GCN Circular 22586

V. D'Elia (ASDC), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A.
Tohuvavohu (PSU), S.L. Gibson (U. Leicester), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A.
Melandri (INAF-OAB) and A.Y. Lien report on behalf of the Swift-XRT
team:

We have analysed 5.4 ks of XRT data for GRB 180402A (Lien et al. GCN
Circ. 22580
Not found
 
 
), from 83 s to 18.3 ks after the  BAT trigger. The data
comprise 24 s in Windowed Timing (WT) mode 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 = 251.9336,
-14.9686 which is equivalent to:

RA (J2000): 16 47 44.06
Dec(J2000): -14 58 06.9

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.94 (+/-0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.46 (+0.28, -0.26). The
best-fitting absorption column is  4.6 (+2.2, -1.7) x 10^21 cm^-2,
consistent with the Galactic value of 2.9 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 5.6 x 10^-11 (7.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.6 (+2.2, -1.7) x 10^21 cm^-2
Galactic foreground: 2.9 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.46 (+0.28, -0.26)

If the light curve continues to decay with a power-law decay index of
1.94, the count rate at T+24 hours will be 1.3 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.3 x
10^-16 (9.1 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00821103.

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