HEXE MIR-KVANT OBSERVATIONS OF THE BURSTING X-RAY PULSAR GRO J1744-28/

We present the results of a timing aid spectral analysis of the bursti ng X-ray pulsar GRO J1744-28 performed by the HEXE instrument of the M ir-Kvant Observatory. In early 1996, we carried out two series of obse rvations separated by a month and detected no significant prolonged lu minosity variations. The mean luminosity in the energy range 20-100 ke V was similar to 2 x 10(36)d(2) erg s(-1) and similar to 1.5 x 10(36)d (2)erg s(-1) during the first and second series of observations, respe ctively (assuming that a point source is located at distance d kpc). T he period of the pulsar, similar to 0.46706 s, and the rate of its evo lution match the BATSE values, The constructed pulse profile is nearly sinusoidal in shape, and the pulse fraction changes from similar to 2 0% (rms approximate to 13.8 +/- 0.6% at energies up to 30 keV to simil ar to 25% (rms approximate to 17.5 +/- 1.2%) in harder energy channels . During the observations, four strong bursts of duration 8-10 s were detected, The burst luminosity at maximum was a factor of 5-10 higher than the mean value for the source. After each of the bursts, the flux from the source rapidly decreased to similar to 80% of the normal lev el and then began to slowly rise. However, the temporal properties of these processes differ for different bursts. Within the limits of the available statistics, the 20-80-keV burst spectra are similar to the s pectrum of the primary component. The averaged spectrum of the source turned out to be harder than the spectrum predicted by the ''canonical '' model and exhibits a considerable excess of photons in the band 60- 120-keV, which can be offset either by introducing an additional power -law component or (in part) by introducing an absorption gyroline at e nergy E-1 similar to 45 keV. However, because of the high density of s ources in the region of the Galactic center, the additional component cannot be uniquely attributed to GRO J1744-28.