Evidence for Quasi-periodic X-Ray Dips from an Ultraluminous X-Ray Source: Implications for the Binary Motion

TL;DR

This study presents long-term X-ray observations of NGC 5408 X-1, revealing quasi-periodic dips and modulations that suggest binary motion and high inclination, providing new insights into the system's orbital period and accretion processes.

Contribution

It provides the first evidence of a 243-day binary orbital period in NGC 5408 X-1 based on long-term X-ray monitoring, expanding understanding of its accretion dynamics.

Findings

Discovery of 243-day quasi-periodic dips in X-ray intensity.

Detection of a 112.6-day energy-dependent X-ray modulation.

Spectral evidence indicating increased absorption during the observation period.

Abstract

We report results from long-term (approximately 1240 days) X-ray (0.3-8.0 keV) monitoring of the ultraluminous X-ray source NGC 5408 X-1 with the Swift/X-Ray Telescope. Here we expand on earlier work by Strohmayer (2009) who used only a part of the present data set. Our primary results are: (1) the discovery of sharp, quasi-periodic, energy-independent dips in the X-ray intensity that recur on average every 243 days, (2) the detection of an energy-dependent (variability amplitude decreases with increasing energy), quasi-sinusoidal X-ray modulation with a period of 112.6+-4 days the amplitude of which weakens during the second half of the light curve and (3) spectral evidence for an increase in photoelectric absorption during the last continuous segment of the data. We interpret the X-ray modulations within the context of binary motion in analogy to that seen in high-inclination accreting X-ray binaries. If correct, this implies that NGC 5408 X-1 is in a binary with an orbital period of 243+-23 days, in contrast to the 115.5 day quasi-sinusoidal period previously reported by Strohmayer (2009). We discuss the overall X-ray modulation within the framework of accretion via Roche lobe overflow of the donor star. In addition, if the X-ray modulation is caused by vertically structured obscuring material in the accretion disk, this would imply a high value for the inclination of the orbit. A comparison with estimates from accreting X-ray binaries suggests an inclination > 70 degrees. We note that, in principle, a precessing accretion disk could also produce the observed X-ray modulations.