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 (firstname.lastname@example.org)
Subjects: X-ray, Gamma Ray, Transient
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
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