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GCN Circular 2731

Subject
GRB040912(=H3557): Chandra Candidate X-ray Afterglow
Date
2004-09-22T14:23:36Z (20 years ago)
From
Nat Butler at MIT/CSR <nrbutler@space.mit.edu>
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_E1>  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, "<Cts_E1>" 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.

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