  Here we report the detection of a 626 s periodic modulation from the X-ray
source 2XMM J174016.0-290337 located in the direction of the Galactic center.
We present temporal and spectral analyses of archival XMM-Newton data and
photometry of archived near-infrared data in order to investigate the nature of
this source. We find that the X-ray light curve shows a strong modulation at
626 +/- 2 s with a confidence level > 99.9% and a pulsed fraction of 54%.
Spectral fitting demonstrates that the spectrum is consistent with an absorbed
power law. No significant spectral variability was observed over the 626 s
period. We have investigated the possibility that the 626 s period is orbital
in nature (either that of an ultra-compact X-ray binary or an AM CVn) or
related to the spin of a compact object (either an accretion powered pulsar or
an intermediate polar). The X-ray properties of the source and the photometry
of the candidate near-infrared counterparts are consistent with an accreting
neutron star X-ray binary on the near-side of the Galactic bulge, where the 626
s period is most likely indicative of the pulsar spin period. However, we
cannot rule out an ultra-compact X-ray binary or an intermediate polar with the
data at hand. In the former case, if the 626 s modulation is the orbital period
of an X-ray binary, it would be the shortest period system known. In the latter
case, the modulation would be the spin period of a magnetic white dwarf.
However, we find no evidence for absorption dips over the 626 s period, a low
temperature black body spectral component, or Fe Kalpha emission lines. These
features are commonly observed in intermediate polars, making 2XMM
J174016.0-290337 a rather unusual member of this class if confirmed. We instead
suggest that 2XMM J174016.0-290337 could be a new addition to the emerging
class of symbiotic X-ray binaries.
