Chandra imaging spectroscopy of 1E 1740.7-2942

In 2000 August, we observed the black hole candidate 1E 1740.7–2942, the brightest persistent hard X-ray source within a few degrees of the Galactic Centre, for 10 ks with Chandra (ACIS-I). Attempting to compensate for pile-up effects, we found that the spectra were well-fitted by an absorbed power law, with photon indices Γ= 1.54+0.42−0.37 (readout streak) and Γ= 1.42+0.14−0.14 (annulus), consistent with a black hole low/hard state. We have analysed a public observation performed by Chandra which utilized short frames in order to avoid severe pile-up effects. Subtracting the core point spread function from the whole image, we did not find evidence for any elongated feature perpendicular to the radio jet axis, as reported in a recent analysis of the same data. Moreover, comparing the radial profiles with those of an unscattered X-ray point source, we found an indication of an extended, previously undetected, X-ray scattering halo. The measured halo fractional intensity at 3 keV is between 30 and 40 per cent within 40 arcsec but drops below detectable levels at 5 keV. Finally, by placing a limit on the X-ray flux from the radio-emitting lobe, which has been identified as the hotspot at the end of the northern jet of 1E 1740.7–2942, we are able to constrain the magnetic energy density in that region.