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GCN Circular 30994

GRB 211024B: Swift-XRT refined Analysis
2021-10-25T15:12:58Z (2 years ago)
Phil Evans at U of Leicester <>
P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), E. Ambrosi 
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

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

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

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