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GCN Circular 14430

GRB 130420B: Swift-XRT refined Analysis
2013-04-21T05:10:08Z (11 years ago)
Kim Page at U.of Leicester <>
K.L. Page (U. Leicester) and S. R. Oates (UCL-MSSL) report on behalf of
the Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 130420B (Oates  et al. GCN
Circ. 14411),  from 39 s to 39.9 ks after the  BAT trigger. The data
comprise 18 s in Windowed Timing (WT) mode (the first 8 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 2638 s of PC mode data and 4 UVOT images, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 183.12838, +54.39054
which is equivalent to:

RA (J2000): 12h 12m 30.81s
Dec(J2000): +54d 23' 26.0"

with an uncertainty of 1.8 arcsec (radius, 90% confidence).

The light curve can be modelled with  a power-law decay with a decay
index of alpha=1.11 (+0.09, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.0 (+/-0.4). The
best-fitting absorption column is  6.8 (+9.6, -5.4) x 10^20 cm^-2,
consistent with the Galactic value of 1.4 x 10^20 cm^-2 (Kalberla et
al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.5 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:	     6.8 (+9.6, -5.4) x 10^20 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.0 (+/-0.4)

If the light curve continues to decay with a power-law decay index of
1.11, the count rate at T+24 hours will be 2.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.1 x
10^-14 (8.5 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at

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