GCN Circular 35218

D.N. Burrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E.
Ambrosi  (INAF-IASFPA)	and P.A. Evans report on behalf of the
Swift-XRT team:

We have analysed 7.0 ks of XRT data for GRB 231129A, from 115 s to 43.8
ks after the  BAT trigger. The data comprise 174 s in Windowed Timing
(WT) mode with the remainder in Photon Counting (PC) mode. The refined
XRT position is RA, Dec = 317.5437, +41.5206 which is equivalent to:

RA (J2000): 21 10 10.49
Dec(J2000): +41 31 14.2

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=4.86 (+/-0.21). At T+165 s  the decay
steepens to an alpha of 6.44 (+0.28, -0.24) before breaking again at
T+308 s to a final decay with index alpha=0.83 (+0.05, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.83 (+/-0.07). The
best-fitting absorption column is  1.09 (+/-0.08) x 10^22 cm^-2, in
excess of the Galactic value of 5.9 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.18 (+0.31, -0.29)
and a best-fitting absorption column of 1.6 (+/-0.4) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 5.0 x 10^-11 (1.2 x 10^-10) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+/-0.4) x 10^22 cm^-2
Galactic foreground: 5.9 x 10^21 cm^-2
Excess significance: 4.5 sigma
Photon index:	     2.18 (+0.31, -0.29)

If the light curve continues to decay with a power-law decay index of
0.83, the count rate at T+24 hours will be 0.014 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.2 x
10^-13 (1.8 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01199764.

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