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 \sim 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 18 ~ { } 857.63 s ; 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 .