Swift/BAT (15–50 keV) dynamic power spectrum (DPS) analysis of 4U 1909+07 over the entire observation period (MJD 53416–60670). Left panel: DPS in the frequency range 0.05–0.25 d⁻¹. The vertical dotted lines indicate the time range of the XRT campaign discussed in this article. Right panel: Orbital period (4.4 d, indicated by the black arrow), the superorbital period of 15.196 d (red arrow, ∼0.06 d⁻¹), and its second harmonic (blue arrow). The 99.9% and 99.99% significance levels for white noise are shown by the dashed blue and green lines, respectively. Lower panel: Relative strengths of the orbital period (black), superorbital period (red), and its second harmonic (blue).