Understanding the Inner Structure of Accretion disk in GX 17+2: AstroSat's Outlook

TL;DR

This study uses AstroSat data to analyze timing and spectral features of GX 17+2, revealing correlated X-ray lags, detection of HBOs, and suggesting corona structural variations as key to understanding the Z track in the HID.

Contribution

First report of HBOs at ~25 Hz and ~33 Hz with their harmonics in GX 17+2 using AstroSat LAXPC data, linking corona structure to Z track features.

Findings

Detection of correlated and anti-correlated X-ray lags.

Observation of HBOs at ~25 Hz and ~33 Hz with harmonics.

Inner disk close to the last stable orbit with no systematic variation.

Abstract

We performed the timing and spectral studies of a Z source GX 17+2 observed from Astrosat LAXPC instrument. Cross-Correlation function (CCF) was performed using soft (3-5 keV) and hard (16-40 keV) X -ray bands across the hardness intensity diagram and found correlated/anti-correlated hard and soft lags which seems to be a common feature in these sources. We performed spectral analysis for few of these observations and found no consistent variation in the spectral parameters during the lags, however 10-40% change was noticed in diskbb and power-law components in few of observations. For the first time, we report the detection of HBOs around $\sim$25 Hz and $\sim$ 33 Hz along with their harmonics using AstroSat LAXPC data. On comparison with spectral results of HB and other branches, we found that inner disk front is close to the last stable orbit and as such no systematic variations are observed. We suggest that the detected lags are readjustment time scales of corona close to the NS and constrained its height to be around few tens to hundreds of km. The detected lags and no significant variation of inner disk front across the HID strongly indicate that structural variation in corona is the most possible cause of Z track in HID.