TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5009 SUBJECT: GRB060418: Swift XRT Team Further Refined Analysis DATE: 06/04/27 17:05:58 GMT FROM: Abe Falcone at PSU/Swift A. D. Falcone, D. N. Burrows, D. Morris, J. Racusin (PSU), P. T. O'Brien, J. P. Osborne (U Leicester), N. Gehrels (GSFC) report on behalf of the Swift XRT team: We have performed more analysis on recent Swift XRT data from GRB 060418 (Falcone et al., GCN4966). As reported by Falcone et al. (GCN4973), the early afterglow decay has bright flaring. This flaring made it difficult to estimate the afterglow decay index at the time of the previous report, which made use of only the initial flare-dominated data. Here we give a better estimate of this decay, along with some preliminary spectral information. From 78 s until about 115 s after the burst trigger time, the light curve had a steep power law decay index of alpha = 4.6 +/- 0.2. A large flare begins at T+115 s, peaking at T+135 s (where T is the BAT trigger time) with a maximum count rate of ~560 c/s. Flaring continues to at least T+6000 s. No flares are evident after T+40,000 s, at which point the count rate has dropped to less than 0.01 c/s. The temporal decay of the underlying afterglow was fit using two time regions that showed no evidence of flaring (T+350 to T+530 s, and T+40,000 to T+190,000 s). The underlying afterglow has a power law decay index of alpha = 1.4 ± 0.1 from T+350 s until the last detection at T+700,000 s. There is no evidence for a break in the underlying decay curve up to 9 days after the trigger. In particular, we do not detect the break predicted by Ghisellini et al. (GCN 4991), although we have only a single 2 sigma data point after the predicted break time. Due to a normal orbital gap, we have no data during the time of the potential second burst reported by Konus-Wind (Golenetskii et al. GCN4989). A preliminary light curve can be viewed at http://www.astro.psu.edu/users/afalcone/grb060418/falcone_grb060418.gif The observed spectra of the underlying afterglow can be described by a simple absorbed power law, with photon index 2.04 +/- 0.13 and total N_H = (19 +/- 4)e20 cm^-2, with a chi2/dof of 1.22 (75 dof). This fit assumes all absorption is local to the observer. The Galactic N_H at this position is ~9e20 cm^-2 (Dickey and Lockman 1990). We fit the time period during the large flare with a spectral model that was the sum of two power laws; one was the underlying afterglow frozen to the values shown above (with the normalization changed based on the measured temporal decay) and the other was a power law that was free to vary. This fit resulted in a chi2/dof of 1.07 (321 dof), with the following flare power law component parameters: photon index 2.04 +/- 0.05 and N_H = (29 +/-2)e20 cm^-2. For the flare component, we also tried a cutoff power law, a Band function, and a blackbody model. In all cases, the fits were equivalently acceptable, with chi2/dof always falling in the range from 1.05 to 1.07. The cutoff power law was equivalent to the simple power law since the cutoff reached a maximum value in excess of 500 keV. The Band function and the blackbody models both resulted in harder energy spectral indices, with an N_H that was consistent (within 1 sigma error bars) with that of the underlying afterglow. This is in contrast to the simple power law that implies an N_H increase during the flare (relative to the underlying afterglow value). This Circular is an official product of the Swift XRT Team.