# Study of the X-ray properties of the neutron-star binary 4U 1728$-$34 from the soft to hard state

## TL;DR

This study analyzes XMM-Newton observations of the neutron-star binary 4U 1728-34 across different spectral states, revealing variable reflection features, an unexpectedly high iron abundance, and disc inner radius changes contrary to standard models.

## Contribution

It provides the first detailed reflection modeling of 4U 1728-34 across multiple states, highlighting the neutron star surface's role and challenging standard accretion disc predictions.

## Key findings

- Reflection and Comptonized fluxes vary inconsistently.
- High iron abundance likely due to lack of hard photons.
- Disc inner radius decreases from soft to intermediate state.

## Abstract

We studied five XMM-Newton observations of the neutron-star binary 4U 1728$-$34 covering the hard, intermediate and soft spectral states. By jointly fitting the spectra with several reflection models, we obtained an inclination angle of 25$-$53$\deg$ and an iron abundance up to 10 times the solar. From the fits with reflection models, we found that the fluxes of the reflection and the Comptonised components vary inconsistently; since the latter is assumed to be the illuminating source, this result possibly indicates the contribution of the neutron star surface/boundary layer to the disc reflection. As the source evolved from the relatively soft to the intermediate state, the disc inner radius decreased, opposite to the prediction of the standard accretion disc model. We also explore the possible reasons why the supersolar iron abundance is required by the data and found that this high value is probably caused by the absence of the hard photons in the XMM-Newton data.