GCN Circular 6172

K.L. Page (U. Leicester) and S.B. Pandey (UCL-MSSL) report on behalf of 
the Swift-XRT team:

We have analysed the first three orbits of Swift-XRT data obtained for GRB
070306 (Pandey et al., GCN Circ. 6169
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 ), which includes 156 s of Windowed
Timing (WT) data and 3.3 ks of Photon Counting (PC) mode data. Using the
PC data we derive a refined position of RA, Dec = 148.0974, 10.4820, which
is equivalent to

RA(J2000)  =  09 52 23.39
Dec(J2000) = +10 28 55.3

with an estimated error radius of 3.6 arcsec (90% confidence). This lies
67 arcsec and 3.3 arcsec from the onboard BAT and XRT positions given in
GCN Circ. 6169
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 , respectively.

The XRT light-curve initially decays between ~155 and 165 seconds after
the trigger, followed by a small increase in count rate until 180 seconds;  
after this time, the curve shows a steep decay, with alpha_1 = 6.3 +/- 0.2
until 411 +/- 17 seconds post-trigger, at which point the decay flattens
to alpha_2 = 0.22 +/- 0.08. All error bars are at the 90% confidence
level. Note that Swift triggered on a precursor; if T0 for the light-curve
fits were to be referenced to the main peak of the burst, around 100
seconds later, this initial decay would not be so steep.

The WT spectrum can be modelled by a power-law of Gamma = 2.29 +/- 0.05,
with a total absorbing column of 3.9x10^21 cm^-2, compared to the Galactic
column of 2.94x10^20 cm^-2 in this direction. This value of NH was
calculated from the later PC data (Gamma = 2.14 +/- 0.17), when the
spectral evolution apparent during the initial steep decay has ceased. The
mean observed (unabsorbed) flux over 0.3-10 keV for this WT spectrum is
1.73e-9 (3.48e-9) erg cm^-2 s^-1.

If the decay continues with a slope of alpha = 0.22, the count rate is
predicted to be 0.21 count s^-1 at 24 hours. This corresponds to an
observed flux of 1.0e-11 erg cm^-2 s^-1 (over 0.3-10 keV), with the 
unabsorbed value being 1.9e-11 erg cm^-2 s^-1, using the spectral fit to 
the PC data.

This circular is an official product of the XRT team.