XMM-Newton Observations of Flares and a Possible Pulse Dropout in the Supergiant X-ray binary 4U 1909+07

TL;DR

This study presents XMM-Newton observations of the supergiant X-ray binary 4U 1909+07, revealing a pulse dropout possibly caused by the propeller effect, and documents a continued neutron star spin-up trend over two decades.

Contribution

First detection of a pulse dropout in 4U 1909+07 and analysis of spectral softening during this phase, suggesting a possible propeller effect mechanism.

Findings

Neutron star spin period continues to decrease, indicating ongoing spin-up.

Detected a low flux interval with no pulsations, interpreted as a pulse dropout.

Spectral softening observed during pulse dropout, supporting propeller effect hypothesis.

Abstract

We report on a pair of X-ray Multi-Mirror Mission (XMM-Newton) observations of the Supergiant X-ray binary 4U 1909+07, which were performed on 2021 October 3 and 2021 October 8, respectively. We measure the neutron star rotation period in each observation to be $\sim$602.62 s. This continues a long spin-up trend that has persisted since 2001 where the neutron star spin period was found to be $\sim$604.66 s. In our timing analysis, we observe strong variations in the amplitude of the 1--10 keV pulse profile as a function of time, and for the first time we find a low flux interval extending for a single pulse period in which pulsations are no longer detected. We interpret this low flux interval as a pulse dropout similar to those observed in Vela X-1 and GX 301-2, which were each explained by a low-density cavity in the wind driving the propeller effect. In our time-resolved spectral analysis, we observed the spectral continuum, which can be described as an absorbed power law modified by a high-energy cutoff, to significantly soften during the pulse-dropout phase. No evidence of an increasing absorption column density was found. The observed softening in 4U 1909+07 also supports an interpretation that the observed pulse dropout may be driven by the propeller effect, but the quasi-spherical settling accretion regime cannot be ruled out.