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Swift catches a new outburst from the Supergiant Fast X-ray Transient IGRJ16479-4514

ATel #1435; P. Romano (INAF-IASF Palermo), L. Sidoli (INAF-IASF Milano), V. Mangano (INAF-IASF Palermo), J. A. Kennea, D. N. Burrows (PSU), H. Krimm (GSFC), N. Gehrels (GSFC), S. Vercellone (INAF-IASF Milano), G. Cusumano (INAF-IASF Palermo), A. Paizis (INAF-IASF Milano)
on 20 Mar 2008; 15:03 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Pat Romano (romano@ifc.inaf.it)

Subjects: X-ray, Gamma Ray, Transient

Referred to by ATel #: 1920, 1929

A new outburst from the transient IGR J16479-4514 was caught by Swift satellite: the Swift Burst Alert Telescope (BAT) triggered twice on March 19, first at 22:44:45 UT (Swift trigger 306829, Barthelmy et al. 2008, GCN 7466) and then again on a stronger peak at 22:59:57 UT (trigger 306930). Swift immediately slewed to the target, so that the NFI onboard Swift started observing it about 115s after each trigger.

For the first peak, the time-averaged spectrum in the BAT from T-70.7 to T-0.9 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.76+-0.61. The fluence in the 15-150 keV band is (3.0+-0.9)E-7 erg/cm^2. This corresponds to 350 mCrab in the 15-50 keV band. For the second peak, the time-averaged spectrum from T+63.5 to T+159.5 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.67+-0.57, and Epeak of 9.2+-9.4 keV (chi squared 44.28 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is (1.3+-0.1)E-6 erg/cm^2. A fit to a simple power law gives a photon index of 2.88+-0.14 (chi squared 54.71 for 57 d.o.f.). For the second, brighter, peak the 15-50 keV flux reached 0.228+-0.013 counts/sec/cm2 (equal in intensity to the Crab). All the quoted errors are at the 90% confidence level.

Swift/XRT observations started on March 19, at 22:46:47 UT. The initial WT data (120 to 1003s) lightcurve shows an increasing count rate, from 15 to 60 count/s (0.3-10 keV), with several flares. The following PC data light curve (4240-4600s) lightcurve shows that the source has declined to about 0.5 count s^-1, with a large dynamic range of at least two orders of magnitude.

The brightest X-ray emission (WT) reaches an observed 2-10 keV flux of 5E-9 erg/cm2/s and displays an hard spectrum, well fit with an absorbed powerlaw (NH= (5.6+-0.3)E+22 cm-2; photon index of 0.99+-0.07, redchi2/d.o.f.=0.940/563), or alternatively with a cutoff powerlaw with a NH=(4.8+-0.6)E+22 cm-2, a photon index of 0.22+-0.48 and a high energy cutoff at 7.6-3.0+13.0 keV (redchi2/d.o.f.=0.929/562).

The X-ray spectrum of the fainter emission (PC, ~5E-11 erg/cm2/s, observed) is significantly softer: fitted with a powerlaw model, the resulting photon index is 2.67-0.90+1.00 and an absorbing column density of (6.0-2.2+2.8)E+22 cm-2.

Both of these peaks are more intense than any of the eight other short-lived outbursts above 150 mCrab and 3 sigma significance during the BAT mission, including the two previously reported (Markwardt et al, ATel #816, Kennea et al, ATel #599). IGR J16479-4514 is normally just detected in the BAT transient monitor at a level of 4 mCrab. The IGR J16479-4514 light curve from the BAT monitor can be found at http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ16479-4514/

IGRJ16479-4514 is a High Mass X-ray Binary belonging to the new class of Supergiant Fast X-ray Transients, where wind accretion (the actual mechanism is still debated) from the supergiant donor causes "short" outbursts. A previous bright outburst was observed with Swift satellite as well, on 2005-08-30 (Kennea et al., 2005, ATel #599).

Scaled Map Transient Analysis for IGR J16479-4514