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

Some updates on the role of Magnetic Fields in Cataclysmic Variables

In this review talk I cover some recent developments in understanding the role that magnetic fields play in cataclysmic variables. I discuss the recent DNO--QPO unification models; the disk--magnetosphere boundary; some issues concerning the soft blackbody component and the nature of the X-ray spectra in MCVs; whether the SW Sex stars are magnetic, and finally I mention the weird behavior of FS Aur and HS2331+3905.Comment: To appear in ``Compact Binaries in the Galaxy and Beyond'', RevMexA

An outburst of the magnetic cataclysmic variable XY Arietis observed with RXTE

We report the first observed outburst of the magnetic cataclysmic variable XY Ari. X-ray observations show a flux increase by an order of magnitude the day after the first signs of outburst. During the 5-d duration the X-ray spin pulse is greatly enhanced and the X-ray spectrum far more absorbed. We suggest that the inner disc pushes inwards during outburst, blocking the view to the lower accreting pole, breaking the symmetry present in quiescence, and so producing a large pulsation. The observations are consistent with a disc instability as the cause of the outburst, although we can't rule out alternatives. We draw parallels between our data and the UV delay and dwarf nova oscillations seen in non-magnetic dwarf novae.Comment: 10 pages, 9 figures, LaTeX, also at http://www.astro.keele.ac.uk/~ch

XMM-Newton observations of the complex spin pulse of the intermediate polar PQ~Geminorum

The intermediate polar PQ Geminorum shows a complex pulsation, caused by a spinning white dwarf, which varies markedly with wavelength. We report XMM-Newton observations, including the soft and hard X-ray bands and the first UV lightcurves of this star. We update the ephemeris for PQ Gem allowing us to align these data with a compilation of lightcurves from the optical to the X-ray. Building on work by previous authors, we show how a model in which accretion flows along skewed field lines, viewed at the correct inclination, can explain the major features of the lightcurves in all bands. We discuss how the skew of the field lines relates to the spinning down of the white-dwarf rotation.Comment: 7 pages, 7 figures, accepted for publication in MNRA

On the orbital period distribution of cataclysmic variables

Using the latest compilation of cataclysmic variable orbital periods by Ritter & Kolb we argue against Verbunt's conclusion that the period gap is not significant for nova-like variables. We also discuss the relation of the VY Scl stars to the dwarf novae.Comment: 3 pages, 3 figures, LaTeX, To appear in MNRAS. Also at http://www.astro.keele.ac.uk/~ch

TESS Hunt for Young and Maturing Exoplanets (THYME). IV. Three Small Planets Orbiting a 120 Myr Old Star in the Pisces-Eridanus Stream

Young exoplanets can offer insight into the evolution of planetary atmospheres, compositions, and architectures. We present the discovery of the young planetary system TOI 451 (TIC 257605131, Gaia DR2 4844691297067063424). TOI 451 is a member of the 120 Myr old Pisces–Eridanus stream (Psc–Eri). We confirm membership in the stream with its kinematics, its lithium abundance, and the rotation and UV excesses of both TOI 451 and its wide-binary companion, TOI 451 B (itself likely an M-dwarf binary). We identified three candidate planets transiting in the Transiting Exoplanet Survey Satellite data and followed up the signals with photometry from Spitzer and ground-based telescopes. The system comprises three validated planets at periods of 1.9, 9.2, and 16 days, with radii of 1.9, 3.1, and 4.1 R ?, respectively. The host star is near-solar mass with V = 11.0 and H = 9.3 and displays an infrared excess indicative of a debris disk. The planets offer excellent prospects for transmission spectroscopy with the Hubble Space Telescope and the James Webb Space Telescope, providing the opportunity to study planetary atmospheres that may still be in the process of evolving