arXiv subjects

G. A. Wynn

Publications and source records attributed to G. A. Wynn.

43 records · Page 3Linked to original sources

arXiv2003

Fe K emission and absorption features in XMM-Newton spectra of Mkn 766 - evidence for reprocessing in flare ejecta

We report on the analysis of a long XMM-Newton EPIC observation in 2001 May of the Narrow Line Seyfert 1 galaxy Mkn 766. The 3-11 keV spectrum exhibits a moderately steep power law continuum, with a broad emission line at ~6.7 keV, probably blended with a narrow line at ~6.4 keV, and a broad absorption trough above ~8.7 keV. We identify both broad spectral features with reprocessing in He-like Fe. An earlier XMM-Newton observation of Mkn 766 in 2000 May, when the source was a factor ~2 fainter, shows a similar broad emission line, but with a slightly flatter power law and absorption at a lower energy. In neither observation do we find a requirement for the previously reported broad 'red wing' to the line and hence of reflection from the innermost accretion disc. More detailed examination of the longer XMM-Newton observation reveals evidence for rapid spectral variability in the Fe K band, apparently linked with the occurrence of X-ray 'flares'. A reduction in the emission line strength and increased high energy absorption during the X-ray flaring suggests that these transient effects are due to highly ionised ejecta associated with the flares. Simple scaling from the flare avalanche model proposed for the luminous QSO PDS 456 (Reeves etal. 2002) confirms the feasibility of coherent flaring being the cause of the strong peaks seen in the X-ray light curve of \mkn.

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arXiv2003

The spin periods of magnetic cataclysmic variables

We have used a model of magnetic accretion to investigate the rotational equilibria of magnetic cataclysmic variables (mCVs). This has enabled us to derive a set of equilibrium spin periods as a function of orbital period and magnetic moment which we use to estimate the magnetic moments of all known intermediate polars. We further show how these equilibrium spin periods relate to the polar synchronisation condition and use these results to calculate the theoretical histogram describing the distribution of magnetic CVs as a function of P_spin / P_orb. We demonstrate that this is in remarkable agreement with the observed distribution assuming that the number of systems as a function of white dwarf magnetic moment is distributed according to N(mu_1) d mu_1 ~ mu_1^{-2} d mu_1.

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arXiv2002

AE Aquarii: how CVs descend from supersoft binaries

AE Aquarii is a propeller system. It has the shortest spin period among cataclysmic variables, and this is increasing on a 10^7 yr timescale. Its UV spectrum shows very strong carbon depletion vs nitrogen and its secondary mass indicates a star far from the zero-age main sequence. We show that these properties strongly suggest that AE Aqr has descended from a supersoft X-ray binary. We calculate the evolution of systems descending through this channel, and show that many of them end as AM CVn systems. The short spindown timescale of AE Aqr requires a high birthrate for such systems, implying that a substantial fraction of cataclysmic variables must have formed in this way. A simple estimate suggests that this fraction could be of order one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the CIV1550 / NV1238 ratio, in determining how large the observed fraction is.

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arXiv2001

On the effects of the stellar magnetic field on the structure of T Tauri accretion discs

The structure of accretion discs around magnetic T Tauri stars is numerically calculated using a particle hydrodynamical code, in which magnetic interaction is included in the framework of King's diamagnetic blob accretion model. Setting up the calculation so as to simulate the density structure of a quasi-steady disc in the equatorial plane of a T Tauri star, we find that the central star's magnetic field typically produces a central hole in the disc and spreads out the surface density distribution. We argue that this result suggests a promising mechanism for explaining the unusual flatness (IR excess) of T Tauri accretion disc spectra.

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arXiv2001

The low mass white dwarf companion to Beta Crateris

We present FUSE H Lyman series spectroscopy of the hot white dwarf companion to the 4th magnitude A1III star Beta Crateris, which shows that is has an unusually low mass, 0.43Msun, and has almost certainly evolved through binary interaction. This system could be a long-sought remnant of Algol-type evolution, although radial velocity measurements appear to show that the pair are not close. Instead, micro-variations in Beta Crt's proper motion as measured by Hipparcos suggests that the period could be as high as 10 years. However, a low mass white dwarf in a system with a period >3 years is difficult to explain by conventional models for binary evolution. We speculate on alternative models for the evolution of this system which involve an eccentric binary or multiple components.

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arXiv2001

A ROSAT WFC observation of SW UMa: the EUV behaviour of dwarf novae in superoutburst explained

During re-processing and analysis of the entire ROSAT Wide Field Camera (WFC) pointed observations database, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 October superoutburst. Although long outbursts in this SU UMa-type system are known to occur every ~450 days, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT. The WFC observations began just ~13 hr after the optical rise was detected. With a peak count rate of 4.5 count/s in the S1 filter, SW UMa was temporarily the third brightest object in the sky in this waveband. Over the next ~19 hr the measured EUV flux dropped to <2 count/s while the optical brightness remained essentially static at V~11. Similar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst (Mauche and Raymond 2000). In contrast, U Gem-type dwarf novae show no such early EUV dip during normal outbursts. Therefore, this feature may be common in superoutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki (1994, 1995) and Mauche and Raymond (2000) and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities which affect the accretion disks in these systems.

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arXiv1999

The Spin Period of EX Hydrae

We show that the spin period of the white dwarf in the magnetic CV EX Hydrae represents an equilibrium state in which the corotation radius is comparable with the distance from the white dwarf to the inner Lagrange point. We also show that a continuum of spin equilibria exists at which Pspin is significantly longer than \sim 0.1 Porb. Most systems occupying these equilibrium states should have orbital periods below the CV period gap, as observed.

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