Constraining the geometry of the dipping atoll 4U 1624-49 with X-ray spectroscopy and polarimetry

TL;DR

This study combines X-ray spectroscopy and polarimetry to analyze the geometry of the dipping atoll 4U 1624-49, revealing high polarization levels and insights into the system's inclination and accretion processes.

Contribution

It provides the first polarization measurement of 4U 1624-49 and links polarization degree to system inclination and accretion disk reflection.

Findings

4U 1624-49 shows the highest polarization among Atoll sources observed with IXPE.

Polarization degree increases marginally with energy during non-dip phases.

Dips are associated with increased hydrogen column density and decreased ionization.

Abstract

We present the spectro-polarimetric results obtained from simultaneous X-ray observations with IXPE, NuSTAR and NICER of the dipping neutron star X-ray binary 4U 1624-49. This source is the most polarized Atoll source so far observed with IXPE, with a polarization degree of 2.7% $\pm$ 0.9% in the 2-8 keV band during the non-dip phase and marginal evidence of an increasing trend with energy. The higher polarization degree compared to other Atolls can be explained by the high inclination of the system ($i \approx 60${\deg}). The spectra are well described by the combination of a soft thermal emission, a Comptonized component, plus reflection of soft photons off the accretion disk. During the dips, the hydrogen column density of the highly-ionized absorber increases while the ionization state decreases. The Comptonized radiation seems to be the dominant contribution to the polarized signal, with additional reflected photons which significantly contribute even if their fraction in the total flux is not high.