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

We present the results on broad-band 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 modelling of both sources after eliminating the contamination in their LAXPC spectra from nearby X-ray sources. Preliminary spectral modelling using Comptonization and line emission (~6.4 keV) models suggest that GRS 1758-258 occupies both dim-soft state (kT_bb = 0.37 ± 0.01 keV, Γ ~ 5.9, $L_{bol}=1{{\ \rm per\ cent}}$ of Eddington luminosity L_Edd) and hard state (Γ = 1.64-2.22, kT_e = 4-45 keV, L_bol = 1-5 per cent L_Edd) that requires a multicolour disc blackbody model (kT_in = 0.54 ± 0.01 keV) occasionally. 1E 1740.7-2942 instead is found only in hard state (Γ = 1.67-2.32, kT_e = 5-16 keV, L_bol = 1-2 per cent L_Edd). Reflection properties of both sources are studied by applying relativistic reflection model RELXILL to the broad-band spectra. Our results from AstroSat and NuSTAR consistently unveiled the presence of a Comptonizing region along with an ionized reflection region (ionization parameter log ξ = 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 modelling revealed GRS 1758-258 to have a high metal abundance ($A_{fe}=3.9^{+0.4}_{-0.3}$ times solar metal abundance) and inclination angle (i) of 61 ± 2°. In case of 1E 1740.7-2942, i is constrained to be 55 ± 1°. Finally, we discuss the implication of our findings in the context of accretion dynamics by comparing our results with the previous studies.