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GCN Circular 30994

Subject
GRB 211024B: Swift-XRT refined Analysis
Date
2021-10-25T15:12:58Z (2 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR),
J. D. Gropp (PSU), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), K.L.
Page (U. Leicester) and J.D. Gropp report on behalf of the Swift-XRT
team:

We have analysed 6.2 ks of XRT data for GRB 211024B (Gropp et al. GCN
Circ. 30980), from 105 s to 24.7 ks after the  BAT trigger. The data
comprise 6 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. Using 6214 s
of PC mode data and 12 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 = 154.71268, +24.56825 which is equivalent
to:

RA (J2000): 10h 18m 51.04s
Dec(J2000): +24d 34' 05.7"

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

The late-time light curve (from T0+5.6 ks) can be modelled with a
power-law decay with a decay index of alpha=2.69 (+0.13, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.08 (+0.11, -0.10). The
best-fitting absorption column is  9.7 (+2.7, -2.5) x 10^20 cm^-2, in
excess of the Galactic value of 2.1 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 3.2 x 10^-11 (3.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.7 (+2.7, -2.5) x 10^20 cm^-2
Galactic foreground: 2.1 x 10^20 cm^-2
Excess significance: 5.0 sigma
Photon index:	     2.08 (+0.11, -0.10)

If the light curve continues to decay with a power-law decay index of
2.69, the count rate at T+24 hours will be 5.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.6 x
10^-13 (2.0 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/01081073.

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