Coronal Vertical Structure Variations in Normal Branch of GX 17+2 : AstroSa\v{t}s SXT and LAXPC perspective

We performed spectro-temporal analysis in the 0.8--50 keV energy band of the neutron star Z source GX 17+2 using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data. The source was found to vary in the normal branch of the Hardness Intensity Diagram. Cross-correlation studies of LAXPC light curves in soft and hard X-ray band unveiled anticorrelated lags of the order of few hundred seconds. For the first time, Cross-correlation studies were performed using SXT soft and LAXPC hard lightcurves and they exhibited correlated and anti-correlated lags of the order of a hundred seconds. Power density spectrum displayed NB oscillations of 6.7--7.8 Hz (quality factor 1.5--4.0). Spectral modeling resulted in inner disk radius of $\sim$ 12--16 km with $\Gamma$ $\sim$ 2.31--2.44 indicating that disk is close to the ISCO and a similar value of disk radius was noticed based on the reflection model. Different methods were used to constrain the corona size in GX 17+2. Using the detected lags, corona size was found to be 27-46 km ($\beta$ = 0.1, $\beta$ = v$_{corona}$/v$_{disk}$) and 138--231 km ($\beta$ = 0.5). Assuming the X-ray emission to be arising from the Boundary Layer (BL), its size was determined to be 57--71 km. Assuming that BL is ionizing the disk's inner region, it's size was constrained to $\sim$ 19--86 km. Using NBO frequency, the transition shell radius was found to be around 32 km. Observed lags and no movement of the inner disk front strongly indicates that the varying corona structure is causing the X-ray variation in the NB of Z source GX 17+2.

Malu S, K. Sriram, V. K. Agrawal · 2020