GCN Circular 9449

Sbarufatti B., Mangano V. (INAF-IASFPA) and T. Sakamoto (NASA/UMBC)
report on behalf of the Swift Team:

We have analyzed the first eleven orbits of Swift-XRT data of
GRB 090529 (trigger 353540; Sakamoto, et al., GCN Circ. 9430
Not found
 
 
),
comprising 148 s taken in Windowed Timing (WT) mode, from T+203 s
to T+352 s, and 8.2 ks in Photon Counting (PC)
mode from T+353 s to T+46.7 ks.

The best position for the X-ray afterglow is the XRT UVOT-enhanced
position:RA, Dec = 212.46850, +24.45919 which is equivalent
to:

RA (J2000): 14h 09m 52.44s
Dec (J2000): +24d 27' 33.1"

with an uncertainty of 1.8 arcsec (radius, 90% confidence), as given
by Osborne, et al., GCN Circ 9433.

The 0.3-10 keV X-ray light curve is best fitted by a broken power-law
wit early  decay index -3.5+/-0.1, late decay index -0.7+/-0.2 and the
break at (1100+180-140) s after the trigger. Faint flaring activity is
detected along the decay. If decaying at the present rate, the
predicted rate for T+ 48h is 2.8e-3 counts/s. However, we remark that
a second break followed by a steepening of the light-curve is
expected. The issue will be clarified when new data from the Swift
observations (resumed today at 14:35 UT) will be available.

The average spectrum of the steep decay part of the afterglow (WT + PC
from T+203s to T+1.1 ks) is best fit by a power-law with indices
2.5+/-0.12 for the WT data and 2.2+/-0.15 for the PC data. The
absorbing column is NH = (2.3-1.5+1.7)E20 cm^-2 slightly in excess
with respect to the Galactic value of 1.61E20 cm^-2 (Kalberla et
al. 2005). The average observed (unabsorbed) fluxes are
4.5(5.1)E-10 ergs cm^-2 s^-1 for the WT data and 1.5(1.6)E-11 for the
PC data.
The average spectrum of the flat decay part of the afterglow,
consisting of 7.5 ks of PC data, observed from T+1.1 ks to T+46.7 ks
(containing 120 photons) was fitted using Cash statistics with an
absorbed power-law model. The absorbing column NH is (9 -7 +9)E20
cm^-2, consistent with the Galactic value, the photon index is
2.3+/-0.5. The observed (unabsorbed) flux is 5.3(7.2)E-13 ergs cm^-2
s^-1. The C-statistic is 66.4 with 101 bins.
The count-rate to flux conversion factor is 4.7E-11.
All quoted errors are at 90% confidence level.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00353540.

This circular is an official product of the Swift-XRT team.