INTEGRAL observes further hard X-ray activity of IGR J17586-2129

ATel #2194; C. Sanchez-Fernandez, E. Kuulkers (ESAC/ESA), A. J. Bird (Southampton), C. Ferrigno (ISDC/IAAT), G. Belanger (ESAC/ESA), A. Bazzano, M. Del Santo, L. Natalucci, P. Ubertini (INAF/IASF Roma), P. R. den Hartog (HEPL/KIPAC/Stanford), D. Gotz, L. Prat (CEA Saclay), J.-C. Leyder (IAG, Liege), A. Lohfink (UMCP/CRESST/NASA-GSFC), T. Oosterbroek (ESTEC/ESA), S. Piraino (INAF/IASF Palermo), K. Pottschmidt (UMBC/CRESST/NASA-GSFC) & K. Watanebe (Florida Gulf Coast)
on 11 Sep 2009; 11:50 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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An outburst of IGR J17586-2129 (CXOU J175834.5-212321/Swift J175834.6-212331), a possible highly obscured high-mass X-ray binary (Tomsick et al. 2009, ApJ 701, 811), has been reported using Swift observations. The outburst started by the end of June 2009 and an initial decline was seen in mid-August 2009 (ATel #2156). Since then it has continued to decline to near the Swift/BAT detection limits at the end of August 2009. However, recent activity is again seen in the Swift/BAT data. This is confirmed by INTEGRAL IBIS/ISGRI observations, as part of the Galactic bulge monitoring program (see ATel #438) and the Key Program on the Galactic Center obtained between Aug 23 and Sep 6, 2009. Unfortunately, during these observations the source was not within the JEM-X field of view.

The source is detected by IBIS/ISGRI with the following fluxes in the 18-40 keV band: 10.7 ± 0.4 (Aug 23-26), 11 ± 2 (Aug 31), 13 ± 2 (Sep 3), 14 ± 2 (Sep 6) mCrab, with an average of 11.2 ± 1.0 mCrab when combining the observations from Aug 31 - Sep 6. This shows the flux to be rather stable over the last 2 weeks.

We have performed joint fits to the average spectra obtained on Aug 23-26 and on Aug 31 - Sep 6. The 18-150 keV spectra can be well fitted (chi^2/red = 1.4 for 14 degrees of freedom) with a power-law with photon index of 3.0 ± 0.3, steeper than that found by Swift/XRT (ATel #2156), i.e., 1.14 (-0.86, +1.00) in the 0.3-10 keV band. If the source spectral behaviour has remained unchanged since the Swift observation this would suggest the presence of a cut-off in the spectrum in the 10-20 keV range. Indeed, a good fit (chi^2/red = 1.0 for 13 degrees of freedom) is also achieved with a cut-off power-law with photon index of 1.5 ± 0.4 and a cut-off energy of 20 (-4, +5) keV, where the photon index is consistent with that found by the Swift/XRT. We derive a flux in the 20-100 keV band of 1.1 (+/-0.1) e-10 erg/cm2/s.

Our IBIS/ISGRI refined position of the source is RA, Dec (J2000) = 269.653, -21.391 degrees with a 90% confidence error of 1.4 arcmin, as determined during the Aug 23-26 observations (total exposure time of about 120 ksec). This is consistent with both the Swift/XRT and BAT positions, as well as the position quoted in Bird et al. (2007, ApJS 170, 175). We therefore suggest the INTEGRAL and Swift source to be the same. Our position is also consistent with the infrared and Chandra X-ray counterparts reported by Tomick et al. (2009).