GCN Circular 13989

A. Melandri (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), G. Stratta
(ASDC), J.A. Kennea (PSU), M.C. Stroh (PSU), D.N. Burrows (PSU), A.P.
Beardmore (U. Leicester), P.A. Evans (U. Leicester), O.M. Littlejohns
(U. Leicester) and E.A. Helder report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 121123A (Helder  et al. GCN
Circ. 13982
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 ), from 129 s to 23.8 ks after the  BAT trigger. The data
comprise 669 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The refined XRT position is RA, Dec =
307.3183, -11.85898 which is equivalent to:

RA (J2000): 20 29 16.39
Dec(J2000): -11 51 32.3

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

The late-time light curve (from T0+4.7 ks) can be modelled with  a
power-law decay with a decay index of alpha=0.50 (+/-0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.448 (+0.023, -0.022). The
best-fitting absorption column is  8.9 (+/-0.6) x 10^20 cm^-2, in
excess of the Galactic value of 4.0 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.97 (+/-0.11) and a
best-fitting absorption column of 5.8 (+2.3, -1.8) x 10^20 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (4.2 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.8 (+2.3, -1.8) x 10^20 cm^-2
Galactic foreground: 4.0 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.97 (+/-0.11)

If the light curve continues to decay with a power-law decay index of
0.50, the count rate at T+24 hours will be 0.051 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x
10^-12 (2.1 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/00539358.

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