GCN Circular 36443

C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB),
J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), D.N. Burrows (PSU), K.L.
Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 8.5 ks of XRT data for GRB 240511A, from 94 s to 36.0
ks after the  BAT trigger. The data comprise 345 s in Windowed Timing
(WT) mode (the first 6 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The best available XRT position
 (using the promptly downlinked event data, the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec =
336.67568, 8.51317 which is equivalent to:

RA (J2000): 22 26 42.16
Dec(J2000): +08 30 47.4

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

The late-time light curve (from T0+5.2 ks) can be modelled with a
power-law decay with a decay index of alpha=0.76 (+/-0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.07 (+/-0.04). The
best-fitting absorption column is  2.60 (+/-0.16) x 10^21 cm^-2, in
excess of the Galactic value of 8.5 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.08 (+0.16, -0.15)
and a best-fitting absorption column of 1.6 (+0.5, -0.4) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.4 x 10^-11 (4.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+0.5, -0.4) x 10^21 cm^-2
Galactic foreground: 8.5 x 10^20 cm^-2
Excess significance: 2.8 sigma
Photon index:	     2.08 (+0.16, -0.15)

If the light curve continues to decay with a power-law decay index of
0.76, the count rate at T+24 hours will be 0.030 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x
10^-12 (1.4 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/01227767.

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