2019ApJS..244....5S

Neutron star Z-type sources provide a unique platform in order to understand the structure of accretion disk-corona geometry emitting close to the Eddington luminosity. Using RXTE and Nuclear Spectroscopic Telescope Array Mission (NuSTAR) satellite data, we performed crosscorrelation function (CCF) studies in GX 17+2 in order to constrain the size of the corona responsible for hard X-rays. From the RXTE data, we found that during horizontal and normal branches, the CCFs show anticorrelated hard (16-30 keV) and soft (2-5 keV) X-ray delays of the order of a few tens to hundred seconds with a mean correlation coefficient of 0.42 ± 0.11. Few observations shows correlated lags and, on one occasion, coincident with radio emission. We also report an anticorrelated hard X-ray delay of 113 ± 51 s using the NuSTAR data of GX 17+2. Based on RXTE data, we find that soft and hard X-ray fluxes are varying, indicating the changes in the disk-corona structure during delays. We bridle the size of the corona using relativistic precession, transition layer models, and boundary layer models. Assuming the delays to be a readjustment timescale of the disk-corona structure, the height of the corona was estimated to be ∼17-100 km. Assuming that the inner region of the truncated disk is occupied by the corona, we constrain the coronal readjustment velocities (v_corona = βv_disk, where v_disk is the radial velocity component of the disk) of the order of β = 0.06-0.12. This study indicates that the observed delays are primarily dependent on the varying coronal readjustment velocities.