Swift/XRT monitoring of the orbital and superorbital modulations in 4U 1909+07

TL;DR

This study used Swift/XRT observations to analyze the orbital and superorbital X-ray modulation in the wind-fed supergiant X-ray binary 4U 1909+07, confirming orbital variability but not superorbital changes during the observation period.

Contribution

It provides detailed observational evidence of orbital variability and assesses the superorbital modulation, offering insights into the accretion process in wind-fed supergiant X-ray binaries.

Findings

Orbital modulation confirmed and characterized in intensity and absorption.

Superorbital modulation was not significantly detected during observations.

Results support the corotating interaction region model for superorbital variability.

Abstract

We report on an observational campaign performed with Swift/XRT on the wind-fed supergiant X-ray binary 4U 1909+07 to investigate the nature of the orbital and superorbital modulation of its X-ray emission. A total of 137 XRT observations have been carried out, summing up to a total effective exposure time of 114 ks and covering a total of 66 orbital and 19 superorbital cycles of the source. The XRT data folded on the orbital period of the source confirmed and improved the previously reported variability in intensity and absorption column density, which can be ascribed to the neutron star accreting from the wind of its B supergiant companion across a fairly circular orbit. The XRT data folded on the superorbital period did not provide evidence of significant variations in either the absorption column density and/or the power-law photon index. This may be due to a significant weakening of the superorbital modulation during the times when the XRT observations were carried out, as confirmed by the BAT dynamic power spectrum. We discuss the implications of these findings within the corotating interaction region model proposed to interpret the superorbital variability in wind-fed supergiant X-ray binaries.