J. D. Gropp (PSU), A. Tohuvavohu (PSU), J.A. Kennea (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G.
Bernardini (INAF-OAB) and A. D'Ai report on behalf of the Swift-XRT
We have analysed 6.7 ks of XRT data for GRB 190821A (D'Ai et al. GCN
Circ. 25427), from 84 s to 33.9 ks after the BAT trigger. The data
comprise 290 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Goad et al. (GCN Circ. 25436).
The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=7.82 (+0.18, -0.87). At T+140 s the decay
flattens to an alpha of -0.4 (+/-0.5) before breaking again at T+1047 s
to a final decay with index alpha=1.06 (+0.17, -0.16).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.75 (+0.18, -0.11). The
best-fitting absorption column is consistent with the Galactic value
of 3.3 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum is 4.4 x 10^-11 (5.9 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 3.3 (+/-1.0) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.75 (+0.18, -0.11)
If the light curve continues to decay with a power-law decay index of
1.06, the count rate at T+24 hours will be 0.015 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.8 x
10^-13 (9.0 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.