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Swift observations of the SFXT IGR J11215-5952 at P/4

ATel #1444; L. Sidoli (INAF-IASF Milano), P. Romano (INAF-IASF Palermo), S. Vercellone (INAF-IASF Milano)
on 27 Mar 2008; 12:12 UT
Credential Certification: Lara Sidoli (sidoli@mi.iasf.cnr.it)

Subjects: X-ray, Binary, Transient

We performed two observations with Swift/XRT of the transient IGR J11215-5952, the unique member of the class of the Supergiant Fast X-ray Transients (SFXTs) which undergoes periodic outbursts.

At first, the outbursts were found to be equally spaced by P=329 days (Sidoli et al. 2006, A&A,450,L9; Smith et al. 2006,ATel #766), but during a monitoring campaign with Swift/XRT at the supposed apastron passage, a new unexpected outburst was observed (Romano et al. 2007, ATel #1151; Sidoli et al. 2007, A&A,476,1307), after 165 days from the previous outburst, at half the period reported by Sidoli et al (2006,A&A,450,L9). This is probably the true orbital period of the system, although a period of 329 days is not completely ruled out (see Sidoli et al., 2007, A&A, 476, 1307, for a discussion on the two possible geometries).

The current observation occurred at a quarter of the period reported by Sidoli et al (2006,A&A,450,L9), or half the period reported in Sidoli et al (2007,A&A,476,1307).

The first observation lasted 1.8 ks, starting on 2008-03-25 at 00:16:42 UT, the second lasted 1.8 ks, starting on 2008-03-27 at 00:26:47 UT.

The source was not detected in either observation. The 3sigma upper limits on the unabsorbed flux (obtained assuming the spectral parameters based on the 2007 February outburst reported in Romano et al. 2007,A&AL,469,L5; photon index=1.0, column density=1.0E+22 cm-2) are 1.7E-12 and 2.3E-12 erg/cm2/s (1-10 keV). The 3sigma upper limit on the unabsorbed flux of the cumulative observation is 1.1E-12 erg/cm2/s (1-10 keV), which translates into a 1-10 keV luminosity of 5E+33 erg/s at 6.2 kpc. The 3sigma upper limit on the unabsorbed flux can be compared with the 2.3E-12, 9.2E-12, 9.5e-12, erg/cm2/s obtained during the 2007 February outburst at T-48 hours, T-24 hours T+24 hours, respectively.

We would like to thank the Swift Team for making these observations possible, in particular N. Gehrels, D. Burrows, the duty scientists as well as the science planners.