TITLE:   GCN CIRCULAR
NUMBER:  19334
SUBJECT: GRB 160422A: Swift-XRT afterglow detection
DATE:    16/04/23 05:15:14 GMT
FROM:    Alessandro Maselli at INAF/IASF Palermo  <maselli@ifc.inaf.it>

A. Maselli  (INAF-IASFPA), A. Melandri (INAF-OAB), B. Sbarufatti
(INAF-OAB/PSU), D.N. Burrows (PSU), T.G.R. Roegiers (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), C. Pagani (U. Leicester), P.
D'Avanzo (INAF-OAB) 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 160422A (Yassine et al. GCN Circ. 19329),
collecting 4.9 ks of Photon Counting (PC) mode data between T0+32.6 ks
and T0+45.8 ks. 

Three uncatalogued X-ray sources are detected inside or close to the
Fermi/LAT error region, 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=42.0947, -57.8757 which is equivalent to:

RA (J2000): 02:48:22.74
Dec(J2000): -57:52:32.6

with an uncertainty of 3.6 arcsec (radius, 90% confidence). This
position is 2.0 arcmin from the Fermi/LAT position and 2.6 arcsec from
the Swift-UVOT candidate optical afterglow (Marshall, GCN Circ. 19333).


The light curve can be modelled with a power-law decay with a decay
index of alpha=1.3 (+/-1.0).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.3 (+0.4, -0.3). The
best-fitting absorption column is  2.4 (+1.3, -1.0) x 10^21 cm^-2, in
excess of the Galactic value of 2.3 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.1 x 10^-11 (5.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+1.3, -1.0) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^20 cm^-2
Excess significance: 3.5 sigma
Photon index:	     2.3 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of
1.3, the count rate at T+24 hours will be 0.017 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.2 x
10^-13 (8.5 x 10^-13) 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/00020605/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020605.

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