C. Pagani (U. Leicester), V. Mangano (INAF-IASFPA), A. Melandri
(INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), J.A.
Kennea (PSU), M.C. Stroh (PSU), A.P. Beardmore (U. Leicester), P.A.
Evans (U. Leicester) and V. D'Elia report on behalf of the Swift-XRT
We have analysed 9.0 ks of XRT data for GRB 121102A (D'Elia et al. GCN
Circ. 13938), from 53 s to 18.8 ks after the BAT trigger. The data
comprise 203 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The best available XRT position (using the
promptly downlinked event data, the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 270.9009,
-16.9576 which is equivalent to:
RA (J2000): 18 03 36.22
Dec(J2000): -16 57 27.5
with an uncertainty of 2.0 arcsec (radius, 90% confidence).
The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.33 (+0.22, -0.24). At T+111 s the decay
flattens to an alpha of 1.59 (+0.18, -0.11) before breaking again at
T+696 s to a final decay with index alpha=1.04 (+0.08, -0.07).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.95 (+/-0.13). The
best-fitting absorption column is 8.9 (+/-1.0) x 10^21 cm^-2, in
excess of the Galactic value of 5.3 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.74 (+0.23, -0.22)
and a best-fitting absorption column of 7.7 (+1.9, -1.6) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 5.9 x 10^-11 (9.6 x 10^-11) erg cm^-2
A summary of the PC-mode spectrum is thus:
Total column: 7.7 (+1.9, -1.6) x 10^21 cm^-2
Galactic foreground: 5.3 x 10^21 cm^-2
Excess significance: 2.4 sigma
Photon index: 1.74 (+0.23, -0.22)
If the light curve continues to decay with a power-law decay index of
1.04, the count rate at T+24 hours will be 2.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (2.7 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.