GCN Circular 3920

T. Mineo(INAF-IASF), V. Mangano(INAF-IASF), V. La Parola(INAF-IASF),
G. Cusumano (INAF-IASF), L. Angelini (GSFC), J. Cummings (GSFC/NRC),
D. N. Burrows (PSU), F. Marshall (GSFC), P. Boyd (GSFC-UMBC)
report on behalf of the Swift XRT team:

We have analyzed the Swift XRT data from the first observation
of GRB 050904 (Cummings et al. 2005, GCN 3910
Not found
 
 
) consisting
of four orbits (about 20 ks).
The refined coordinates of the X-ray afterglow are:

RA(J2000)  =   0h 54m 50.6s
Dec(J2000) = +14d 05' 04.5"

with an estimated uncertainty is of 6 arcseconds radius (90%
containment).
This position is 37 arcsec from the revised BAT position given
in GCN 3918
Not found
 
 
 (Palmer et al. 2005), 4.5 arcseconds from
the XRT position determined on board and 11 arcseconds
from the SOAR position (Haislip et al., GCN 3913
Not found
 
 
).

The 0.2-10 keV light curve, that starts in Windowed Timing (WT) mode
169 seconds after the BAT trigger (T0), shows a fading behaviour.
Moreover two flares are clearly detected in the first orbit:
the first at T0+466 s in WT data and the second at at T0+1240 s
in  Photon Counting (PC) mode data.
The light curve decay index, obtained excluding the flare
time intervals, is -2.08+/-0.03.
The remaining three orbits show irregular rate variations
likely due to other flares.
  
A preliminary spectral fit to WT and PC data of the first orbit,
excluding the flare time intervals, shows an evidence of spectral
evolution from hard to soft:

Time interval(s)  Photon Index  NHx10^20(cm^-2) Flux(erg cm^-2 s^-1)
  T0+170-370         1.33+/-0.04    15+/-1         1.8x10^-9
  T0+580-1760        1.70+/-0.08    5.6+/-1.0      4.6x10^-11

The Galactic absorption  in the GRB direction is 5x10^20 cm^-2.
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