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

TL;DR

This study analyzes the broad band X-ray spectra of the atoll source 4U 1636-536 using NuSTAR and Swift data, revealing complex emission components, spectral state variations, and a relativistic iron line, providing insights into accretion processes.

Contribution

First detailed broadband spectral analysis of 4U 1636-536 combining NuSTAR and Swift data, revealing spectral components and state-dependent variations.

Findings

Detection of a broad iron line fitted with a relativistic model.

Variation in inner disc radius, electron temperature, and optical depth with flux.

Presence of a non-thermal tail contributing 10% in the soft state.

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 $\times$ 10$^{-9}$ to 4.36 $\times$ 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 $kT_e \sim 3$ keV and optical depth $\tau \sim $ 16. We also detect a non-thermal tail with index $\sim$ 2.4, contributing $\sim$ 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.