GCN Circular 2523

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
Not found
 
 
).

Of the seven sources reported in GCN2502
Not found
 
 
 (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
Not found
 
 
, GCN 2513
Not found
 
 
) with a decay
index (0.21 +/- 0.05, GCN 2513
Not found
 
 
) 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"



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