GCN Circular 36631

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), D.N.
Burrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU), A.P. Beardmore (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
Swift/BAT-GUANO-detected burst GRB 240606B, collecting 4.0 ks of Photon
Counting (PC) mode data between T0+22.9 ks and T0+35.1 ks. 

Four uncatalogued X-ray sources are detected consistent with being
within 493 arcsec of the BAT-GUANO position, of which one ("Source 1")
is fading with 2.1 sigma significance and is undetected by eRosita to
deeper upper limits. It is likely the afterglow.

Using 3507 s of PC mode data and 3 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 = 40.29507, -60.61201 which is
equivalent to:

RA (J2000): 02h 41m 10.82s
Dec(J2000): -60d 36' 43.2"

with an uncertainty of 2.4 arcsec (radius, 90% confidence). This
position is 53 arcsec from the BAT-GUANO position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.2 (+0.9, -1.0).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.7 (+0.4, -0.3). The
best-fitting absorption column is  6 (+11, -4) x 10^20 cm^-2,
consistent with the Galactic value of 2.4 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 4.1 x 10^-11 (4.5 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6 (+11, -4) x 10^20 cm^-2
Galactic foreground: 2.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.7 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of
1.2, the count rate at T+24 hours will be 0.013 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.2 x
10^-13 (5.7 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021695.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021695.

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