GCN Circular 16305

A. Maselli  (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows
(PSU), J.A. Kennea (PSU), V. Mangano (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), C. Pagani (U. Leicester) and A.
Melandri report on behalf of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 140518A (Melandri  et al.
GCN Circ. 16298
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 ),  from 58 s to 19.5 ks after the  BAT trigger. The
data comprise 92 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 4543 s of PC mode data and 6 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 = 227.25249, +42.41821
which is equivalent to:

RA (J2000): 15h 09m 00.60s
Dec(J2000): +42d 25' 05.6"

with an uncertainty of 1.5 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.42 (+0.22, -0.19). At T+251 s  the decay
flattens to an alpha of 0.25 (+0.15, -0.18) before breaking again at
T+2747 s to a final decay with index alpha=1.51 (+0.32, -0.27).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.66 (+/-0.10). The
best-fitting absorption column is  2.5 (+1.7, -1.6) x 10^22 cm^-2, at a
redshift of 4.707, in addition to the Galactic value of 1.6 x 10^20
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 1.94 (+/-0.12) and a best-fitting absorption column of 2.9 (+1.7,
-1.6) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV
flux conversion factor deduced from this spectrum  is 3.4 x 10^-11 (3.8
x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.6 x 10^20 cm^-2
Intrinsic column:    2.9 (+1.7, -1.6) x 10^22 cm^-2 at z=4.707
Photon index:	     1.94 (+/-0.12)

If the light curve continues to decay with a power-law decay index of
1.51, 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 6.9 x
10^-14 (7.8 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00599287.

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