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

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy
² Catholic University of America, 620 Michigan Ave NE, Washington, DC 20064, USA
³ Center for Space Science and Technology, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Department of Astronomy, Université de Genève, 16 chemin d’Écogia, 1290 Versoix, Switzerland
⁶ George Washington University, Washington, DC 20052, USA
⁷ CRESST and CSST, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁸ Maryland Institute College of Art, 1300 W Mt Royal Ave, Baltimore, MD 21217, USA
⁹ Department of Physics and Astronomy, Howard University, Washington, DC 20059, USA
¹⁰ CRESST/Astroparticle Physics Laboratory, Code 661, NASA Goddard Space Flight Center, Greenbelt Road, MD 20771, USA
¹¹ University of Maryland, Baltimore County, MD 21250, USA

^⋆ Corresponding author: patrizia.romano@inaf.it

Received: 18 March 2025
Accepted: 2 May 2025

Abstract

We report on an observational campaign conducted with Swift/X-ray Telescope (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 were carried out, resulting in a total effective exposure time of 114 ks and covering 66 orbital and 19 superorbital cycles of the source. The XRT data folded on the orbital period of the source confirm and improve 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 nearly circular orbit. The XRT data folded on the superorbital period do not show 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 period of the XRT observations, as confirmed by the Burst Alert Telescope 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.