Suzaku Observations of the Galactic Center Microquasar 1E 1740.7-2942

TL;DR

This study presents Suzaku X-ray observations of the Galactic center microquasar 1E 1740.7-2942, revealing persistent high-energy emission, a Comptonized corona, and an accretion disc close to the innermost stable circular orbit, challenging previous models.

Contribution

First detailed Suzaku observations of 1E 1740.7-2942 in the hard state showing high-energy emission and disc properties close to the ISCO.

Findings

Detected emission up to 300 keV without spectral break.

Spectra consistent with a Comptonized corona and a ~0.4 keV accretion disc.

Inner disc radius near the ISCO, contradicting truncated disc models.

Abstract

We present two Suzaku observations of the Galactic center microquasar 1E 1740.7-2942 separated by approximately 700 days. The source was observed on both occasions after a transition to the spectrally hard state. Significant emission from 1E 1740.7-2942 is detected out to an energy of 300 keV, with no spectral break or turnover evident in the data. We tentatively measure a lower limit to the cut-off energy of ~ 380 keV. The spectra are found to be consistent with a Comptonized corona on both occasions, where the high energy emission is consistent with a hard power-law (\Gamma ~ 1.8) with a significant contribution from an accretion disc with a temperature of ~ 0.4 keV at soft X-ray energies. The measured value for the inner radius of the accretion disc is found to be inconsistent with the picture whereby the disc is truncated at large radii in the low-hard state and instead favours a radius close to the ISCO (R_in ~ 10 - 20 R_g).