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GCN Circular 17630

Subject
GRB 150323C: Swift-XRT refined Analysis
Date
2015-03-24T05:15:04Z (9 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
V. D'Elia (ASDC), M. de Pasquale (INAF-IASFPA), J.A. Kennea (PSU), B.
Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), K.L. Page (U.
Leicester), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), A.
Maselli  (INAF-IASFPA) and A. Amaral-Rogers report on behalf of the
Swift-XRT team:

We have analysed 6.1 ks of XRT data for GRB 150323C (Amaral-Rogers et
al. GCN Circ. 17621), from 160 s to 12.6 ks after the  BAT trigger. The
data comprise 312 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 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 = 192.6169, 50.1918 which
is equivalent to:

RA (J2000): 12 50 28.05
Dec(J2000): +50 11 30.4

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

The late-time light curve (from T0+5.1 ks) can be modelled with a
power-law decay with a decay index of alpha=1.0 (+/-0.5).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.34 (+/-0.05). The
best-fitting absorption column is  1.55 (+0.13, -0.12) x 10^21 cm^-2,
in excess of the Galactic value of 1.2 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.52 (+0.25, -0.23)
and a best-fitting absorption column of 6.5 (+4.5, -3.9) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 2.7 x 10^-11 (3.5 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.5 (+4.5, -3.9) x 10^20 cm^-2
Galactic foreground: 1.2 x 10^20 cm^-2
Excess significance: 2.2 sigma
Photon index:	     2.52 (+0.25, -0.23)

If the light curve continues to decay with a power-law decay index of
1.0, the count rate at T+24 hours will be 3.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.8 x
10^-14 (1.3 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/00636005.

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