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ATEL # 1151; P. Romano (INAF-OABrera), V. Mangano (INAF-IASF Palermo), S. Mereghetti, A. Paizis, L. Sidoli, S. Vercellone (INAF-IASF Milano)
on 24 Jul 2007; 14:38 UT
Password Certification: Pat Romano (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Transients
We are performing a campaign with Swift/XRT aimed at observing the Supergiant Fast X-ray Transient IGR J11215-5952 (Sidoli et al. 2006,A&A,450,L9; Smith et al.2006,ATEL #766) in quiescence and close to the expected apastron. The campaign started on June 5 and continued with a ~2ks snapshot per week.
The source was not detected in seven observations performed between June 5 and July 17 with 3 sigma upper limits of ~ 3E-13 erg/cm2/s (1-10 keV). The source was detected on July 24, when Swift/XRT observed it from 00:01:42 to 00:17:57 UT. The data show an unabsorbed flux that reached 1.4E-10 erg/cm2/s in the energy range 1-10 keV, comparable with the flux reached at the peak of the February 2007 outburst. The spectrum can be fit with a single power-law with a photon index of 0.87-0.26+0.27, absorbed with a column density of (1.08-0.29+0.35)x10E22 cm^-2, consistent with the spectral parameters determined on February 2007 (ATEL #995).
The current outburst occurred on the 165th day since the previous one in February 2007 (Romano et al. 2007,A&A,469,L5), i.e., at half the period reported by Sidoli et al (2006,A&A,450,L9). Further observations are ongoing in order to discriminate among different possible models for the X-ray emission, related to the eccentricity of the binary system (see Sidoli et al 2007,A&A,submitted; "http://www.iasfbo.inaf.it/simbolx/Pres/Sidoli.ppt").
We would like to thank the Swift Team for making these observations possible, in particular N. Gehrels, the duty scientists as well as the science planners.