GCN Circular 17792

D.N. Burrows (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), C. Pagani (U.
Leicester), A. D'a� (INAF-IASFPA), A. Maselli  (INAF-IASFPA), A.
Melandri (INAF-OAB) and V. D'Elia report on behalf of the Swift-XRT
team:

We have analysed 10 ks of XRT data for GRB 150430A (D'Elia et al. GCN
Circ. 17789
Not found
 
 
), from 84 s to 23.7 ks after the  BAT trigger. The data
comprise 207 s in Windowed Timing (WT) mode (the first 8 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 8645 s of PC mode data and 15 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 = 326.47988, -27.90181
which is equivalent to:

RA (J2000): 21h 45m 55.17s
Dec(J2000): -27d 54' 06.5"

with an uncertainty of 1.4 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.97 (+/-0.17). At T+233 s  the decay
flattens to an alpha of 0.73 (+0.07, -0.22) before breaking again at
T+4365 s to a final decay with index alpha=1.89 (+/-0.15).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.27 (+0.10, -0.09). The
best-fitting absorption column is  4.3 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.5 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.10 (+/-0.14) and a
best-fitting absorption column of 3.8 (+/-0.6) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (6.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.8 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 2.5 x 10^20 cm^-2
Excess significance: 9.7 sigma
Photon index:	     2.10 (+/-0.14)

If the light curve continues to decay with a power-law decay index of
1.89, the count rate at T+24 hours will be 1.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.2 x
10^-14 (8.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00639433.

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