Broad Band X-ray Spectra of Atoll Source 4U 1636-536: NuSTAR and Swift Results

Abstract

In this work we investigate broad band (1-79 keV) spectral nature of the atoll source 4U 1636-536 using the combined Nu-STAR and SWIFT-XRT data. The spectra are complex and have emission components from the disc, boundary-layer and corona. In addition to that a broad iron line is also observed. A relativistic line model assuming Schwarzchild metric fits this feature. The total flux varies from 1.4 × 10^(−9) to 4.36× 10^(−9)ergs/s/cm^2. At the highest flux level the source was found in the soft state. In this state the Comptonized component has temperature kT_e ∼ 3 keV and optical depth τ ∼ 16. We also detect a non-thermal tail with index ∼ 2.4, contributing ∼ 10 % of the total flux in the soft state. We also find that the inner disc radius, electron temperature and optical depth vary with the total 0.1-100 keV unabsorbed flux. We discuss the implication of the results in this paper.