TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2655 SUBJECT: GRB 040812: Second epoch CXO Observations DATE: 04/08/25 01:53:25 GMT FROM: Chryssa Kouveliotou at MSFC S. Patel (USRA/NSSTC), C. Kouveliotou (NASA/MSFC,NSSTC), S. Mereghetti, D. Gotz (IASF-Milano), N. Lund, J. Chenevez (Danish Space Research Institute), P. Ubertini (IASF-Rome), A. Levan (U. Leicester), E. van den Heuvel (University of Amsterdam), K. Hurley (UCB), report on behalf of a larger Integral team: We observed with Chandra/ACIS-S the error box of the Integral GRB 040812 (Gotz et al. 2004, GCN 2640) for a second time starting on 2004-08-22 9:55 UT, approximately 10 days after the trigger for 10 ks. We have compared the sources reported in our earlier GCN on the first CXO observation (Patel et al., 2004, GCN 2648) with the second CXO image, applying a SNR>2 selection threshold. The results are shown in the Table below. No. RA Dec Counts(1)* SNR Counts(2) SNR 1 16 26 0.47 -44 41 22.6 9 2.8 -- -- 2 16 26 2.25 -44 43 49.4 57 6.9 38 5.4 3 16 26 6.48 -44 43 20.8 10 2.9 10 2.7 4 16 26 8.44 -44 41 25.2 33 5.0 33 5.2 5 16 26 1.71 -44 43 27.9 -- --- 16 3.7 *0.5-8.0 keV band All coordinates are given in J2000. We do not detect source #1 above SNR=2.0; a search in the corresponding error region in the second epoch CXO image reveals 5 net counts at this location. We conclude that there is no significant variation in that source within statistics. Source #2 decayed with an index of approximately 0.6 between the two CXO observations; this source, thus, seems to be most likely the candidate of the GRB 040812 afterglow, as also pointed out by Campana et al. 2004 (GCN 2649). Finally, a new source (#5) appeared within the IBIS error box; inspection of the first image reveals 3 net counts at this location, indicating a variable source. We detect two of the radio sources (#2 and #4) reported by Ishwara Chandra et al. 2004 (GCN # 2653) at the same flux level in both CXO observations. We are grateful to the Chandra Science Data Center for processing the data rapidly and in particular to Harvey Tananbaum and Andrea Prestwich for approving this DDT observation.