SIGMA and XTE observations of the soft X-ray transient XTEJ1755-324

We present observations of the X-ray transient XTEJ1755-324 performed during summer 1997 with the XTE satellite and with the SIGMA hard X-ray telescope onboard the GRANAT observatory. The source was first detected in soft X-rays with XTE on July 25 1997 with a rather soft X-ray spectrum and its outburst was monitored in soft X-rays up to November 1997. On September 16 it was first detected in hard X-rays by the French soft gamma ray telescope SIGMA during a Galactic Center observation. The flux was stronger on September 16 and 17 reaching a level of about 110 mCrab in the 40-80 keV energy band. On the same days the photon index of the spectrum was determined to be alpha =-2.3 +/- 0.9 (1 sigma error) while the 40-150 keV luminosity was about 8 x 10^{36} erg/s for a distance of 8.5 kpc. SIGMA and XTE results on this source indicate that this source had an ultrasoft-like state during its main outburst and a harder secondary outburst in September. These characteristics make the source similar to X-Nova Muscae 1991, a well known black hole candidate.Comment: 19 pages LaTeX, 6 Postscript figures included, Accepted by Astrophysical Journa

SIGMA Observations of the Bursting Pulsar GRO J1744-28

We present the results of the GRANAT/SIGMA hard X-/soft gamma-ray long-term monitoring of the Galactic Center (GC) region concerning the source GRO J1744-28, discovered on 1995 Dec. 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 March 1996 and March 1997, GRO J1744-28 was detected with a confidence level greater than 5(sigma) 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)E-11 and (44.7 +/- 6.4)E-11 ergs cm^-2 s^-1 in the 35-75 keV energy band, for the March 1996 and March 1997 observing sessions, respectively. Combining the March 1997 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(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^-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 candidateComment: Accepted for publication in the Astrophysical Journal, 4 pages, 4 figure