Spectral-Timing Analysis of the Lower kHz QPO in the Low-Mass X-ray   Binary Aquila X-1
  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. 
Jon S. Troyer, Edward M. Cackett
astro-ph.HE