Constraining the Coronal Heights and Readjustment Velocities Based on the Detection of a Few Hundred Seconds Delays in the Z Source GX 17+2

TL;DR

This study uses RXTE and NuSTAR data to measure delays in X-ray emissions from GX 17+2, constraining the coronal height and readjustment velocities, revealing insights into accretion disk-corona dynamics near neutron stars.

Contribution

It provides the first estimates of coronal height and readjustment velocities based on observed X-ray delays in a neutron star Z source.

Findings

Correlated and anti-correlated X-ray delays of tens to hundreds of seconds were observed.

Coronal height was estimated to be between 17 and 100 km.

Coronal readjustment velocities are constrained to 6-12% of disk radial velocity.

Abstract

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 NuSTAR satellite data, we performed cross correlation function (CCF) studies in GX 17+2 in order to constrain the size of corona responsible for hard X-rays. From the RXTE data, we found that during horizontal and normal branches, the CCFs show anti-correlated 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$\pm$0.11. Few observations shows correlated lags and on one occasion coincident with radio emission. We also report an anti-correlated hard X-ray delay of 113$\pm$51 s using the NuSTAR data of GX 17+2. Based on RXTE data, it was found that soft and hard X-ray fluxes are varying indicating the changes in the disk-corona structure during delays. We bridle the size of corona using relativistic precession, transition layer and boundary layer models. Assuming the delays to be readjustment time scale of disk-corona structure, height of the corona was estimated to be $\sim$ 17--100 km. Assuming that inner region of the truncated disk is occupied by the corona, we constrain the coronal readjustment velocities (v$_{corona}$=$\beta$ v$_{disk}$, where v$_{disk}$ is the radial velocity component of the disk) of the order of $\beta$=0.06-0.12. This study indicates that the observed delays are primarily dependent on the varying coronal readjustment velocities.