TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2523 SUBJECT: GRB031220: 2nd Chandra observation and X-ray afterglow candidate DATE: 04/01/29 18:46:51 GMT FROM: Luigi Piro at IAS/CNR Frascati GRB031220: 2nd Chandra observation and X-ray afterglow candidate B. Gendre, M. De Pasquale, L. Piro, E. Costa, M. Feroci (IASF-INAF/Rome), M. Garcia (Harvard, CFA), L. A. Antonelli (OaR/INAF-Rome), G. Garmire (PSU), G.Ricker (MIT), G. Tagliaferri (Oss.Astr.Merate/INAF-Mi), J. in 't Zand (SRON-Utrecht) report: On 17 Jan. 14:47 UT, the Chandra Observatory targeted the field of GRB031220 for a second time for a total exposure time of 20 ksec. We have performed a detailed analysis of both observations (preliminary results of the 1st observation were reported in GCN2502). Of the seven sources reported in GCN2502 (i.e. the brightest contained in the HETE2 error box) we find that only 2 of them show a decrease by more than a factor of 2 (at 2 sigma level). In particular source #1 is not detected (with a 2 sigma upper limit of about 6 counts) and source #7 is marginally detected. In the following table we report for sources #1 and #7 the observed counts (corrected for the point spread function) in the two observations. Note that the first observation was twice as long. # RA DEC Cts (obs1) Cts (obs2) variation factor 1 4:39:44.35 7:20:36.99 31.4+/- 6.5 ND >2.6 7 4:39:46.19 7:22:56.17 31.9+/- 6.5 7+/- 4 2.3 Source #7 showed an optical variation (GCN 2503, GCN 2513) with a decay index (0.21 +/- 0.05, GCN 2513) consistent, within the errors, with the observed X-ray slope. This slope is much flatter than usually observed in afterglows (unless the optical flux is being dominated by a constant contribution by e.g. the host galaxy). Assuming a (rather flat) decay slope of -1, the expected variation between the two observations is a factor of 4.5. The most likely afterglow candidate on the basis of X-ray data is then source #1. We also note that the suspect source #37 reported in the previous GCN near the border of the CCD chip is excluded by this refined analysis. In addition to the bright sources found in the error box reported in the previous GCN, we list the fainter sources detected with this refined analysis in the error box during the 1st observation and not detected in the second one. Note, however, that we cannot derive any significant conclusion on the variability of these sources. # RA DEC Cts(Obs1) 82 4:38:55.23 7:24:57.79 10.3+/- 3.7 66 4:39:27.23 7:23:51.04 13.3+/- 4.1 55 4:39:29.20 7:23:14.88 18.6+/- 5.2 78 4:39:34.49 7:21:24.16 13.4+/- 4.4 58 4:40:12.58 7:19:45.20 22.6+/- 6.2 Finally, we list all the sources outside the error box that underwent a decrease by more than a factor of 2. We add in table their distance from the nearest side of the error box # RA Dec Cts(Obs1) Cts(Obs2) variation D E28 4:39:27.62 7:24:49.04 37.4+/-7.3 9.2+/- 3.2 2.0 22" E10 4:39:07.31 7:21:23.07 76.2+/-9.8 10.7+/-4.8 3.5 1' E21 4:39:12.65 7:26:14.00 43.2+/-7.7 10.6+/-3.5 2.0 1' E5 4:39:57.00 7:18:20.92 110.9+/-11.7 23.4+/-5.7 2.4 1'10" E22 4:39:31.37 7:26: 1.26 33.7+/- 6.6 6.0+/-2.6 2.8 1'40" E4 4:39:39.38 7:16:55.21 122.3+/-12.0 25.5+/-6.5 2.4 3'30" E2 4:39: 6.88 7:17: 1.27 244.6+/-17.9 54.2+/-9.2 2.2 5'5" E13 4:39:20.23 7:16: 3.25 61.6+/- 9.2 9.1+/-4 3.4 5'15" This message may be cited.