GCN Circular 17086

D.N. Burrows (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U.
Leicester), P.A. Evans (U. Leicester), B. Sbarufatti (INAF-OAB/PSU), P.
D'Avanzo (INAF-OAB), V. D'Elia (ASDC), J.A. Kennea (PSU), V. Mangano
(PSU) and A.Y. Lien report on behalf of the Swift-XRT team:

We have analysed 7.3 ks of XRT data for GRB 141121A (Lien  et al. GCN
Circ. 17075
Not found
 
 
),  from 355 s to 58.1 ks after the	BAT trigger. The data
comprise 305 s in Windowed Timing (WT) mode (the first 5 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Evans et
al. (GCN Circ. 17080
Not found
 
 
).

The late-time light curve (from T0+3.8 ks) can be modelled with a
series of power-law decays. The initial decay index is alpha=1.50
(+/-0.13). At T+15.8 ks  the decay steepens to an alpha of 4.3 (+1.2,
-0.7) before breaking again at T+25.6 ks to a final decay with index
alpha=0.1 (+0.6, -1.5).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.52 (+/-0.04). The
best-fitting absorption column is  6.2 (+/-1.0) x 10^21 cm^-2, at a
redshift of 1.47, in addition to the Galactic value of 4.3 x 10^20
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 1.93 (+/-0.12) and a best-fitting absorption column of 3.9 (+2.2,
-2.0) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV
flux conversion factor deduced from this spectrum  is 3.4 x 10^-11 (4.1
x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 4.3 x 10^20 cm^-2
Intrinsic column:    3.9 (+2.2, -2.0) x 10^21 cm^-2 at z=1.47
Photon index:	     1.93 (+/-0.12)

If the light curve continues to decay with a power-law decay index of
0.1, the count rate at T+24 hours will be 0.032 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.3 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/00619182.

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