TITLE:   GCN CIRCULAR
NUMBER:  7653
SUBJECT: GRB 080430: Swift-XRT refined analysis
DATE:    08/05/01 10:25:39 GMT
FROM:    Cristiano Guidorzi at INAF-OAB  <cristiano.guidorzi@brera.inaf.it>

C. Guidorzi, J. Mao (INAF-OAB), P. Evans and K.L. Page (Leicester)
report on behalf of the Swift-XRT team:

The Swift-XRT began observing GRB 080430 (trigger=310613, Guidorzi et
al., GCN Circ. 7647) in Window Timing mode, 55 s after the BAT
trigger. Using 2 ks of overlapping XRT and UVOT data, we can improve
the UVOT-enhanced XRT position given by Evans in al. (GCN Circ. 7652)
to RA, Dec = 165.31104, +51.68558 which is equivalent to:

RA (J2000): 11h 01m 14.65s
Dec (J2000): +51d 41' 08.1"

with an uncertainty of 1.4 arcsec (radius, 90% confidence).

The light curve of the first five orbits, from 55 s to 24.7 ks,
totalling 10.3 ks of exposure, can be modelled with a double broken
power law with the following best-fitting parameters:
alpha_1=2.34+/-0.16, t_break1=309 (-33,+58) s, alpha_2=0.38+/-0.06,
t_break2=10.3 (-2.2,+3.8) ks, alpha_3=0.65+/-0.17 (chisq/dof=77/81).

The WT mode spectrum spanning from 55 to 138 s can be fit by an absorbed
power-law model, with a photon index of 2.42 (-0.14, +0.27) and column
density of 4.6(-2.6, +3.2)e20 cm^-2, which is in excess of the average
Galactic column density in this direction of 9.6e19 cm^-2.
The PC mode spectrum of orbits 2 to 5, spanning from 5.6 to 24.7 ks,
is fit with a higher column density, (2.0+/-0.4)e21 cm^-2 and
a power-law index of 2.3+/-0.2.
The corresponding observed (unabsorbed) 0.3-10 keV flux is 5.0e-12
(8.5e-12) erg cm^-2 s^-1.

If the burst continues to decay at the same rate, we predict an
XRT count rate of 3.8e-2 count/s at T+24 hours, which corresponds
to an observed 0.3-10 keV flux of approximately 1.7e-12 erg cm^-2 s^-1.

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