  Spectral-timing products of kilohertz quasi-periodic oscillations (kHz QPOs)
in low-mass X-ray binary (LMXB) systems, including energy- and
frequency-dependent lags, have been analyzed previously in 4U 1608-52, 4U
1636-53, and 4U 1728-34. Here, we study the spectral-timing properties of the
lower kHz QPO of the neutron star LMXB Aquila X-1 for the first time. We
compute broadband energy lags, as well as energy-dependent lags and the
covariance spectrum using data from the Rossi X-ray Timing Explorer (RXTE). We
find very similar characteristics to other previously studied systems,
including soft lags of $\sim$30 $\mu$s between the 3.0 - 8.0 keV and 8.0 - 20.0
keV energy bands at the average QPO frequency. We also find lags that show a
nearly monotonic trend with energy, with the highest energy photons arriving
first. The covariance spectrum of the lower kHz QPO is well fit by a thermal
Comptonization model, though we find a higher seed photon temperature compared
to the mean spectrum, which was also seen in Peille et al., (2015), and
indicates the possibility of a composite boundary layer emitting region.
Lastly, we see in one set of observations, an Fe K component in the covariance
spectrum at 2.4-$\sigma$ confidence which may raise questions about the role of
reverberation in the production of lags.
