A Further Drop into Quiescence by the Eclipsing Neutron Star 4U 2129+47

TL;DR

This study analyzes recent XMM-Newton observations of the neutron star binary 4U 2129+47, confirming a long-term period change and flux decrease, supporting the hypothesis of a hierarchical triple system and suggesting cooling of the neutron star.

Contribution

It provides new timing and spectral data that support the triple system hypothesis and explores the neutron star's cooling post-accretion.

Findings

Confirmed the 2005 time delay in eclipse epochs.

Observed a significant decrease in X-ray flux compared to 2000.

Supported the existence of a third body with a period of at least 175 days.

Abstract

The low mass X-ray binary 4U 2129+47 was discovered during a previous X-ray outburst phase and was classified as an accretion disk corona source. A 1% delay between two mid-eclipse epochs measured ~22 days apart was reported from two XMM-Newton observations taken in 2005, providing support to the previous suggestion that 4U 2129+47 might be in a hierarchical triple system. In this work we present timing and spectral analysis of three recent XMM-Newton observations of 4U 2129+47, carried out between November 2007 and January 2008. We found that absent the two 2005 XMM-Newton observations, all other observations are consistent with a linear ephemeris with a constant period of 18857.63s; however, we confirm the time delay reported for the two 2005 XMM-Newton observations. Compared to a Chandra observation taken in 2000, these new observations also confirm the disappearance of the sinusoidal modulation of the lightcurve as reported from two 2005 XMM-Newton observations. We further show that, compared to the Chandra observation, all of the XMM-Newton observations have 40% lower 0.5--2 keV absorbed fluxes, and the most recent XMM-Newton observations have a combined 2--6 keV flux that is nearly 80% lower. Taken as a whole, the timing results support the hypothesis that the system is in a hierarchical triple system (with a third body period of at least 175 days). The spectral results raise the question of whether the drop in soft X-ray flux is solely attributable to the loss of the hard X-ray tail (which might be related to the loss of sinusoidal orbital modulation), or is indicative of further cooling of the quiescent neutron star after cessation of residual, low-level accretion.