{
  "bibcode": "2004GCN..2731....1F",
  "body": "GRB040912(=H3557): Chandra Candidate X-ray Afterglow\n\nP. Ford, N. Butler, G. Ricker, R. Vanderspek, J. Villasenor,\nP. Csatorday (MIT); D.  Lamb (U. Chicago); and J.G. Jernigan (UCB), on \nbehalf of a Chandra GRB ToO Team,\n\nwrite:\n\nOn 21 September 2004, the Chandra Observatory targeted the field of \nGRB040912, an X-ray flash localized by the HETE satellite (=H3557; \nButler et al., GCN 2701; Olive et al., GCN 2723).  This was a 20 ksec \nfollowup observation to the observation beginning on 15 September \nreported by Butler et al. (GCN 2716).  It spanned the interval 10:56 UT \nto to 17:01 UT on September 21, 8.86-9.12 days after the burst.  The \nWXM error circle from Butler et al. (GCN 2701) was completely contained \nwithin the field-of-view of the Chandra ACIS-I array.\n\nAt the location of the 22 X-ray sources reported in our Epoch 1 (E1)\nobservation (GCN 2716), we find:\n\n #    Chandra Name         <Cts_E1>  Cts_E2  Null_Prob\n 1 CXOU J235631.0-005946     55        53     0.421\n 2 CXOU J235700.0-010009     35        40     0.703\n 3 CXOU J235712.2-005942     39        37     0.404\n 4 CXOU J235642.9-005520     36        15     0.003\n 5 CXOU J235645.7-005655     30        20     0.068\n 6 CXOU J235707.6-005713     30        21     0.108\n 7 CXOU J235716.4-010109     33        41     0.791\n 8 CXOU J235639.1-005556     19        30     0.921\n 9 CXOU J235651.2-005513     15        13     0.359\n10 CXOU J235636.7-005431     18        27     0.874\n11 CXOU J235711.1-005637     15         8     0.090\n12 CXOU J235637.3-010160      7         9     0.675\n13 CXOU J235656.4-005839     10         8     0.325\n14 CXOU J235700.1-005713     13        19     0.848\n15 CXOU J235659.8-005903     12         9     0.259\n16 CXOU J235649.1-010344     10         3     0.047\n17 CXOU J235700.4-010018     16         7     0.042\n18 CXOU J235646.4_005621      3         3     0.386\n19 CXOU J235647.0-010045     16         7     0.042\n20 CXOU J235648.8-010319     10         7     0.297\n21 CXOU J235704.0-005859      3         1     0.213\n22 CXOU J235707.2-010420     12         3     0.032\n\nIn this table, \"<Cts_E1>\" refers to the number of counts detected in\nE1, scaled to the exposure in E2.  The number of counts detected in E2 for \neach source is reported in the column labeled \"Cts_E2,\" and the probability \nthat the source remained constant given the E1 and E2 detected count rates \nis presented under \"Null_Prob.\"  \n\nWe note that source #13 has not faded between our two epochs, and this \nstrongly dis-favors the candidate OT of Butler et al. (GCN 2728).\n\nSeveral sources appear to have faded at or beyond the 2-sigma significance\nlevel (#'s 4,17,19,and 22).  However, given the 22 trials conducted to find \nthe candidate X-ray afterglow, only source #4 exhibits a statistically \nsignificant decline in brightness (3-sigma single trial, ~2-sigma in 22 \ntrials).  The decrease in count rate from source #4 between E1 and E2 \nimplies a temporal decay index of alpha=-0.9+/-0.3.  We determine a refined \nposition for this source of:\n\nRA= 23h 56m 42.96s, Dec= -0d 55' 19.8\" (J2000), \n\nwith an uncertainty of 1\".  A faint (R~24) optical source, spatially \ncoincident with this position, was reported by Butler et al. (GCN 2728).  \nHowever, this source did not appear to decrease in flux between t_burst \n+ 13.57 hours and t_burst + 38.65 hours.\n\nWe gratefully acknowledge the timely assistance of the observatory staff \nat the Chandra Science Center in arranging for the acquisition and \npreliminary processing of these data.\n\nThis message may be cited.",
  "circularId": 2731,
  "createdOn": 1095863016000,
  "email": "nrbutler@space.mit.edu",
  "subject": "GRB040912(=H3557): Chandra Candidate X-ray Afterglow",
  "submitter": "Nat Butler at MIT/CSR  <nrbutler@space.mit.edu>",
  "eventId": "GRB 040912"
}