted, and its flux attenuated, particularly at soft X-ray energies, by material in the magnetospheric accretion flow. Any reprocessing site located beyond the magnetosphere would inevitably view the illuminating source through this flow once per beat cycle, possibly witnessing even greater attenuation than we experience if the source is seen through the low-velocity, dense material near the binary plane. If the soft X -ray flux is a significant power source for the reprocessing site, we might expect the beat signal to contain a dip reflecting the reduction in the irradiating flux when the accretion flow shadows the site. That we do not see any evidence of this effect may be a clue that the reprocessing region does not lie beyond the magnetosphere. This argument might be weakened if we experience a grazing viewing geometry in which our line of sight traverses a long pathlength through the pre-shock flow during the dip (as suggested by the polarimetry of Potter et al. 1997). This would reduce the inferred accretion rate in the flow (possibly below that needed to explain the X-ray luminosity) and therefore the absorbing column


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in front of the reprocessing site. None the less, it still seems likely that any reprocessing site beyond the magnetosphere and confined to the binary plane would be shadowed by parts of the flow where the column exceeds _1021 cm-2.

4.2.2 The lines

Most (>80 per cent) of the photons emitted in the soft X-ray component of PQ Gem [for a kT - 50 eV (5.8 x 105 K) blackbody] lie above 54 eV (228 A) and 48 eV (259 A), the ionization thresholds of He rr and C III respectively (these values fall to about 8 per cent if the temperature of the blackbody is nearer 105 K). Thus the soft X-ray spectrum produced at the accretion site of the white dwarf is readily capable of producing the ion species observed in our HST spectra by photoionization. As the beam sweeps on to the reprocessing site, the number of such ions increases with a corresponding rise in the number of line transitions occurring per unit volume.

The observed line flux changes around the beat cycle can be exploited to obtain an order of magnitude estimate of the size of the line-emitting region. Based on the flat-bottomed shape of the C N

beat modulation, we assume that the reprocessing site is illuminated (by an isotropic source), only between phases 0.93 and l.43. The C N Al550 line is predominantly powered by collisional excitation whereas Herr A1640 is generated via recombination. The estimate for the C N emission follows the approach outlined by Peterson (1994), assuming the gas to be optically thin in the lines. The illuminating X-ray/EUV photons can penetrate and maintain the reprocessing material in an ionized state to a depth comparable to the Stromgren length, ..:ir9 - 8 x 103Q43Id~n~12aB_13' Here Q43 is the number of photon S-1 in units of 1043 , emitted isotropically by the source, that can ionize the element to the stage concerned, aB-13 is the case B recombination coefficient for the ion in units of 10-13 cm3 S-I, ne12 is the electron density in units of 1012 cm-3 and d9 is the separation between the reprocessing material and the radiation source (all lengths are in units of 109 cm). We employ a 50-eV blackbody spectrum with a luminosity of 1034 erg S-1 (see Section 3.3.2) as the ionizing continuum source [Q - 3.9 x 1043

(number of photon S-I) above 54 eV]. The recombination coefficient for He rr was taken as 9 x 10-13 . Considering the line-emitting region to be a shell overlying an effectively neutral volume of radius r, the CIV flux is

LeIV = 27rr2..:ir nenCIV q hp, (6)

where q is the collisional excitation rate for the A1550 transition. We approximate the number density of C N ions, ncIV , by nJdcIV

wherefc is the number density of carbon relative to hydrogen (taken as 3 x 10-4 - e.g. Allen 1973) andfc1v' the fraction of carbon in its triply ionized state, is simply taken to be 1. On substituting for ..:ir in equation (6) (noting that the n~ terms cancel), we obtain

(7)

As an example, for a region at d = 7 x 109 cm (- 10Rwd ), the C N

line flux is 4.4 X 1030 r~ erg s -1. The observed change in the C N flux between maximum and minimum of the beat cycle is -2.8 x 10-13 erg s-1 cm-2 which translates to a luminosity of 3.2 x 1029 DIoo erg S-I. Thus this flux could be radiated by a shell of radius r - 2.7 X 108 D100 cm, where D100 is the distance to the system in units of 100 pc. For a density of say ne = 1014 cm -3, the shell thickness, ..:ir, would be 7 x 106 cm. We also considered the possibility that the line flux might arise from the secondary star. If we locate the line-emitting site at a distance a - 0.5Rsec = 9.55 x 1010 cm (-140 Rwd) from the white dwarf,

where a is the binary separation and Rsec is the mean radius of the secondary star, and again equate equation (7) to the observed beat period luminosity variation of the 1550-A line, we deduce a radius of 3.7 X 109 D100 cm for the emitting region. Put another way, if the C N A1550 line emission does arise on the secondary star, only -0.3DIoo per cent of the surface area is producing the line.

