GCN Circular 34400

A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato
(INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), A. Tohuvavohu
(U. Toronto), D.N. Burrows (PSU), J.P. Osborne (U. Leicester) and P.A.
Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/GBM-detected burst GRB 230812B in a series of observations tiled
on the sky. The total exposure time is 8.3 ks, distributed over 27
tiles; the maximum exposure at a single sky location was 4.6 ks. The
data were collected between T0+25.4 ks and T0+38.1 ks, and are entirely
in Photon Counting (PC) mode. 

Four uncatalogued X-ray sources are detected, of which one ("Source 7")
is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow. Using 4926 s of PC mode data and 4
UVOT images, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 249.13196, +47.85892 which is equivalent to:

RA (J2000): 16h 36m 31.67s
Dec(J2000): +47d 51' 32.1"

with an uncertainty of 2.1 arcsec (radius, 90% confidence). This
position is 106.5 arcmin from the Fermi/GBM position, but only 6.7
arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.8 (+/-0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.82 (+/-0.15). The
best-fitting absorption column is  9.6 (+4.3, -3.9) x 10^20 cm^-2, in
excess of the Galactic value of 2.0 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 3.4 x 10^-11 (4.0 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.6 (+4.3, -3.9) x 10^20 cm^-2
Galactic foreground: 2.0 x 10^20 cm^-2
Excess significance: 3.2 sigma
Photon index:	     1.82 (+/-0.15)

If the light curve continues to decay with a power-law decay index of
1.8, the count rate at T+24 hours will be 0.038 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
10^-12 (1.5 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/00021589.
The results of the full analysis of the tiled XRT observations are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00115.

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