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Swift observation of IGR J08408-4503

ATel #1727; V. Mangano (INAF-IASF Palermo), P. Romano (INAF-IASF Palermo), L. Sidoli (INAF-IASF Milano), B. Sbarufatti (INAF-IASF Palermo), J. A. Kennea (PSU), S. Vercellone (INAF-IASF Milano), D. N. Burrows (PSU), G. Cusumano (INAF-IASF Palermo), N. Gehrels (GSFC)
on 21 Sep 2008; 22:40 UT
Credential Certification: Vanessa Mangano (vanessa@ifc.inaf.it)

Subjects: X-ray, Gamma Ray, Binary, Neutron Star

IGR J08408-4503 (Gotz et al. 2006, ATel #813), triggered Swift BAT on 2008-09-21 at 07:55:08 UT.

The Swift/BAT light curve is consistent with a constant level, and the time-averaged spectrum from T-10 to T+730s is best fit by a powerlaw with an exponential cutoff. This fit gives a photon index of 1.3 (-1.1, +0.9), and a cut off energy of 22.7 (-10, +38) keV (chi squared 43.4 for 55 d.o.f.). A fit to a simple power law gives a photon index of 2.87 +/- 0.16 (chi squared 52.0 for 56 d.o.f.). The BAT average flux in the 15-150 keV band is 3.9E-9 erg/cm2/s.

The Swift/XRT light curve shows a bright flare with flat top phase reaching about 40 counts/s and lasting for all the first orbit (about 1 ks). After the first orbital gap, about 5 ks from the trigger, the source had already decayed to the 1 counts/s level.

The XRT/WT spectrum (T+523 to T+1653s since the trigger, first orbit) can be fit with an absorbed powerlaw with a photon index of 0.85 +/- 0.02 and an absorbing column density of NH=(2.9 +/- 0.1) E+21 cm-2 (chi2_red=1.3 for 660 dof). The peak of the flare reached 2.6E-9 erg/cm2/s (unabsorbed, 2-10 keV), which translates into a luminosity of 2E+36 erg/s (assuming the optical counterpart distance of 2.7 kpc, Leyder et al. 2007; A&A, 465, L35). The XRT/PC spectrum (T+4980 to T+10900s from the trigger) can be fit with an absorbed powerlaw with a photon index of 0.8 +/- 0.2 and an absorbing column density of NH=3.7 (-1.5, +2.2) E+21 cm-2 (chi2_red=1.5 for 25 dof). It has an average flux of 7.6E-11 erg/cm2/s (unabsorbed, 2-10 keV) or a luminosity of 6E+34 erg/s.

All the quoted errors are at the 90% confidence level.

Further Swift observations are underway and we encourage multifrequency observations.