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."