GCN Circular 22632

S.L. Gibson (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N.
Burrows (PSU), A. Tohuvavohu (PSU), J.P. Osborne (U. Leicester) and Z.
Liu report on behalf of the Swift-XRT team:

We have analysed 9.3 ks of XRT data for GRB 180411A (Liu et al. GCN
Circ. 22630
Not found
 
 
), from 72 s to 29.8 ks after the  BAT trigger. The data
comprise 200 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The refined XRT position is RA, Dec =
356.8540, +66.7785 which is equivalent to:

RA (J2000): 23 47 24.97
Dec(J2000): +66 46 42.5

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

The late-time light curve (from T0+4.4 ks) can be modelled with a
power-law decay with a decay index of alpha=1.09 (+/-0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.68 (+/-0.10). The
best-fitting absorption column is  1.54 (+0.16, -0.15) x 10^22 cm^-2,
in excess of the Galactic value of 8.3 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.01 (+/-0.16) and a
best-fitting absorption column of 1.91 (+0.25, -0.23) x 10^22 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 5.9 x 10^-11 (1.3 x 10^-10) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.91 (+0.25, -0.23) x 10^22 cm^-2
Galactic foreground: 8.3 x 10^21 cm^-2
Excess significance: 7.7 sigma
Photon index:	     2.01 (+/-0.16)

If the light curve continues to decay with a power-law decay index of
1.09, 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 1.9 x
10^-12 (4.1 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/00824451.

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