Observations of the Galactic center region black hole candidate1E 1740.7-2942 have been carried out using the Caltech Gamma-Ray Imaging Payload(GRIP), the Röntgensatellit (ROSAT) and the Very Large Array (VLA). Thesemultiwavelength observations have helped to establish the association between abright emitter of hard X-rays and soft γ-rays, the compact core of a double radio jetsource, and the X-ray source, 1E 1740.7-2942. They have also provided informationon the X-ray and hard X-ray spectrum.
The Galactic center region was observed by GRIP during balloon flights fromAlice Springs, NT, Australia on 1988 April 12 and 1989 April 3. These observationsrevealed that 1E 1740.7-2942 was the strongest source of hard X-rays within ~10°of the Galactic center. The source spectrum from each flight is well fit by a singlepower law in the energy range 35-200 keV. The best-fit photon indices and 100 keVnormalizations are: γ = (2.05 ± 0.15) and K_(100) = (8.5 ± 0.5) x 10^(-5) cm^(-2) s^(-1) keV^(-1)and γ = (2.2 ± 0.3) and K_(100) = (7.0 ± 0.7) x 10^(-5) cm^(-2) s^(-1) keV^(-1) for the 1988 and1989 observations respectively. No flux above 200 keV was detected during eitherobservation. These values are consistent with a constant spectrum and indicate that1E 1740.7-2942 was in its normal hard X-ray emission state. A search on one hourtime scales showed no evidence for variability.
The ROSAT HRI observed 1E 1740.7-2942 during the period 1991 March20-24. An improved source location has been derived from this observation. Thebest fit coordinates (J2000) are: Right Ascension = 17^h43^m54^s.9, Declination = -29°44'45".3, with a 90% confidence error circle of radius 8".5. The PSPC observationwas split between periods from 1992 September 28- October 4 and 1993 March23-28. A thermal bremsstrahlung model fit to the data yields a column density ofN_H = 1.12^(+1.51)_(0.18) x cm^(-2) , consistent with earlier X- ray measurements.
We observed the region of the Einstein IPC error circle for 1E 1740.7-2942with the VLA at 1.5 and 4.9 GHz on 1989 March 2. The 4.9 GHz observation revealedtwo sources. Source 'A', which is the core of a double aligned radio jet source (Mirabelet al. 1992), lies within our ROSAT error circle, further strengthening its identificationwith 1E 1740.7-2942.
