GCN Circular 33737

D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), P.A. Evans (U. Leicester), V. D'Elia (SSDC & INAF-OAR), B.
Sbarufatti (INAF-OAB), A. D'Ai (INAF-IASFPA), J. D. Gropp (PSU), S.
Dichiara (PSU) report on behalf of the Swift-XRT team:

We have analysed 7.9 ks of XRT data for GRB 230506B, from 68 s to 50.9
ks after the  BAT trigger. The data comprise 124 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. Using 335 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 = 326.98168, +44.51111 which is equivalent to:

RA (J2000): 21h 47m 55.60s
Dec(J2000): +44d 30' 40.0"

with an uncertainty of 2.6 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=1.83 (+0.16, -0.17), followed by a break at T+146 s to
an alpha of 4.87 (+0.29, -0.24).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.45 (+/-0.13). The
best-fitting absorption column is  5.8 (+1.3, -1.1) x 10^21 cm^-2, in
excess of the Galactic value of 3.8 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 5.6 x 10^-11 (7.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     5.8 (+1.3, -1.1) x 10^21 cm^-2
Galactic foreground: 3.8 x 10^21 cm^-2
Excess significance: 3.0 sigma
Photon index:	     1.45 (+/-0.13)

If the light curve continues to decay with a power-law decay index of
4.87, the count rate at T+24 hours will be 5.0 x 10^-13 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x
10^-23 (3.6 x 10^-23) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01167266.

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