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The low mass X-ray binary 4U 2129+47 was discovered during a previous X-rayoutburst phase and was classified as an accretion disk corona source. A 1%delay between two mid-eclipse epochs measured ~22 days apart was reported fromtwo XMM-Newton observations taken in 2005, providing support to the previoussuggestion that 4U 2129+47 might be in a hierarchical triple system. In thiswork we present timing and spectral analysis of three recent XMM-Newtonobservations of 4U 2129+47, carried out between November 2007 and January 2008.We found that absent the two 2005 XMM-Newton observations, all otherobservations are consistent with a linear ephemeris with a constant period of18857.63s; however, we confirm the time delay reported for the two 2005XMM-Newton observations. Compared to a Chandra observation taken in 2000, thesenew observations also confirm the disappearance of the sinusoidal modulation ofthe lightcurve as reported from two 2005 XMM-Newton observations. We furthershow that, compared to the Chandra observation, all of the XMM-Newtonobservations have 40% lower 0.5--2 keV absorbed fluxes, and the most recentXMM-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 isin a hierarchical triple system (with a third body period of at least 175days). The spectral results raise the question of whether the drop in softX-ray flux is solely attributable to the loss of the hard X-ray tail (whichmight be related to the loss of sinusoidal orbital modulation), or isindicative of further cooling of the quiescent neutron star after cessation ofresidual, low-level accretion.

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