Twisted accretion curtains in the intermediate polar FO Aquarii

We report on a ~37-ks XMM-Newton observation of the intermediate polar FO Aquarii, presenting X-ray and UV data from the EPIC and OM cameras. We find that the system has changed from its previously reported state of disc-overflow accretion to one of purely disc-fed accretion. We detect the previously reported `notch' feature in the X-ray spin pulse, and explain it as a partial occultation of the upper accretion pole. Maximum flux of the quasi-sinusoidal UV pulse coincides with the notch, in keeping with this idea. However, an absorption dip owing to the outer accretion curtains occurs 0.27 later than the expected phase, which implies that the accretion curtains are twisted, trailing the magnetic poles. This result is the opposite of that reported in PQ Gem, where accreting field lines were found to lead the pole. We discuss how such twists relate to the accretion torques and thus the observed period changes of the white dwarfs, but find no simple connection.Comment: 7 pages, 5 figures. Accepted by MNRA

Fast spectroscopy and imaging with the FORS2 HIT mode

The HIgh-Time resolution (HIT) mode of FORS2 has 3 sub-modes that allow for imaging and spectroscopy over a range of timescales from milliseconds up to seconds. It is the only high time resolution spectroscopy mode available on an 8m class telescope. In imaging mode, it can be used to measure the pulse of pulsars and spinning white dwarfs in a variety of high throughput broad- and narrow-band filters. In spectroscopy mode it can take up to 10 spectra per second using a novel ''shift-and-wait'' clocking pattern for the CCD. It takes advantage of the user-designed masks which can be inserted into FORS2 to allow any two targets within the 6.8' x 6.8' field of view of FORS2 to be selected. A number of integration, or more precisely 'wait', times are available, which together with the high throughput GRISMs can observe the entire optical spectrum on a range of timescales.Comment: 19 pages, 7 figures, to appear in "High Time Resolution Astrophysics, Galway 2006

Stream-field interactions in the magnetic accretor AO Piscium

UV spectra of the magnetic accretor AO Psc show absorption features for half the binary orbit. The absorption is unlike the wind-formed features often seen in similar stars. Instead, we attribute it to a fraction of the stream that overflows the impact with the accretion disk. Rapid velocity variations can be explained by changes in the trajectory of the stream depending on the orientation of the white-dwarf's magnetic field. Hence we are directly observing the interaction of an accretion stream with a rotating field. We compare this behavior to that seen in other intermediate polars and in SW Sex stars.Comment: Accepted for ApJ; 6 page

The size of the accretion region in intermediate polars: eclipses of XY Arietis observed with RXTE

RXTE observed 20 eclipse egresses of the intermediate polar XY Ari in order to study the size and structure of the X-ray emitting accretion regions. The spin-phase averaged egress lasts 26 s, implying a white dwarf radius of 4.3-7.0 10^8 cm. The individual egresses occur later in orbital phase with later spin phase, as expected if the white dwarf spins in the same sense as the orbital motion. The eclipse times trace out the motion of the upper pole across the white dwarf face; then, when the upper pole disappears over the white dwarf limb and the lower pole appears, they trace the motion of the lower pole across the face. Aligning all the egresses shows that the majority of the X-ray flux emerges in <2 s, implying accretion regions with area, f, < 0.002 as a fraction of the white dwarf surface. Using only the spin-phase to align the egresses, however, gives a longer (~5 s) time for the emergence of the majority of the flux, implying that the accretion regions wander over an area of f < 0.01. There is also evidence that a minority of the flux emerges from a much larger area, or that we see accretion regions at both poles simultaneously at some spin phases.Comment: 11 pages, 10 figures, LaTeX, To appear in MNRAS. Also at http://www.astro.keele.ac.uk/~ch

Stream-fed Accretion in Intermediate Polars

I review the observational evidence for stream-fed accretion in intermediate polars. Recent work on the discless system V2400 Oph confirms the pole-flipping model of stream-fed accretion, but this applies only to a minority of the flow. The bulk of the flow is in the form of blobs circling the white dwarf, a state which might have been a precursor to disc formation in other IPs. I also discuss work on the systems with anomalously long spin periods, V1025 Cen and EX Hya. There are arguments both for and against stream-fed accretion in V1025 Cen, and further work is necessary before reaching a conclusion about this system.Comment: 10 pages, 6 figures, To appear in 'The Physics of Cataclysmic Variables and Related Objects', Goettingen, August 5-10, 2001; includes low-res figures to reduce siz

Why does the intermediate polar V405 Aurigae show a double-peaked spin pulse?

V405 Aurigae is an intermediate polar showing a double-peaked pulsation in soft X-rays and a single-peaked pulsation in harder X-rays. From XMM-Newton observations we find that the soft band is dominated by blackbody emission from the heated white-dwarf surface. Such emission is at a maximum when either magnetic pole points towards us, explaining the double-peaked pulsation. The symmetry of the pulses requires that the angle between the magnetic and spin axes be high. The single-peaked pulsation in harder X-rays is explained in the usual way, as a result of opacity in the accretion curtains. However, the high dipole inclination means that the accretion curtains are nearly in the plane. Thus the outer regions of the curtains do not cross the line of sight to the accretion footprints, explaining the absence of the deep absorption dip characteristic of many intermediate polars. The sawtooth profile of this pulsation requires that the magnetic axis be offset from the white-dwarf centre. We remark also on the double-peaked optical emission in this star. We suggest that the difference between V405 Aur's spin pulse and those of other intermediate polars is the result of its high dipole inclination.Comment: 7 pages, 6 figures, accepted for publication in MNRA

Outbursts of EX Hydrae: mass-transfer events or disc instabilities?

We present the 45-yr record of EX Hya's lightcurve and discuss the characteristics of its 15 observed outbursts. We then concentrate on the 1998 outburst, reporting the first outburst X-ray observations. We discover an X-ray beat-cycle modulation, indicating that an enhanced accretion stream couples directly with the magnetosphere in outburst, confirming our previous prediction. Optical eclipse profiles late in outburst show that the visible light is dominated by an enhanced mass-transfer stream overflowing the accretion disc. We are uncertain whether the enhanced mass transfer is triggered by a disc instability, or by some other cause. While in outburst, EX Hya shows some of the characteristics of SW Sex stars.Comment: To appear in MNRAS (8 pages; 9 figs

On the magnetic accretor GK Persei in outburst

RXTE made 5 X-ray observations of the magnetic accretor GK Per during its 1996 outburst, recording a count rate of ten times the quiescent level. The 351-s spin pulse shows a deep, nearly sinusoidal modulation, in contrast to the weaker, double-humped profile of quiescence. The spectrum shows absorption increased by two orders of magnitude over quiescence. We explain these differences in terms of the changing accretion geometry as the outbursting disc forces the magnetosphere inwards, and discuss the 5000-s X-ray QPOs seen during GK Per's outbursts.Comment: To appear in MNRAS; 5 page