X-Ray Eclipse Time Delays in 4U2129+47

TL;DR

This paper analyzes X-ray eclipse timing in 4U 2129+47 and finds evidence supporting its classification as a triple system through observed eclipse delays and orbital motion analysis.

Contribution

It presents the first timing and spectral analysis of XMM-Newton data that supports the triple system hypothesis for 4U 2129+47.

Findings

Detected a delay between two mid-eclipse epochs.

Eclipse delay explained by orbital motion in a triple system.

Supports the hypothesis that 4U 2129+47 is part of a triple system.

Abstract

4U 2129+47 was discovered in the early 80's and classified as an accretion disk corona source due to its broad and partial X-ray eclipses. The 5.24 hr binary orbital period was inferred from the X-ray and optical light curve modulation, implying a late K or M spectral type companion star. The source entered a low state in 1983, during which the optical modulation disappeared and an F8 IV star was revealed, suggesting that 4U 2129+47 might be part of a triple system. The nature of 4U 2129+47 has since been investigated, but no definitive conclusion has been reached. Here, we present timing and spectral analyses of two XMM-Newton observations of this source, carried out in May and June, 2005. We find evidence for a delay between two mid-eclipse epochs measured ~22 days apart, and we show that this delay can be naturally explained as being due to the orbital motion of the binary 4U 2129+47 around the center of mass of a triple system. This result thus provides further support in favor of the triple nature of 4U 2129+47.