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Spectrum and pulse profile formation in strong-field X-ray pulsars

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Galloway, Duncan K.(Duncan Kenneth) (2001) Spectrum and pulse profile formation in strong-field X-ray pulsars. PhD thesis, University of Tasmania.

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Abstract

I present the results of an analytic and modelling study of X-ray pulsar emission.
Satellite X-ray observations of the binary pulsars GX 1+4 and RX J0812.4-3114
made using the Rossi X-ray Timing Explorer (RXTE) and the Burst and Transient
Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory
(CGRO) were analysed to quantify source variations with time and pulse phase.
A numerical model simulating X-ray emission from pulsars in terms of Compton
scattering of photons within a neutron star accretion column has been developed to
test the consistency of the analysis results and to further investigate the spectral
and pulse profile formation.
Mean Proportional Counter Array (PCA) spectra of both pulsars over the range
2-40 keV are adequately fitted with a Comptonization model, with blackbody source
spectrum To1-1.3 keV, plasma temperature Te 6-10 keV, and optical depth T P..-
2-6. The source spectrum temperature is consistent with an origin at the neutron
star polar cap, with Compton scattering taking place primarily in the hot plasma of
the accretion column. Both the fitted optical depth and plasma temperature vary
significantly with the source flux. The wide range of source luminosity spanned by
archival observations of GX 1+4 offers evidence for two distinct spectral states above
and below Lx1.4 x 10 erg s' (2-20 keV, assuming a source distance of 10 kpc).
GX 1+4 additionally exhibits dramatic hourly variations in neutral column density
nHindicative of density variations in the stellar wind from the giant companion.
Pulse profiles from GX 1+4 vary dramatically over timescales as short as 6 h.
Low-order Fourier decomposition of pulse profiles in the 20-50 keV band from
BATSE monitoring of the source, in addition to the pulse profiles observed by RXTE
over a broader energy range have been used to investigate the relationship between
source flux and profile asymmetry suggested on the basis of previously published
profiles. The asymmetry of the pulse profiles (as measured by the asymmetry parameter
a, Greenhill et al., 1998) exhibits large variation which decreases as the flux
increases. No significant relationship is suggested by the recent data, although the
span in flux is significantly narrower than from historical profiles which cover the
1970s during which the source was consistently very bright.
Pulse-phase spectroscopy of RXTE data from GX 1+4 and RX J0812.4-3114
support the interpretation of the sharp dips in the pulse profile as 'eclipses' of the
emission region by the accretion column. The dip phase corresponds with the closest
approach of the column axis to the line of sight, and the additional optical depth experienced by photons escaping from the column in this direction gives rise to both
the decrease in flux and increase in the fitted T measured at this phase. Analysis
of the arrival time of individual dips in GX 1+4 provides the first measurement
of azimuthal wandering of a neutron star accretion column. The column longitude
varies stochastically with standard deviation 2-6° depending on the source luminosity.
Measurements of the phase width of the dip both from mean pulse profiles and
individual eclipses demonstrates that the dip width is proportional to the flux. The
variation is consistent with that expected if the azimuthal extent of the accretion
column depends only upon the Keplerian velocity at the inner disc radius, which
varies as a consequence of the accretion rate M.
A numerical model using a Monte-Carlo approach has been developed to investigate
the behaviour of the emission model suggested by the analysis results. The
model simulates Comptonization of a source of blackbody photons emitted from the
polar cap of a canonical neutron star in a semi-infinite cylindrical accretion column.
The mean spectra vary with both inclination angle and magnetic colatitude (the
source aspect). For certain ranges of aspect the pulse profiles exhibit dips similar
to those observed in GX 1+4 and RX J0812.4-3114, although not as sharp. Pulse
phase spectroscopy of the model data confirms the increase in fitted T coincident
with the dip. Latitudinal variations in density across the column, suggested as a
possible source of profile asymmetry, result in significant asymmetry only if the
brightness of the two poles is unequal.

Item Type: Thesis (PhD)
Keywords: Pulsars, X-ray spectroscopy, X-ray astronomy
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author – The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2001. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:06
Last Modified: 17 Aug 2016 00:40
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