Chandra imaging spectroscopy of 1E 1740.7-2942

Abstract

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.

Confirming the thermal Comptonization model for black hole X-ray emission in the low-hard state

“…Observations with ASCA (Sakano et al 1999) found an average value of 9.7 ± 0.4 in eight observations. A Chandra/HETGS study found 11.8±0.6 (Cui et al 2001), whereas a Chandra/ACIS-I found 10.5 ± 0.6 (Gallo & Fender 2002). The average in our three observations is 13.6 ± 0.1.…”

“…The lower energy threshold is due to the low count rate and signal-to-noise ratio (S/N) below ∼2 keV for both cameras (PN and MOS1) according to the pileup analysis. We also note that ∼2 keV was the lower limit in energy used by ASCA (Sakano et al 1999) and Chandra on two occasions (Cui et al 2001;Gallo & Fender 2002). With our spectra also starting around 2 keV we left the hydrogen column density as a free parameter in our fits.…”

“…While the ASCA (Sakano et al 1999) and Chandra (Gallo & Fender 2002) studies on 1E 1740.7−2942 have made use of an absorbed powerlaw to fit the 2-20 keV spectra, the broadband (2-200 keV) Suzaku study (Reynolds & Miller 2010) used a combination of models. On the other hand, the INTEGRAL 1 http://xmm.esa.int/sas/ 2 http://www.isdc.unige.ch/integral/ hard X-ray spectrum of 1E 1740.7−2942 has been fitted with a Comptonization model (Bouchet et al 2009).…”

“…The intense optical extinction toward the Galactic Center still prevents the identification of the counterpart of 1E 1740.7−2942 at optical and infrared wavelengths, despite all efforts (Del Santo et al 2005;Smith et al 2002;Martí et al 2000). ASCA (Sakano et al 1999) as well as Chandra (Gallo & Fender 2002) that were helpful in determining the column density of hydrogen (N H ). Recent INTEGRAL observations have shown that the source can be clearly detected up to ∼600 keV (Bouchet et al 2009).…”

1E 1740.7–2942: Temporal and spectral evolution from INTEGRAL and RXTE observations

“…The high hydrogen column density toward the source, recently estimated by Chandra as N H 1.05 × 10 23 cm −2 (Gallo & Fender 2002), does not permit any identification of its optical counterpart.…”