Spectral properties of accreting millisecond pulsars obtained with the Comptonization model COMPPS

Abstract

The neutron stars are unique objects to study properties of matter under extreme conditions. In fact, they are as dense as nuclear nuclei, at the same time they possess the highest magnetic fields in the Universe. Accreting millisecond pulsars are neutron stars in binary systems with optical companions. The companion feeds the compact object with matter, which accelerate the pulsar’s rotational velocity. It finally reaches a period of a few milliseconds.

In this thesis, I performed a systematic spectral analysis of the full sample of accreting millisecond X-ray pulsars (AMXPs) observed in the broad energy range. Particularly, I reduced all available data obtained by the NuSTAR observatory. To widen the energy range, observations done by Swift/XRT or XMM-Newton observatories were added. The spectra are modelled with Comptonization model compPS. It allowed to fit successfully spectra of different AMXPs with only a few exceptions. For the sources where the model did not work correctly, it was possible to have a better result simply by adding a diskbb component to the compPS one without the necessity to complicate the final model with more components. It was possible to show some correlation between spectral parameters and the source luminosity.

Moreover, as the compps model has a gravitational redshift z as a parameter, I compared the values obtained when it was fixed at 0 and when it was fixed at a more physically motivated value of 0.26. The redshift change increased the seed photon temperature kTbb by approximately 30%. Also, the normalisation parameter decreased because the estimated radius source decreased with the redshift.