GCN Circular 33757

P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), A. D'Ai
(INAF-IASFPA), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), S.
Dichiara (PSU), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), K.L.
Page (U. Leicester) report on behalf of the Swift-XRT team:

We have analysed 6.3 ks of XRT data for GRB 230510A, from 52 s to 23.0
ks after the  BAT trigger. The data comprise 250 s in Windowed Timing
(WT) mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+4.2 ks) can be modelled with a
power-law decay with a decay index of alpha=0.92 (+/-0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.19 (+/-0.05). The
best-fitting absorption column is  2.53 (+0.17, -0.16) x 10^21 cm^-2,
in excess of the Galactic value of 1.9 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.99 (+0.13, -0.08)
and a best-fitting absorption column of 1.938 (+0.454, -0.025) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.5 x 10^-11 (4.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.938 (+0.454, -0.025) x 10^21 cm^-2
Galactic foreground: 1.9 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.99 (+0.13, -0.08)

If the light curve continues to decay with a power-law decay index of
0.92, the count rate at T+24 hours will be 0.035 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-12 (1.7 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/01167973.

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