University of Leicester
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X-ray spectroscopy of quasars observed with ASCA

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posted on 2014-12-15, 10:40 authored by James. Reeves
This thesis presents results from a sample of quasars observed with the X-ray astronomy satellite ASCA. The ASCA sample consists of 68 quasars, 35 of which are radio-loud and 33 radio-quiet, covering a range of quasar redshift and luminosity. Differences were found between the radio-loud and radio-quiet quasars; the radio-loud quasars tend to have harder (or flatter) X-ray spectra (~1.5-1.6) and are more X-ray luminous, compared to the softer (~1.9), lower luminosity, radio-quiet quasars. In addition it is found that the line or reflection component observed in some of the radio-quiet quasars is diminished (or absent) in the radio-loud quasars. These observations are consistent with a Doppler-boosted component from a relativistic jet being present in the radio-loud quasars, but not the radio-quiet quasars.;Evidence for reprocessing of the primary X-ray emission is also seen in several quasars. Iron line and reflection features are found predominantly in the radio-quiet quasars. The rest energy and ionisation state of the line emission was found to increase towards the higher luminosity quasars; this was interpreted in terms of the increased ionisation of the inner accretion disk with luminosity and accretion rate. Indeed the highest luminosity radio-quiet quasars show no line or disk reflection features, the result being a featureless power-law continuum, which can be explained if the quasars accrete near the Eddington limit. X-ray absorption is also found in several of the quasars; a trend is found for the absorber column density to increase with quasar redshift. When corrected for calibration effects and local origins, this absorbing material must either reside locally to the quasars or originate from line-of-sight matter.


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University of Leicester

Qualification level

  • Doctoral

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  • PhD



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