  The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an
accreting neutron star. It shows highly ionized absorption features, with a
blueshift of $\sim$ 400 km s$^{-1}$ and an outflow-mass rate similar to the
accretion rate. Many other X-ray sources exhibit warm absorption features, and
they all show periodic dipping behavior at the same time. Recently, a dipping
periodicity has also been determined for GX 13+1 using long-term X-ray folded
light-curves, leading to a clear identification of one of such periodic dips in
an archival Chandra observation. We give the first spectral characterization of
the periodic dip of GX 13+1 found in this archival Chandra observation
performed in 2010. We used Chandra/HETGS data (1.0-10 keV band) and
contemporaneous RXTE/PCA data (3.5-25 keV) to analyze the broadband X-ray
spectrum. We adopted different spectral models to describe the continuum
emission and used the XSTAR-derived warm absorber component to constrain the
highly ionized absorption features. The 1.0-25 keV continuum emission is
consistent with a model of soft accretion-disk emission and an optically thick,
harder Comptonized component. The dip event, lasting $\sim$ 450 s, is
spectrally resolved with an increase in the column density of the neutral
absorber, while we do not find significant variations in the column density and
ionization parameter of the warm absorber with respect to the out-of-dip
spectrum. We argue that the very low dipping duty-cycle with respect to other
sources of the same class can be ascribed to its long orbital period and the
mostly neutral bulge, that is relatively small compared with the dimensions of
the outer disk radius.
