SIGMA Observations of the Bursting Pulsar GRO J1744−28

We present the results of the GRANAT/SIGMA hard X-/soft gamma-ray long-termmonitoring of the Galactic Center (GC) region concerning the source GROJ1744-28, discovered on 1995 Dec. 2 by CGRO/BATSE. SIGMA observed the regioncontaining the source in 14 opportunities between 1990 and 1997. In two ofthese observing sessions, corresponding to March 1996 and March 1997, GROJ1744-28 was detected with a confidence level greater than 5(sigma) in the35-75 keV energy band without detection in the 75-150 keV energy band. For theother sessions, upper limits of the flux are indicated. The particular imagingcapabilities of the SIGMA telescope allow us to identify, specifically, thesource position in the very crowded GC region, giving us a mean flux of (73.1+/- 5.5)E-11 and (44.7 +/- 6.4)E-11 ergs cm^-2 s^-1 in the 35-75 keV energyband, for the March 1996 and March 1997 observing sessions, respectively.Combining the March 1997 SIGMA and BATSE observations, we found evidencepointing to the type-II nature of the source bursts for this period. For thesame observing campaigns, spectra were obtained in the 35 to 150 keV energyband. The best fit corresponds to an optically thin thermal Bremsstrahlung withF(50 keV)=(3.6 +/- 0.6)E-4 phot cm^-2 s^-1 keV^-1 and kT(Bremss)=28 +/- 7 keV,for the first campaign, and F(50 keV)=(2.3 +/- 0.7)E-4 phot cm^-2 s^-1 keV^-1and kT(Bremss)=18 (+12/-7) keV, for the second. This kind of soft spectrum istypical of binary sources containing a neutron star as the compact object, incontrast to the harder spectra typical of systems containing a black holecandidateComment: Accepted for publication in the Astrophysical Journal, 4 pages, 4 figure