2011ATel.3174....1R

Following the MAXI Nova alert 5609524433 reporting a possible detection of IGR J16418-4532 on 2011-02-17 15:18:12 UT, Swift has performed, starting on 2011-February-18 at 01:59:43 UT, a 2ks target of opportunity observation of the field that contains the candidate supergiant fast X-ray transient IGR J16418-4532 (Tomsick et al. 2004,

Atel #224; Sguera et al. 2006, ApJ, 646, 452). Utilizing the 2ks 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, 2009, 397, 1177),

we find IGR J16418-4532 at the following location: RA,Dec (J2000) = 250.46143,-45.53997, which is equivalent to:

RA(J2000) = 16h 41m 50.74s,

Dec(J2000) = -45d 32m 23.9s,

with an estimated error of 2.3 arcsec radius (90% confidence), which is within 2.9 arcsec from the best position of this source obtained with XMM (Walter et al. 2006 A&A, 453, 133) at RA(J2000) = 16h 41m 51.0s, Dec(J2000) = -45d 32m 25s (4 arcsec error).

Within the XRT error circle there are two IR sources: 2MASS J16415078-4532253, the favored IR candidate in Chaty et al. 2008

(A&A, 484, 783) at 1.5 arcsec, and a second one at 1.94 arcsec.

During the Swift observations IGR J16418-4532 is bright, with a flux of

of 1E-10 erg/s/cm2 (0.3-10 keV, uncorrected for absorption). The light curve shows the decaying portion of a flare that reached 3 counts/s, with a dynamic range of about 70. The PC mode spectrum (starting from T+10.7h) can be fit by an absorbed

power-law model with a photon index of 2.0±0.5 and an absorbing column density of NH=(8±2)E22 cm-2 (in excess of the Galactic value,

1.59E22 cm-2; Kalberla et al. 2005). The average 0.3-10 keV unabsorbed flux is 3.7E-10 erg/cm2/s.

The XMM 2004 data (Walter et al. 2006) showed the source peaking at 2 counts/s (and a mean count rate of 0.2count/s). Assuming the above spectral model and this XMM normalization, the observed 0.3-10 keV flux would be roughly 2E-11 erg/cm2/s, which is a few times fainter than the present XRT observation. We can therefore conclude that the source is quite bright, although we may have simply caught one of the flares that characterize these sources.

We thank the Swift Team for making these observations possible, in particular the duty scientists and the science planners.