A. Moretti (INAF-OAB), P. Romano (Univ Bicocca&INAF-OAB),
C. Guidorzi (Univ Bicocca&INAF-OAB), S.D. Vergani (DIAS-DCU)
report on behalf of the Swift team:
We have analysed the first three orbits of XRT data on GRB 070520B
(Moretti et al., GCN Circ. 6425) with total observing times of 284 s
in Windowed Timing mode (from T+104 to T+388 s) and 5.2 ks in Photon
Counting mode (from T+388 s). The Photon Counting mode image provides
a refined XRT position at RA,DEC(J2000) = 121.87964,+57.60896, which is
RA(J2000) = 08h 07m 31.11s
Dec(J2000) =+57d 36' 32.3"
with an uncertainty of 4.0 arcsec (radius, 90% containment). This is
1.0 arcsec from the initial X-ray position, and 76 arcsec from the
refine BAT position (Sakamoto et al., GCN Circ. 6429).
The XRT light curve (LC) exhibits an initial mildly steep decay (slope
2.5) in the interval (100,170) seconds from the trigger. An intense
FRED-like flare is present between 170 and 390 seconds. In the second
and third orbits the LC shows a decay with a slope is 1.0+/-0.3.
The first WT spectrum, extracted before the flare, can be fit with an
absorbed power law with a photon index of 2.5+/-0.2 and a column
density of (2.1+/-0.4)E21 cm^-2 significantly in excess of the
Galactic value (4.22E20 cm^-2; Dickey & Lockman, 1990). The mean WT
spectrum, extracted during the flare, can be fit with an absorbed
power law with a photon index of 2.6+/-0.1 and a column density of
(2.5+/-0.2)E21 cm^-2 again significantly in excess of the Galactic
value. The absorbed (unabsorbed) 0.3-10.0keV mean flare flux is
9.4E-10 (2.1E-9) ergs cm^-2 s^-1.
Errors are quoted at 90% confidence level.
Assuming the source continues to decay at the present rate, we
predict an XRT count rate of 1E-3 counts/s at T+24 hours,
which corresponds to an unabsorbed flux of 5E-14 ergs cm^-2 s^-1.
This circular is an official product of the Swift XRT team.