Broadband X-ray properties of black holes GRS 1758-258 and 1E 1740.7-2942: AstroSat and NuSTAR results

Abstract

We present the results on broadband X-ray properties of persistent black hole binaries GRS 1758$-$258 and 1E 1740.7$-$2942 using AstroSat, NuSTAR and Swift-XRT observations carried out during 2016$-$2022. We perform spectral modeling of both sources after eliminating the contamination in their \textit{LAXPC} spectra from nearby X-ray sources. Preliminary spectral modelling using Comptonization and line emission ($\sim$ 6.4 keV) models suggest that GRS 1758$-$258 occupies both dim-soft state ($k{T}_{bb}=0.37\pm 0.01$ keV, $\Gamma \sim 5.9$, $L_{bol}=1$ of Eddington luminosity L${}_{Edd}$) and hard state ($\Gamma =1.64-2.22$, $k{T}_e$=4$-$45 keV, $L_{bol}$=1$-$5 % L${}_{Edd}$) that requires a multi-colour disc blackbody model ($k{T}_{in}=0.54\pm 0.01$ keV) occasionally. 1E 1740.7$-$2942 instead is found only in hard state ($\Gamma$=1.67$-$2.32, $k{T}_e$=5$-$16 keV, $L_{bol}$=1$-$2 % L${}_{Edd}$). Reflection properties of both sources are studied by applying relativistic reflection model RELXILL to the broadband spectra. Our results from \textit{AstroSat} and \textit{NuSTAR} consistently unveiled the presence of a Comptonizing region along with an ionized reflection region (ionization parameter $log\xi$=2.7$-$3.8 and 2.7$-$4.7 erg cm s${}^{-1}$ in GRS 1758$-$258 and 1E 1740.7$-$2942 respectively) in both sources. Reflection modeling revealed GRS 1758$-$258 to have a high metal abundance ($A_{fe}=3.9_{-0.3}^{+0.4}$ times solar metal abundance) and inclination angle ($i$) of $61\pm 2^{\circ }$. In case of 1E 1740.7$-$2942, $i$ is constrained to be $55\pm 1^{\circ }$. Finally, we discuss the implication of our findings in the context of accretion dynamics by comparing our results with the previous studies.

Accepted for publication in MNRAS

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