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INTEGRAL Galactic bulge monitoring observations of GRO J1750-27 (AX J1749.1-2639), H1743-322 and SLX 1746-331

ATEL # 1385; E. Kuulkers (ESA/ESAC, Spain), V. Beckmann (ISDC, Switzerland), S. Shaw (Southampton, UK/ ISDC, Switzerland), S. Brandt (DNSC, Denmark), J. Chenevez (DNSC, Denmark), T.J.-L. Courvoisier (ISDC, Switzerland), A. Domingo (LAEFF/INTA, Spain), K. Ebisawa (ISAS, Japan), P. Jonker (SRON, The Netherlands), P. Kretschmar (ESA/ESAC, Spain), C. Markwardt (GSFC, USA), T. Oosterbroek (ESA/ESTEC, The Netherlands), A. Paizis (INAF-IASF, Italy), D. Risquez (LAEFF/INTA, Spain), C. Sanchez-Fernandez (ESA/ESAC, Spain), R. Wijnands (UvA, The Netherlands)
on 12 Feb 2008; 16:06 UT
Password Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

Subjects: X-ray, Gamma Ray, Binaries, Black Holes, Neutron Stars, Pulsars, Transients, Variables
Referred to by ATEL #: 1387

A new season of the INTEGRAL Galactic Bulge monitoring program (see ATels #438, #874, #1005; Kuulkers et al. 2007, A&A 466, 595) started, with observations on UT 11 Feb 2008, 16:33-18:07. We here report on results from three currently active transient sources.

The IBIS/ISGRI and JEM-X1 images show a bright source near GX 3+1, coincident with the position of the transient accreting X-ray pulsar GRO J1750-27 (AX J1749.1-2639). This X-ray pulsar became active recently (ATel #1376, GCN Circ #7249, #7274). The JEM-X1 3-10 keV, 10-25 keV, and IBIS/ISGRI 18-40 keV, 40-100 keV fluxes are about 230 mCrab, 350 mCrab, 190 mCrab and 25 mCrab, respectively. In JEM-X1 we see a pulsed signal at 4.454+/-0.002 s, consistent with that found previously (Scott et al. 1997, ApJ 488, 831). In IBIS/ISGRI the source is seen up to about 70 keV. The JEM-X1 (3-35 keV) and IBIS/ISGRI (18-70 keV) spectra, can be simultaneously well fit (reduced chi-squared = 1.2 for 181 degrees of freedom) by a high-energy cut-off power-law model, with a photon index of 0.25+/-0.07 and a cut-off energy of 6.7+/-0.2 keV, subject to a fixed absorption column of 1.4e22 atoms/cm2. (Errors quoted are from single parameters error scans using Delta_Chi^2 = 1.)

In addition, the Galactic black-hole X-ray transient H1743-322 (IGR J17464-3213) has become active again since the beginning of the year (ATel #1348, #1349, #1352). Just prior to our observations the source entered a low state (ATel #1378, #1384). The IBIS/ISGRI 18-40 keV and 40-100 keV fluxes are 13+/-2 mCrab and 25+/-4 mCrab, respectively, indicating the source's spectrum is hard. The source is seen with IBIS/ISGRI all the way up to about 150 keV. A single power-law fit to the IBIS/ISGRI (18-150 keV) spectrum gives a reduced chi-squared = 0.7 for 8 degrees of freedom, with a photon index of 1.6+/-0.2. It is not seen with JEM-X1, corresponding to a 3-sigma upper limit of about 10 mCrab (3-25 keV). The observations are consistent with the source being in a low state.

We also detected the Galactic black-hole X-ray transient SLX 1746-331, which is in outburst since October 2007 (ATel #1235). It is only seen in the JEM-X1 3-10 keV band, with a flux of 57+/-12 mCrab. It was not seen at higher energies (with 3-sigma upper limits of about 15 mCrab [10-25 keV], 10 mCrab [18-40 keV] and 15 mCrab [40-100 keV]), indicating the source's spectrum is soft.

The next INTEGRAL Galactic bulge monitoring observation is foreseen on UT 20 Feb 2008, 13:45-17:27.

Finally, we note that a re-analysis of all consolidated data of the INTEGRAL Galactic Bulge monitoring program has been performed, using the latest software available (OSA 7.0). To increase the sensitivity, and to improve the quality of the output, we have changed the IBIS/ISGRI energy bands used; they are now 18-40 keV and 40-100 keV. Continuing the service to the scientific community, the near-real-time results (JEM-X and IBIS/ISGRI light curves and mosaic images) are made publicly available as soon as possible after the observations have been performed at http://isdc.unige.ch/Science/BULGE/.


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