2002ApJ...566..387M

We present the results of the GRANAT/SIGMA hard X-/soft γ-ray long-term monitoring of the Galactic center (GC) region concerning the source GRO J1744-28, discovered on 1995 December 2 by CGRO/BATSE. SIGMA observed the region containing the source in 14 opportunities between 1990 and 1997. In two of these observing sessions, corresponding to 1996 March and 1997 March, GRO J1744-28 was detected with a confidence level greater than 5 σ in the 35-75 keV energy band without detection in the 75-150 keV energy band. For the other sessions, upper limits of the flux are indicated. The particular imaging capabilities of the SIGMA telescope allow us to identify, specifically, the source position in the very crowded GC region, giving us a mean flux of (73.1±5.5)x10^–11 and (44.7±6.4)x10^–11 ergs.cm^–2.s^–1 in the 35-75 keV energy band, for the 1996 March and 1997 March observing sessions, respectively. Combining the 1997 March SIGMA and BATSE observations, we found evidence pointing to the type II nature of the source bursts for this period. For the same observing campaigns, spectra were obtained in the 35 to 150 keV energy band. The best fit corresponds to an optically thin thermal Bremsstrahlung with F_50keV=3.6(±0.6)x10^–4 photons cm^–2.s^–1.keV^–1 and kT_Bremss=28±7 keV for the first campaign, and F_50keV=2.3(±0.7)x10^–4 photons cm^–2 .s^–1.keV^–1 and kT_Bremss=18⁺¹²_–7 keV for the second. This kind of soft spectrum is typical of binary sources containing a neutron star as the compact object, in contrast to the harder spectra typical of systems containing a black hole candidate.