J.P. Osborne (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC &
INAF-OAR), V. D'Elia (SSDC & INAF-OAR), J. D. Gropp (PSU), J.A. Kennea
(PSU), A. Tohuvavohu (U. Toronto), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester) and M.J. Moss report on behalf of the
We have analysed 4.8 ks of XRT data for GRB 210420B (Moss et al. GCN
Circ. 29844), from 141 s to 87.5 ks after the BAT trigger. The data
comprise 177 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The refined XRT position is RA, Dec =
254.3227, +42.5696 which is equivalent to:
RA (J2000): 16 57 17.45
Dec(J2000): +42 34 10.6
with an uncertainty of 3.5 arcsec (radius, 90% confidence).
The light curve can be modelled with an initial power-law decay with an
index of alpha=2.23 (+0.11, -0.12), followed by a break at T+425 s to
an alpha of 1.13 (+/-0.07).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.72 (+0.08, -0.07). The
best-fitting absorption column is 9 (+11, -9) x 10^20 cm^-2, at a
redshift of 1.4, in addition to the Galactic value of 2.1 x 10^20 cm^-2
(Willingale et al. 2013). The PC mode spectrum has a photon index of
1.61 (+0.08, -0.07) and a best-fitting absorption column consistent
with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV
flux conversion factor deduced from this spectrum is 4.0 x 10^-11 (4.2
x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.1 x 10^20 cm^-2
Intrinsic column: 2.1 (+6.0, -0.0) x 10^20 cm^-2 at z=1.4
Photon index: 1.61 (+0.08, -0.07)
The results of the XRT-team automatic analysis are available at
This circular is an official product of the Swift-XRT team.