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GCN Circular 2630

Subject
XRF040701: Second-Epoch Chandra Observations
Date
2004-07-22T00:23:59Z (20 years ago)
From
Derek Fox at CIT <derekfox@astro.caltech.edu>
D.B. Fox (Caltech) reports on behalf of a larger collaboration: 

"We have observed the HETE localization region for XRF040701 (Barraud
et al., GCN 2620) with the Chandra X-ray Observatory + ACIS for 20.4
ksec beginning at 01:07 UT on July 18, 2004.  The observation aimpoint
and dither pattern are identical to that of our first-epoch
observation (Fox, GCN 2626); however, a change in the roll angle has
resulted in a slightly different exposure pattern due to the chip gaps
of the ACIS-I array.  A comprehensive "wavdetect" analysis provides
the following second-epoch fluxes for sources from the catalog of our
initial observation:  

                                     Ep #1     Ep #2     Variability
   #   RA             Dec          Flux  Unc  Flux  Unc    %  Sigma
  ==================================================================
   1   20 47 20.696  -40 19 41.52  25.6  3.2  20.5  3.0    80  -1.6
   2   20 48 16.097  -40 11 08.83  11.1  2.1   6.4  1.7    58  -2.2
   3   20 47 46.602  -40 13 57.15   9.0  2.0   9.4  2.3   105   0.2
   4   20 47 23.970  -40 10 56.07   8.8  2.2  12.2  2.3   138   1.5
   5   20 47 19.056  -40 12 34.33   8.5  1.8   7.8  1.9    92  -0.4
   6   20 48 05.696  -40 18 23.01   8.1  1.8   7.5  1.8    93  -0.3
   7   20 47 51.810  -40 19 36.90   8.0  1.7   5.1  1.5    63  -1.7
   8   20 47 06.231  -40 14 45.66   7.8  1.8   7.7  2.0    99  -0.0
   9   20 48 05.337  -40 12 33.74   6.1  1.5   7.2  1.7   118   0.8
  10   20 47 36.158  -40 16 23.76   6.0  1.4   5.5  1.4    92  -0.3
  11   20 47 39.249  -40 17 48.32   5.5  1.4   6.2  1.6   112   0.5
  12   20 48 09.981  -40 07 21.19   5.4  1.6   3.8  1.4    71  -1.0
  13   20 47 40.281  -40 12 25.99   4.5  1.3   4.8  1.8   107   0.2
  14   20 48 23.282  -40 18 01.06   4.3  1.7   4.5  1.6   105   0.1
  15   20 47 33.118  -40 14 47.48   4.1  1.2   6.0  1.6   146   1.6
  16   20 47 12.288  -40 11 12.49   4.1  1.3   4.4  1.5   106   0.2
  17   20 48 10.054  -40 19 32.71   4.1  1.3   3.4  1.3    83  -0.5
  18   20 47 51.377  -40 06 03.64   3.9  1.4   1.6  1.0    41  -1.7
  19   20 47 56.453  -40 07 15.01   3.9  1.3   2.5  1.2    65  -1.0
  20   20 47 55.713  -40 13 47.56   3.8  1.1   4.0  1.3   104   0.1
  21   20 47 47.349  -40 15 16.97   3.7  1.1   4.2  1.3   112   0.4
  22   20 47 34.678  -40 10 42.16   3.7  1.1   1.9  0.9    53  -1.6
  23   20 48 09.739  -40 10 05.74   3.6  1.2   3.4  1.2    95  -0.1
  ==================================================================
  24   20 48 16.866  -40 11 46.55   3.0  1.1   4.1  1.2   135   0.9
  25   20 47 49.854  -40 06 27.14   2.8  1.1   2.7  1.2    96  -0.1
  26   20 48 10.259  -40 12 56.78   2.7  1.0   3.2  1.2   121   0.5
  27   20 47 36.341  -40 14 26.55   2.6  1.0   3.6  1.2   135   1.0
  28   20 47 48.624  -40 08 28.52   2.5  1.0   1.6  0.9    64  -0.9
  29   20 47 17.085  -40 08 25.65   2.3  1.0   1.5  0.0    66  -0.7
  30   20 48 03.514  -40 10 28.77   2.0  0.9   3.7  1.4   180   1.7
  31   20 48 03.294  -40 10 47.68   1.8  0.9  <1.5   -    <82 <-0.4
  32   20 48 08.121  -40 18 42.44   1.8  0.9   2.0  1.0   108   0.2
  33   20 47 48.309  -40 15 04.50   1.8  0.8  <1.5   -    <83 <-0.4
  34   20 47 48.316  -40 16 00.24   1.8  0.8   1.6  0.8    89  -0.3
  35   20 47 47.796  -40 15 12.32   1.7  0.6  <1.5   -     88  -0.4
  36   20 47 58.094  -40 17 35.49   1.4  0.8  <1.5   -   <103  <0.1
  37   20 47 28.350  -40 14 29.01   1.4  0.8  <1.5   -   <107  <0.1
  ==================================================================

As in the original catalog, fluxes here are in approximate units of
1E-06 photons/cm^2/s.  Given a flux F1 and flux uncertainty U1 at the
first epoch, and flux (uncertainty) F2 (U2) at the second epoch, we
calculate the "Variability %" as 100*F2/F1, and the "Variability
Sigma" as (F2-F1)/U1.

Our two Chandra observations take place 7.9 and 16.6 days after the
burst, respectively, and we therefore expect a flux decay of
approximately 53% for an afterglow power-law decay with index alpha ~
-1.  The sources showing the strongest evidence for a decrease in flux
of this magnitude are (in decreasing order of confidence) sources #2,
#7, and #18.  Source #18 is 13-arcsec outside the HETE localization.

Given the brightness of Source #2, its fading behavior, and its
association with a z=0.2146 galaxy (Kelson et al., GCN 2627), this
source is our preferred candidate of these three."
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