TITLE:   GCN CIRCULAR
NUMBER:  24706
SUBJECT: GRB 190531B: Swift-XRT afterglow detection
DATE:    19/06/01 07:46:50 GMT
FROM:    Aaron Tohuvavohu at PSU/Swift  <auc444@psu.edu>

P. D'Avanzo (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea (PSU), B.
Sbarufatti (INAF-OAB/PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), M.G. Bernadini (INAF-OAB), E. Ambrosi  (INAF-IASFPA) , 
A. Tohuvavohu (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 190531B (Axelsson et al. GCN Circ. 24701),
collecting 5.0 ks of Photon Counting (PC) mode data between T0+18.2 ks
and T0+29.7 ks. 

Eleven uncatalogued X-ray sources are detected, of which one ("Source
8") is above the RASS limit, and is therefore likely the GRB afterglow.
Using 1291 s of PC mode data and 1 UVOT image, we find an enhanced XRT
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 24.29620, -41.95841 which is
equivalent to:

RA (J2000): 01h 37m 11.09s
Dec(J2000): -41d 57' 30.3"

with an uncertainty of 1.6 arcsec (radius, 90% confidence). This
position is 3.4 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.0 (+/-0.3).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.20 (+0.15, -0.14). The
best-fitting absorption column is  9.1 (+1.2, -1.1) x 10^21 cm^-2, in
excess of the Galactic value of 1.8 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 4.2 x 10^-11 (9.0 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.1 (+1.2, -1.1) x 10^21 cm^-2
Galactic foreground: 1.8 x 10^20 cm^-2
Excess significance: 12.8 sigma
Photon index:	     2.20 (+0.15, -0.14)

If the light curve continues to decay with a power-law decay index of
1.0, the count rate at T+24 hours will be 0.14 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.9 x
10^-12 (1.3 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/00020895/Source8.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020895.

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