TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2347 SUBJECT: XRF030723(=H2777): Chandra Fading X-ray Afterglow Confirmed DATE: 03/08/07 01:27:59 GMT FROM: George Ricker at MIT XRF030723(=H2777): Chandra Fading X-ray Afterglow Confirmed N. Butler, P. Ford, G. Ricker, R. Vanderspek, J. Villasenor (MIT); D. Lamb (U. Chicago); G. Garmire (PSU); L. Piro (CNR); and J.G. Jernigan (UCB), on behalf of a Chandra GRB ToO Team, write: On 4 August 2003, the Chandra Observatory targeted the field of XRF030723, an X-ray flash localized by the HETE satellite (=H2777; Prigozhin et al, GCN 2313). The duration of the observation was 85 ksec, and it was a followup to the 25 ksec observation reported by Butler et al. in GCN 2328. The observation spanned the interval 4 August 22:22 UT to 5 August 22:27 UT, 12.66 to 13.67 days after the burst. The SXC error circle from Prigozhin et al. was completely contained within the field-of-view of the Chandra ACIS-S3 chip. Within the SXC error circle, the three X-ray sources from GCN 2328 are detected. Only source #1 is observed to vary in flux at a significance level of >1 sigma. We detect 74 counts in the 0.5-8.0 keV band, corresponding to a >7-sigma significance decrease (i.e. factor of ~6) in flux since the first epoch observation. We are thus extremely confident that source #1 is the X-ray counterpart to XRF030723 and is the counterpart to the optical source discovered by Fox et al. (GCN 2323). If we jointly fit the counts from the first and second Chandra epochs, we find that the data are well described by a power-law with absorption fixed at the Galactic value in the source direction (chi^2 = 8.9 for 9 [12-3] degrees of freedom). The best fit photon number index (for both epochs) is gamma = 1.9 +/- 0.3, which is a typical value for the X-ray afterglows of long duration GRBs. Using this model, we find that the first epoch flux is ( 2.2 +/- 0.3 ) x 10-14 erg cm-2 s-1 (0.5-8.0 keV band), while the second epoch flux is ( 3.5 +/- 0.5 ) x 10-15 erg cm-2 s-1 (0.5-8.0 keV band). The decrease in flux between the two epochs can be described by a power-law with a decay index of alpha= -1.0 +/- 0.1. This value of alpha is consistent with the power-law decline reported in the optical by Dullighan et al. (GCN 2336) for <~1.5 day after the GRB; however, the index is considerably flatter than the index at t>1.5 days reported by Dullighan et al. This flatter X-ray decay may possibly be related to the rebrightening of the optical afterglow reported by Fynbo et al. (GCN 2345). We gratefully acknowledge the timely assistance of the observatory staff at the Chandra Science Center in arranging for the acquisition and preliminary processing of these data. This message may be cited.