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Swift observations of a new outburst of the SFXT IGR J08408-4503

ATel #2178; S. D. Barthelmy (GSFC), P. Romano (INAF-IASF Palermo), D. N. Burrows (PSU), P. Esposito (INAF-IASF Milano), P. A. Evans (U. Leicester), N. Gehrels (GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), V. La Parola (INAF-IASF Palermo), C. B. Markwardt (GSFC/UMD), C. Pagani (PSU), D. M. Palmer (LANL), L. Sidoli (INAF-IASF Milano), S. Vercellone (INAF-IASF Palermo)
on 29 Aug 2009; 20:39 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Pat Romano (romano@ifc.inaf.it)

Subjects: X-ray, Gamma Ray, Binary, Transient

Referred to by ATel #: 2520, 3586, 5190

Swift BAT triggered twice on the supergiant fast X-ray transient IGR J08408-4503 (trigger 361128 at 22:51:46.96 UT and trigger 361129 at 23:09:22.96 UT; a 1056-s separation). Both were image triggers; 320-s and 64-s, respectively. Swift did not perform an autonomous slew to either of these triggers because the current target was of a higher merit value. However, a TOO was executed 6.1 hours later.

The mask-weighted light curve for the first trigger shows the emission starting at ~T+100 s and continuing at an approximately constant level of 0.03 +- 0.01 ph/cm2/s out to T+800 s where the event-by-event data end from the first trigger. Using event-by-event data from the second trigger, the lightcurve shows the same level of constant emission from T-200 s to T_zero2nd. Then it roughly doubles and is constant out to T+800 s where the data end again.

There were problems with the spectral fitting for the first trigger interval. For the second trigger, the time-averaged spectrum from T-1.9 to T+244.4 s is best fit by a power law with an exponential cutoff. This fit gives a photon index -0.40 +- 1.78, and Epeak of 24.1 +- 3.7 keV (chi squared 48.5 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is (1.1 +- 0.1)E-6 erg/cm2 and the 1-s peak flux measured from T+101.43 s in the 15-150 keV band is 0.5 +- 0.2 ph/cm2/s. A fit to a simple power law gives a photon index of 2.71 +- 0.21 (chi squared 68.8 for 57 d.o.f.). All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/361128/BA/ and http://gcn.gsfc.nasa.gov/notices_s/361129/BA/ .

The Swift XRT data (PC mode, 22.8 to 34.4 ks since the first trigger, 3.6 ks net exposure) show a light curve with a count rate ranging from ~0.2 counts/s (first two orbits) and 0.02 counts/s (third orbit). The spectrum of the first two orbits (22.8-29.9 ks since the first trigger, for 3.1 ks net exposure), fit with an absorbed power-law model, yields a photon index of 1.4 +- 0.4, and a column density of NH=(0.8 +- 0.5)E+22 cm-2 and has an observed (unabsorbed) 0.3-10 keV flux of 1E-11 (1.4E-11) erg/cm2/s.

Previously, Swift observed flares from this source on 2006 October 4 (Gotz D. et al. 2007,ApJ,655,L101; Romano et al. 2009,MNRAS,392,45), 2008 July 05 (Romano et al. 2009,MNRAS,392,45), 2008 September 21 (Sidoli et al. 2009,MNRAS,397,1528). The historical light curve from the BAT hard X-ray transient monitor (Krimm et al, 2006, ATel #904; 15-50 keV) can be found at http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ08408-4503 .