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Title: X-ray emission from intermediate polars and open clusters
Author: Harmer, Sean.
Awarding Body: University of Keele
Current Institution: Keele University
Date of Award: 2003
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We have implemented a full 3-dimensional model to generate the X-ray lightcurves and spectra of intermediate polars using the traditional accretion curtain model as a basis. The model constitutes an improvement over previous works because it includes the effects of tall, structured accretion columns, electron scattering by the post-shock flow, and photoelectric absorption by the pre-shock flow by accurately considering the 3-dimensional dipolar geometry of the accretion curtains. A systematic parameter study has been undertaken using this model revealing that both occultation of the Xray emitting regions and electron scattering can produce energy-dependent modulations. Including the effects of tall, structured accretion columns introduces sensitivities to parameters such as the white dwarf mass that Kim & Beuermann (1995) found to have little effect. The energy-dependent modulation is found to vary for each parameter in a systematic but unique manner. These results imply that it may be possible to fit this model to observational data and extract information about many system parameters. We have analysed 5 RXTE observations taken of GK Per over a 41 day period during a dwarf nova outburst in 1996. We have confirmed the white dwarf spin period of 351s but find no evidence of any quasi-periodic oscillations around the 5 - 6ks period. During the outburst, the spin pulse changes from a complex, low-amplitude (~ 14%), double-peaked shape to a high-amplitude (~61%), flat-bottomed quasi-sinusoid at the peak of the outburst. Spectral fits indicate that the absorption and X-ray flux both increase during the outburst, and that it is not possible to reproduce the X-ray spin pulse with only changes in the absorption. Using the model developed here, we have reproduced the quiescent and outburst lightcurves of GK Per and find that the observed changes in the lightcurve morphology are consistent with an increase in the accretion disc mass transfer rate by a factor of f'V 16. We also suggest that the observed lightcurves are consistent with X-ray emission from complete accretion rings around either one or both poles if the dipole axis is inclined to the spin axis by ~ 5.50 , and is offset from the centre of the white dwarf by ~ 0.12 Rwd. NGC 6633 is a young, open cluster with a similar age to the Hyades and Praesepe, but probably a lower metallicity. We present the results of ROSATHRI observations of an optically selected catalogue of likely members of NGC 6633. Eight out of fifty-one NGC 6633 members have been detected, with main-sequence spectral types A to G, above a threshold X-ray luminosity of ~ 6 -12 x 1028ergs-1• We find that NGC 6633 does not contain cool stars which are as X-ray luminous as the most active objects in the Hyades and that the median X-ray luminosity of F-G stars in NGC 6633 is less than that in the Hyades, but probably greater than in Praesepe. However, when X-ray activity is expressed as the X-ray to bolometric flux ratio we find that NGC 6633 and the Hyades are very similar and display similar peak levels of coronal activity. We attribute this discrepancy to a number of possible wide binary systems with higher X-ray (and bolometric) luminosities in the Hyades sample and either a low metallicity in NGC 6633, which makes its cool stars both X-ray and bolometrically less luminous at the same colour, or that the distance to NGC 6633 has been underestimated, which would decrease stellar X-ray luminosities without changing X-ray to bolometric flux ratios. We have highlighted the need for a model that can be used to calculate the iron Ka line profiles detected in intermediate polars that result from Comptonisation of photons trapped by resonance scattering. Such a model may yield information about the structure of the post-shock accretion columns and in particular details about the transition to optical thickness. Monte Carlo simulations are an ideal candidate because of the complexities involved in solving the equations of radiative transfer in the accretion columns of intermediate polars. We present the theoretical background necessary to construct such a model and the initial implementation of a framework upon which more complicated models may be built.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available