GCN Circular 19558

B. Mingo (U. Leicester), A.P. Beardmore (U. Leicester), A. D'Ai
(INAF-IASFPA), A. Maselli  (INAF-IASFPA), A. Melandri (INAF-OAB), J.A.
Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU) and P.A.
Evans (U. Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 160623A (Vianello et al. GCN Circ. 19553
Not found
 
 
),
collecting 1.9 ks of Photon Counting (PC) mode data between T0+40.3 ks
and T0+45.4 ks. 

Three uncatalogued X-ray sources are detected, of which one ("Source
1") is above the RASS limit, and is therefore likely the GRB afterglow.
 The position of this source is RA, Dec=315.2967, +42.2205 which is
equivalent to:

RA (J2000): 21:01:11.22
Dec(J2000): +42:13:13.7

with an uncertainty of 3.5 arcsec (radius, 90% confidence).  This
position is 2.5 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.3 (+0.5, -1.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.79 (+0.21, -0.20). The
best-fitting absorption column is  1.69 (+0.32, -0.29) x 10^22 cm^-2,
in excess of the Galactic value of 7.2 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 6.3 x 10^-11 (1.1 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.69 (+0.32, -0.29) x 10^22 cm^-2
Galactic foreground: 7.2 x 10^21 cm^-2
Excess significance: 5.5 sigma
Photon index:	     1.79 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
2.3, the count rate at T+24 hours will be 0.12 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.6 x
10^-12 (1.4 x 10^-11) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020666/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020666.

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