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

ATel #3586; V. Mangano (INAF-IASF Palermo), P. Romano (INAF-IASF Palermo), S. D. Barthelmy (GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), D. N. Burrows(PSU), P. Esposito (INAF-OAC), R. Farinelli (INAF-IASF Palermo, U Ferrara), S. Vercellone (INAF-IASF Palermo), N. Gehrels (GSFC)
on 25 Aug 2011; 15:31 UT
Credential Certification: Vanessa Mangano (vanessa@ifc.inaf.it)

Subjects: X-ray, Request for Observations, Binary, Neutron Star, Transient

Referred to by ATel #: 5190


The Swift Burst Alert Telescope (BAT) triggered on a new outburst from the Supergiant Fast X-ray Transient (SFXT) IGR J08408-4503 on 2011 August 25 at 00:53:04 UT (image trigger=501368). Swift executed an immediate slew.
We performed the analysis of the BAT observation using the data set from T-239 to T+2700 s from recent telemetry downlinks. The mask-weighted light curve shows emission from this source when the position came in to the BAT FoV during a planned slew at T-120 s. The flux showed a mostly linear decay to half the initial observed value out to ~T+900 s when it started to increase, peaking at ~T+1500 s, and returning to zero around T+2500 s. Any activity beyond this time is not known, presently. The time-averaged spectrum from T+0.0 to T+64.0 s is best fit by a simple power-law model. The photon index of the time-averaged spectrum is 2.56 +/- 0.50. The fluence in the 15-150 keV band is 2.9 +/- 0.9 x 10^-7 erg cm^-2. The emission was soft throughout the 2700 s of BAT observations. All the quoted errors are at the 90% confidence level.
The XRT began observing the field at 00:55:27.5 UT, 142.8 s after the BAT trigger. The light curve shows several flares and reached 30 counts/s the brightest of which occurred at ~T+1500 s and reached about 25 counts/s and. This flare corresponds to the BAT light curve peak. Utilizing the first 2.4 ks Photon Counting (PC) mode data, and correcting for the astrometric errors by utilizing Swift/UVOT data according to the method described by Evans et al. (2009, MNRAS, 397, 1177), we find IGR J08408-4503 at the following location: RA,Dec (J2000) = 130.19931,-45.05863, which is equivalent to: RA(J2000) = 08h 40m 47.83s, Dec(J2000) = -45d 03m 31.1s, with an estimated error of 1.6 arcsec radius (90% confidence), which is 0.77 arcsec from HD 74194, the optical counterpart (Masetti et al 2006, Atel #815).
We have analysed 2 orbits of XRT data from T+148 s to T+8.4 ks, and extracted an average WT spectrum with 641 s exposure and an average PC spectrum with 4 ks exposure. A joint fit with an absorbed power-law model gives a photon index of 0.91+/-0.13. The best-fitting absorption column is (7.7+/-0.7) x 10^22 cm^-2, in excess of the Galactic value of 9.3 x 10^21 cm^-2 (Kalberla et al. 2005). The average 0.3-10 keV observed (unabsorbed) flux for the PC spectrum is 2.3 (3.9) x 10^-10 erg s^-1 cm^-2. Average observed (unabsorbed) flux for the WT spectrum (roughly the peak of the emission) is 1.7 (2.7) x 10^-9 erg s^-1 cm^-2.
Previously, Swift observed flares from this source on 2006 October 4 (Gotz 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), 2009 August 28 (ATel #2178), and 2010 March 28 (Atel #2520).
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.
We would like to thank the Swift Team for making these observations possible, in particular the duty scientists as well as the science planners.