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BeppoSAX measurements of IGR J17091-3624 = 1SAX J1709-36

ATel #160; J. J.M. in 't Zand, J. Heise, and P. Lowes (Space Research Organization Netherlands, Utrecht), P. Ubertini (CNR Istituto di Astrofisica Spaziale e Fisica Cosmica, Rome)
on 20 May 2003; 14:37 UT
Credential Certification: Jean in' t Zand (jeanz@sron.nl)

Subjects: X-ray, Gamma Ray, Black Hole, Neutron Star, Transient

Referred to by ATel #: 444

On Sept. 20.7 through 21.7 and Sept. 29.8 through Oct. 1.1, 2001 (UT), the Wide Field Cameras on BeppoSAX detected a source which coincides with the recently reported new X-ray source IGR J17091-3624 (Kuulkers et al., ATel #149). From combined data taken during August-October 2001, the source was localized to R.A. = 17h 09m 06s, Decl. = -36d 24' 39", with an error radius of 1.5' (99% confidence level). The position is 0.03' from the INTEGRAL centroid, 0.5' from the COMIS-TTM centroid (Revnivtsev et al., ATel #150), and 1.3' from the reported radio source (Rupen et al., ATel 152). The flux was 14 mCrab and 20 mCrab (2-10 keV) during the two observations. The spectrum is consistent with a 3.0+/-0.4 photon-index power law or kT=4.3+/-1.4 keV for thermal bremsstrahlung; NH was constrained to upper limits of 5E22 and 2E22 cm-2, respectively (90% confidence). A search through the WFC archive revealed a weak detection five years earlier, in the combined data of the 1996 August-Oct WFC campaign of the Galactic center. In 650 ksec of data, the source was detected at an average flux of about 5 mCrab. All X-ray data combined imply that IGR J17091-3624 = 1SAX J1709-36 is a moderately bright variable X-ray source which flared in Oct. 1994 (Mir-COMIS/TTM), Sept. 1996 (BeppoSAX-WFC), Sep. 2001 (BeppoSAX-WFC), and April 2003 (INTEGRAL-IBIS) and most likely is an X-ray binary. We note that the source was followed up by the BeppoSAX Narrow-Field Instruments on Sept. 25, 2001, after the first real-time detection. Therefore, it is listed in the SAX observation catalog under 1SAX J1709-36. Unfortunately, this observation was terminated before useful data could be taken due to problems with the satellite attitude control.