TITLE: GCN CIRCULAR NUMBER: 13921 SUBJECT: GRB 121027A: Swift-XRT refined Analysis DATE: 12/10/29 09:29:21 GMT FROM: Phil Evans at U of Leicester 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), 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). 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) 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.