Spectral and Timing Properties of Atoll Source 4U 1705-44 : LAXPC/AstroSat Results

TL;DR

This study presents the first spectral and timing analysis of the atoll source 4U 1705-44 using AstroSat data, revealing state-dependent spectral features, variability in the hard tail, and correlations between timing and spectral parameters.

Contribution

First detailed spectral and timing analysis of 4U 1705-44 with AstroSat, highlighting state evolution, broad iron line origin, and corona-related correlations.

Findings

Presence of broad Lorentzian feature (Peaked Noise) at 1-13 Hz

Broad iron line with relativistic and Comptonization broadening

Correlation between PN frequency and corona strength

Abstract

In this paper, we present the first results of spectral and timing properties of the atoll source 4U 1705-44 using $\sim$ 100 ks data obtained with Large Area X-ray Proportional Counter (LAXPC) onboard {\it AstroSat}. The source was in the high-soft state during our observations and traced out a {\it banana track} in the Hardness Intensity Diagram (HID). We study {\bf the} evolution of the Power Density Spectra (PDS) and the energy spectra along the HID. PDS show presence of a broad Lorentzian feature (Peaked Noise or PN) centered at $1-13$ Hz and a very low frequency noise (VLFN). The energy spectra can be described by sum of a thermal Comptonized component, a power-law and a broad iron line. The hard tail seen in the energy spectra is variable and contribute $4-30$\% of the total flux. The iron line seen in this source is broad (FWHM $\sim$ 2 keV) and strong (EW $\sim$ $369-512$ eV). Only relativistic smearing in the accretion disc can not explain the origin of this feature and requires other mechanism such as broadening by Comptonization process in the external part of the `Comptonized Corona'. A subtle and systematic evolution of the spectral parameters (optical depth, electron temperature etc.) is seen as the source moves along the HID. We study the correlation between frequency of the PN and the spectral parameters. PN frequency seems to be correlated with the strength of the corona. We discuss the implication of the results in the paper.