  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.
