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GCN Circular 1392

XRF020427: Chandra Identification of a Likely X-ray Counterpart
2002-05-20T19:12:51Z (22 years ago)
Derek Fox at CIT <>
D. W. Fox (Caltech) reports on behalf of the Caltech-NRAO GRB

"We have observed the error region of the X-ray flash XRF020427 (GCN
1383, 1384, 1386) with the Chandra X-ray Observatory for 12.5 ksec
beginning May 14.1939 UT.  Reduction of the data using standard Ciao
procedures, and comparison with the source catalog from our first
observation of May 6.2456 UT (GCN 1387), yields the following results
for the BeppoSAX NFI error circle (Amati et al., GCN 1386):

    ID        IAU Name          CR_1    CR_2    Delta   
     1  CXOU J220928.2-651932   4.2(6)  1.8(4)  -3.3 sigma
     2  CXOU J220925.9-651855   1.2(3)  2.3(4)  +2.1
     3  CXOU J220925.0-651919   1.1(3)  0.8(3)  -0.7

where "CR_1" and "CR_2" are the source count rates per ksec in the 0.3
to 7.0 keV band for the first and second Chandra observations,
respectively, and "Delta" is the signed significance of the change in
count rate from the first to the second observation, expressed in
standard deviations.  Uncertainties in the trailing digits are
indicated in parentheses.  

The brightest source in the first epoch, CXOU J220928.2-651932 (ID 1),
shows a pronounced fading between the two epochs, equivalent to a
power-law decay with index 1.3 referenced to the epoch of the burst
(Apr 27.1588).  We conclude that this source is the likely X-ray
afterglow of XRF020427.  

In addition, identification of three USNO stars on the Chandra images
has allowed us to improve the aspect; the improved position of the
likely afterglow (ID 1) is

    RA  22:09:28.265(46),  Dec -65:19:31.78(32)  (J2000)

where the uncertainties are indicated in parentheses and correspond to
an average radial uncertainty of 0.3 arcsec (one-sigma)."
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