Detection of lags in an atoll source 4U 1728-34 using AstroSat

Hard X-rays from atoll sources either can be emitted from the compact corona or the boundary layer around the neutron star surface. For the first time, we report the lags of the order of a few hundred seconds between soft and hard energy bands observed in an atoll source 4U 1728-34 using the AstroSat LAXPC data. We fitted the spectra with various models and found that the source was in the soft banana state during the observations and the inner disc radius was found to be ∼16 km. Based on a thermal Comptonization model, the electron temperature was found to be low (∼3 keV) with an index of Γ=1.82\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\Gamma = 1.82$\end{document}, indicating that the corona was compact in nature. We noted that the boundary layer emission was relatively more than the disc emission component. Based on the reflection model, we noted that the inner disk front was at Rin∼1.45RISCO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\mathrm{R}_{\mathit{in}} \sim 1.45~\mathrm{R}_{\mathit{ISCO}}$\end{document} with an electron temperature ∼2.72 keV. Based on the observed lags, the coronal height was estimated to be around 3-20 km assuming that the disc is close to the ISCO. We noted that the detected lags are similar to the depletion time scale of the boundary layer. These results indicate that the hard X-ray emission can arrive from the corona or boundary layer and the observed lags are due to readjustment time scales of either or both the physical structures.