The Herr 1640-A line arises from an Ha-like (n=3-2) transition in the recombination cascade. Here we equated the observed variation of the 1640-A line luminosity around the beat cycle (1.1 x 1029DIoo erg s-l) to that expected from a shell-like volume, 27rr2..:ir ne nHellh2, where the emissivity, h2, for the Al640 line at 20 000 K (5.4 x 10-24 erg cm3 S-I) was obtained by scaling from data in Osterbrock (1974). Substituting again for ..:ir and using an He/H number density ratio of 8.5 per cent (assuming all He is essentially fully ionized), this yields r - 7.3 X 108 D100 cm iflocated at d = 7 x 109 cm from the irradiation source, a factor - 3 larger than estimated for the C N line. This difference might be real. However, in part, it could also be because we assumed the limiting case, fC1v =1, whereas other ion states might in fact be dominant. This would then require a larger radius of the emitting region to produce enough C IV line flux [r DC ifC1v>1/2]. It may also indicate optical depth and collisional effects which are ignored in this simple treatment - we are considering far higher densities here than those for which the emissivities are tabulated. These results suggest that the size of the region producing the beat modulation of the lines is :51 ORwd , regardless of where it is located within the system.

4.2.3 Implications of the beat period phenomena

An important issue to address regarding the UV beat period phenomena in PQ Gem is the anti-phasing of the continuum and line variations. In terms of reprocessing models, the most obvious explanations are invoking (1) two reprocessing sites, oppositely located around the white dwarf, one of which is dense and produces the continuum variation, the other more tenuous site generating the line flux, and (2) a single site producing both the continuum and line components but with a phase lag of about half a beat cycle between the two reprocessed signals.

In the context of the two-site scenario, it is often assumed, at least qualitatively, that a plausible reprocessing region in any IP binary is the secondary star or a bulge on the edge of any disc where the stream from the secondary star impacts (although see above). Both are non-axisymmetric, orbitally fixed entities that could be exposed to the rotating X-ray searchlight. It is harder to identify a viable origin for a second reprocessing site. One possibility for the second site could arise if the accretion stream also overflows the disc and directly impacts on the magnetosphere on the far side of the white dwarf from the secondary star. The magnetic field in PQ Gem is thought to be comparable to that in polars (_107 G) (e.g. Piirola et al. 1993). For a plausible white dwarf mass (0.8 Mo) and accretion rate (M - 1017 g s-I), the magnetospheric radius is -l.5 x 1010 cm (21Rwd) (e.g. Hameury, King & Lasota 1986) - the radius varies with M-217 • In comparison, the point of closest approach to the compact star for a free-falling stream (in the absence of a disc) is roughly Rc/2 (Lubow & Shu 1975; see also Lubow 1989), where Rc - 1.6 X 1010 cm (23Rwd) is the circularization radius (e.g. Frank, King & Raine 1992). Thus closest approach would be at r - 8 x 109 cm (-llRwd)' Viscous interaction with the disc surface would remove angular momentum from the stream, bringing it even closer to the accreting star. Thus a stream overflowing the disc would be unlikely to circumnavigate the magnetospheric obstacle but would instead presumably crash into it. The impact region

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would be fixed in the orbital frame and probably subject to irradiation once per beat cycle but its location would make an acute angle to the line of centres and could not therefore explain the half-cycle phase shift. A further important objection applies to the stream overflow picture, even if the stream could get around the magnetosphere. For a magnetic geometry, material fed from an orbitally fixed location (such as the putative stream impact region) is likely to gain access via favoured entry points within the magnetosphere which constantly slips beneath the impact site. This gives rise to a 'gating' phenomenon which effectively modulates the mass flow within the magnetosphere (e.g. Mason, Rosen & Hellier 1988) and could be expected to produce a beat modulation of the X -ray signal. However, no beat period phenomenon has yet been detected in the X-ray emission of PQ Gem. For this same reason we also reject any possibility that the UV beat phenomena are intrinsically powered by magnetospheric gating (i.e. that their emission is related directly to modulation of the accretion rate on to the white dwarf).

