[vsnet-grb-info 21375] GRB 181201A: Swift-XRT afterglow detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  23474
SUBJECT: GRB 181201A: Swift-XRT afterglow detection
DATE:    18/12/01 14:17:02 GMT
FROM:    Kim Page at U.of Leicester  <klp5 at leicester.ac.uk>

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia
(ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), S. J. LaPorte
(PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU) and P.A. Evans
(U. Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the INTEGRAL-detected
burst GRB 181201A (Mereghetti et al. GCN Circ. 23469), collecting 5.0
ks of Photon Counting (PC) mode data between T0+14.3 ks and T0+26.4 ks.


Five uncatalogued X-ray sources are detected, of which one ("Source 5")
is above the RASS limit, and is therefore likely the GRB afterglow.
Using 4968 s of PC mode data and 3 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 = 319.29666, -12.63082 which is
equivalent to:

RA (J2000): 21h 17m 11.20s
Dec(J2000): -12d 37' 51.0"

with an uncertainty of 2.0 arcsec (radius, 90% confidence). This
position is 28 arcsec from the INTEGRAL position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.44 (+/-0.18).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.74 (+0.08, -0.07). The
best-fitting absorption column is  6.1 (+2.0, -0.9) x 10^20 cm^-2,
consistent with the Galactic value of 5.2 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.8 x 10^-11 (4.2 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.1 (+2.0, -0.9) x 10^20 cm^-2
Galactic foreground: 5.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.74 (+0.08, -0.07)

If the light curve continues to decay with a power-law decay index of
1.44, the count rate at T+24 hours will be 0.30 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-11 (1.3 x 10^-11) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020848.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020848.

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