GCN Circular 13921

P. A. Evans, K.L. Page and J.P. Osborne (U Leicester) report on behalf
of the Swift-XRT team:

We have analysed 12 ks of XRT data for GRB 121027A (Evans  et al. GCN
Circ. 13906
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 ), from 57 s to 76.2 ks after the  BAT trigger. The data
comprise 2.0 ks in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Beardmore
et al. (GCN. Circ 13916).

This burst is unusual in showing multiple strong rebrightenings at high
flux in X-rays, a rebrightening has also reported in the IR (Levan et
al GCN 13920
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 ). This behaviour is reminiscent of the tidal disruption
event Swift J1644.

The light curve initially follows a power-law decay with an index of
alpha~1.8. This is interrupted by a flare beginning at T+220 s and
lasting ~300 s, after which the decay appears much steeper: alpha~6.6.
However at T+800 s the count rate jumps sharply, increasing by more
than 2 orders of magnitude in under 300 s; by T+1.1 ks the count rate
is ~30 ct/sec, corresponding to an observed flux of ~1e-9 erg/cm^2/s.
At this point the GRB left Swift's observing window, however during the
next observation (T+5.3 ks to T+6ks) the count rate remained high,
increasing from ~20 ct/s at the start of the observation to ~60 ct/s at
the end. The MAXI observation (GCN 13908
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 ) occurred between these
observations, and the flux reported by MAXI is consistent with the
Swift flux of ~1-2 e-9 erg cm^2/s. Over the next 4 exposures (T+11 ks
to T+30 ks), the count-rate declines with an index of ~3.8 to a level
of ~0.08 ct/s, it then remains at that level until T+76 ks, where our
current observations end. The hardness ratio follows the count-rate
during the rebrightenings (i.e. the emission is harder when brighter),
which is typical of flaring activity.

A spectrum formed from the first observation in WT mode data can be
fitted with an absorbed power-law with a photon spectral index	of 2.55
(+/-0.06). The best-fitting absorption column is  2.80 (+0.16, -0.15) x
10^21 cm^-2, in excess of the Galactic value of 1.5 x 10^20 cm^-2
(Kalberla et al. 2005). The PC mode spectrum has a photon index of 1.94
(+0.29, -0.27) and a best-fitting absorption column of 1.1 (+0.7, -0.6)
x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.8 x 10^-11 (4.8 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.1 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^20 cm^-2
Excess significance: 2.5 sigma
Photon index:	     1.94 (+0.29, -0.27)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00536831.

Multi-wavelength follow-up is strongly recommended.

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