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

• Published in: The Astrophysical journal. Supplement series Vol. 244; no. 1; pp. 5 - 24
• Main Authors: Sriram, K., Malu, S., Choi, C. S.
• Format: Journal Article
• Language: English
• Published: Saskatoon The American Astronomical Society 01.09.2019; IOP Publishing
• Subjects: Accretion; Accretion disks; accretion, accretion disks; binaries: close; Boundary layer models; Boundary layer transition; Boundary layers; Corona; Correlation coefficient; Correlation coefficients; Fluxes; Hard X-rays; Luminosity; Neutron stars; Radial velocity; Radio emission; Satellite data; Spectroscopic telescopes; stars: individual (GX 17+2); Transition layers; X-ray fluxes; X-rays; X-rays: binaries
• ISSN: 0067-0049; 1538-4365
• DOI: 10.3847/1538-4365/ab30e1

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 (vcorona = βvdisk, where vdisk 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.