GCN Circular 22707

A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU),
D.N. Burrows (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U.
Leicester), S.L. Gibson (U. Leicester), A. Melandri (INAF-OAB), P.
D'Avanzo (INAF-OAB) and J.D. Gropp report on behalf of the Swift-XRT
team:

We have analysed 4.9 ks of XRT data for GRB 180510A (Gropp et al. GCN
Circ. 22695
Not found
 
 
), from 77 s to 40.5 ks after the  BAT trigger. The data
comprise 228 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The refined XRT position is RA, Dec = 276.3314, -31.9081 which is
equivalent to:

RA (J2000): 18 25 19.54
Dec(J2000): -31 54 29.1

with an uncertainty of 3.6 arcsec (radius, 90% confidence).

The late-time light curve (from T0+15.8 ks) can be modelled with a
power-law decay with a decay index of alpha=1.2 (+/-0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.94 (+0.08, -0.07). The
best-fitting absorption column is  5.2 (+/-0.5) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.01 (+0.30, -0.28)
and a best-fitting absorption column of 3.5 (+1.4, -1.2) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.8 x 10^-11 (5.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.5 (+1.4, -1.2) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^21 cm^-2
Excess significance: 1.9 sigma
Photon index:	     2.01 (+0.30, -0.28)

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 0.016 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.2 x
10^-13 (9.4 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/00831810.

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