TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2731 SUBJECT: GRB040912(=H3557): Chandra Candidate X-ray Afterglow DATE: 04/09/22 14:23:36 GMT FROM: Nat Butler at MIT/CSR GRB040912(=H3557): Chandra Candidate X-ray Afterglow P. Ford, N. Butler, G. Ricker, R. Vanderspek, J. Villasenor, P. Csatorday (MIT); D. Lamb (U. Chicago); and J.G. Jernigan (UCB), on behalf of a Chandra GRB ToO Team, write: On 21 September 2004, the Chandra Observatory targeted the field of GRB040912, an X-ray flash localized by the HETE satellite (=H3557; Butler et al., GCN 2701; Olive et al., GCN 2723). This was a 20 ksec followup observation to the observation beginning on 15 September reported by Butler et al. (GCN 2716). It spanned the interval 10:56 UT to to 17:01 UT on September 21, 8.86-9.12 days after the burst. The WXM error circle from Butler et al. (GCN 2701) was completely contained within the field-of-view of the Chandra ACIS-I array. At the location of the 22 X-ray sources reported in our Epoch 1 (E1) observation (GCN 2716), we find: # Chandra Name Cts_E2 Null_Prob 1 CXOU J235631.0-005946 55 53 0.421 2 CXOU J235700.0-010009 35 40 0.703 3 CXOU J235712.2-005942 39 37 0.404 4 CXOU J235642.9-005520 36 15 0.003 5 CXOU J235645.7-005655 30 20 0.068 6 CXOU J235707.6-005713 30 21 0.108 7 CXOU J235716.4-010109 33 41 0.791 8 CXOU J235639.1-005556 19 30 0.921 9 CXOU J235651.2-005513 15 13 0.359 10 CXOU J235636.7-005431 18 27 0.874 11 CXOU J235711.1-005637 15 8 0.090 12 CXOU J235637.3-010160 7 9 0.675 13 CXOU J235656.4-005839 10 8 0.325 14 CXOU J235700.1-005713 13 19 0.848 15 CXOU J235659.8-005903 12 9 0.259 16 CXOU J235649.1-010344 10 3 0.047 17 CXOU J235700.4-010018 16 7 0.042 18 CXOU J235646.4_005621 3 3 0.386 19 CXOU J235647.0-010045 16 7 0.042 20 CXOU J235648.8-010319 10 7 0.297 21 CXOU J235704.0-005859 3 1 0.213 22 CXOU J235707.2-010420 12 3 0.032 In this table, "" refers to the number of counts detected in E1, scaled to the exposure in E2. The number of counts detected in E2 for each source is reported in the column labeled "Cts_E2," and the probability that the source remained constant given the E1 and E2 detected count rates is presented under "Null_Prob." We note that source #13 has not faded between our two epochs, and this strongly dis-favors the candidate OT of Butler et al. (GCN 2728). Several sources appear to have faded at or beyond the 2-sigma significance level (#'s 4,17,19,and 22). However, given the 22 trials conducted to find the candidate X-ray afterglow, only source #4 exhibits a statistically significant decline in brightness (3-sigma single trial, ~2-sigma in 22 trials). The decrease in count rate from source #4 between E1 and E2 implies a temporal decay index of alpha=-0.9+/-0.3. We determine a refined position for this source of: RA= 23h 56m 42.96s, Dec= -0d 55' 19.8" (J2000), with an uncertainty of 1". A faint (R~24) optical source, spatially coincident with this position, was reported by Butler et al. (GCN 2728). However, this source did not appear to decrease in flux between t_burst + 13.57 hours and t_burst + 38.65 hours. 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.