TITLE:   GCN CIRCULAR
NUMBER:  5832
SUBJECT: GRB 061121: Swift-XRT Team Refined Analysis
DATE:    06/11/21 21:38:02 GMT
FROM:    Kim Page at U.of Leicester  <kpa@star.le.ac.uk>

K.L. Page, R.L.C. Starling, J.P. Osborne (U. Leicester), E. Troja (U.  
Leicester/INAF-IASFPa) and D. Morris (PSU)  report on behalf of the
Swift-XRT team:

We have analysed the first 2 orbits (up to 8ks after the trigger) of
Swift-XRT data obtained for GRB 061121 (Page et al., GCN Circ. 5823).
Using ~3.2ks of Photon Counting (PC)  data, we derive a refined position
of

RA(J2000) =   09 48 54.54
Dec(J2000) = -13 11 42.4

with an estimated error radius of 3.5 arcsec (90% confidence, using the
updated teldef file as described by Burrows et al. in GCN Circ. 5750).  
This is 4.0 and 0.7 arcsec from the original XRT and UVOT positions (GCN
Circ. 5823) respectively.

Because the BAT triggered on a pre-cursor, XRT was on target for the main
burst. The detected emission peaked at ~74s after the trigger, at a count
rate of ~2400 count s^-1 (corrected for pile-up). After 200s, the decay
follows a slope of alpha_1 = 0.36 +/- 0.07, until about 3ks after the
trigger, when the decay steepens to alpha_2 = 1.4 +/- 0.4.

The Windowed Timing mode data between 200 and 600s after the trigger
(i.e., when the data are clearly no longer piled-up) can be fitted with a
photon index of 2.25 +/- 0.12, with a total absorbing column of 2.3e21
cm^-2; comparison with the PC data from orbit 2 shows no further spectral
evolution. This absorption is significantly in excess of the Galactic
value of 5.1e20 cm^-2. Taking the redshift to be 1.314 from the Keck
measurement (GCN Circ. 5826; Bloom, Perley & Chen), the excess NH is
equivalent to a value of 9.6e21 cm^-2 in the rest-frame of the burst. The
observed (unabsorbed) flux for this spectrum is 2.51e-10 (3.81e-10) erg
cm^-2 s^-1.

If the decay continues with a slope of alpha = 1.4, the estimated count
rate at 24 hours is 0.031 count s^-1. This corresponds to an observed
(unabsorbed) flux of 1.19e-12 (1.80e-12) erg cm^-2 s^-1.

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