Invoking a single reprocessing site allows two obvious variants. In one, the rotating radiation beam could perhaps drive both an emission line and a continuum-reprocessed signal but with a time delay between them. In principle, for example, the line emission might be seen as soon as the beam illuminates the site but if there were a significant time needed for the heat energy dumped on the region to diffuse inward and raise the temperature of the optically thick volume, the continuum pulse could then peak somewhat after the lines. The response of the lines to the ionizing flux depends on the recombination time-scale, T" for ions in the gas: Tr - 10/(cL13nel2) s, where CL13 is the recombination coefficient in units of 10-13 cm3 S-I (e.g. Allen 1973; Osterbrock 1974) and nel2 is the electron density in units of 1012 cm -3. This time-scale is ::;10 s if the gas density is ;:::1011 cm-3 so regions regarded as likely candidates to explain the beat signal probably would generate enhanced line emission almost as soon as the beam started to transit the site. It seems unlikely, however, that the continuum pulse could be delayed by -Pbeat!2 since much of the emission would presumably be produced in the outer layers of the reprocessing site so one would expect the continuum flux to respond quickly to the incident flux too.

The reverse case, i.e. where the line pulse is delayed with respect to the continuum, can be ruled out. To introduce an -430-s delay (if the continuum peaked immediately) would require ne - 2.5x 109 cm -3, which in turn would imply a Stromgren depth _1016 cm; a region of this density that would fit inside the binary could not power the observed line flux.

Finally, a second, single-site possibility exists, at least qualitatively, if the reprocessing site is illuminated by flux from both polar accretion sites on the white dwarf. The one above the disc could then power the line emission by ionizing a shell of gas on the upper, visible surface of the site. Half a beat cycle later, the lower pole would irradiate the portion of the site beneath the disc. Geometric obscuration (by the disc) could conspire to hide the line emission produced beneath the disc so yielding just one line pulse per beat period. However, as the rest of the optically thick volume is heated by the lower pole, we might then observe the blackbody radiation from the upper, visible portion of the reprocessing site. An argument against this idea, though, is that it is difficult to see why we would not also observe continuum (and line) emission from the upper surface when irradiated by the upper pole. Qualitatively, one way of circumventing this problem could be to invoke polar emission regions with very different spectral properties. Since we believe a disc or ring is feeding the magnetosphere in PQ Gem, this

© 1997 RAS, MNRAS 288, 891-902


might require a substantially decentred field configuration. There is certainly evidence for cyclotron emission from two poles (Piirola et al. 1993; Potter et al. 1997). Whether both poles are also X-ray sources is less clear. The X-ray light curves may be consistent with a single X-ray pole hypothesis, thus perhaps supporting the notion of dissimilar emission regions, but we cannot rule out X-ray flux from the lower pole or ascertain whether the two poles possess distinct spectral properties.

In summary, several qualitative possibilities exist to explain the UV beat period phenomena witnessed in our HST observation of PQ Gem but, currently, none comes without significant objections.

5 CONCLUSIONS

We have presented HST FOS UV spectroscopy of the intermediate polar PQ Gem. We find the UV continuum light is modulated on both the spin and the beat periods. The rotational UV light curve contains a dip which, in these observations, occurs near, but not coincident with, the phase of the X-ray dip. Nevertheless, we suspect they are one and the same event. The UV dip is deepest at far-UV wavelengths (at least down to about 1350 A). Based on the colour dependences of the spin and beat pulsations and the spectral behaviour during the rotational dip, we conclude that the spectrum is multicomponent in nature, containing evidence of two spin-modulated continuum components and a beat-modulated continuum contribution. It is important to note that it is not possible to explain the behaviour of the spectrum without invoking multiple components. One of the spin components shows a distinct rise to the blue and appears to be occulted by the accretion flow during the spin cycle. The other shows a red spectral distribution and is not obviously subject to absorption. Our ability to determine the shape of the blue component is sensitive to the amount and the wavelength dependence of absorption that pertains during the dip and is therefore uncertain. Nevertheless, simple blackbody fits suggest a temperature that may be as low as 25 000 K, but could conceivably be consistent with that inferred from previous soft X-ray data although the inferred bolometric flux would then be discrepant (within the context of the simple models adopted). The blue component is very probably associated with the accretion region or cooler surrounding annuli. The cooler red component is more easily attributed to the magnetospheric accretion flow.

The emission lines modulate predominantly on the beat period but, remarkably, vary in antiphase with the beat-modulated continuum component. Both the continuum and the C IV and He II lines seem to originate from regions that are comparable in size to the white dwarf. However, we have been unable to identify a site that could naturally explain the half beat cycle delay between their light curves. Neither a single- nor a two-site scenario provides a consistent, problem-free answer.

ACKNOWLEDGMENTS

We thank Andrew King, Gordon Stewart, Paul O'Brien and Matthew Burleigh for useful discussions. This work was based on observations with the NASAIESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

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