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GCN Circular 1268

Subject
XRF/GRB 011030: Detection of the Probable Host Galaxy
Date
2002-03-12T20:03:45Z (22 years ago)
From
Andrew S. Fruchter at STScI <fruchter@stsci.edu>
A. Fruchter (STScI), S.  Pattel (MSFC), C. Kouveliotou (MSFC), J.
Rhoads (STscI), S. Holland (Notre Dame), I. Burud (STScI) and R. Wijers
(Stony Brook) report for the larger GOSH (Grb Optical Studies with Hst)
collaboration:

A field containing the reported X-ray (GCN 1143) and radio (GCN 1136)
positions of XRF/GRB 011030 (GCN 1118) was observed with HST on 12
December 2001 using the STIS CCD camera both in open mode (50CCD) and
with a red-pass filter (LP).  Due to the low-galactic latitute (b=20.6)
of this field, the estimated foreground extintion is Av=1.2 (Schlegel,
Finkbeiner, Davis 1999).  Nevertheless, a 26th magnitude galaxy is
found to be coincident with both the X-ray and radio positions (within
the reported ~1" errors of both).

With the public release of the second epoch of Chandra observations
(taken on 29 November 2001), we have been able to determine that the
X-ray source fell between the two Chandra epochs approximately as
t^{-2} (using the time of the XRF/GRB as the zero-point), thus confirming
the association of this source with the XRF/GRB.

We have therefore attempted to refine the Chandra error circle.  We
aligned the first, deeper (9-November) Chandra observation with
wide-field R band images obtained on the 1 and 3 November on the WIYN
telescope using six point sources which are bright in both images.  We
find an r.m.s.  positional offset between sources in the two images of
~0."18.  As the XRF/GRB is brighter in the Chandra image than any source used
in the fit (though not visible in the WIYN image), we estimate that the 
error in transforming its position to the optical field should be no larger 
than 0."18, one sigma.   We have also been able to place the WIYN image on
the J2000 coordinate system to an accuracy of ~0."1, using both Naval
Observatory and Tycho standards.

We find that the 0."18 Chandra error circle overlaps the somewhat 
bluer, eastern end of the small irregular galaxy noted above.  The galaxy 
is approximately 0."5 across, and has a total magnitude of V~26.2, or
approximately V=25, after removal of the estimated foreground extinction.
This is comparable to the peak of the distribution of XRF/GRB host
magnitudes (Hogg and Fruchter 1999).  The color of the galaxy (again
after removal of the foreground extinction) is quite blue (consistent
with a power-law of nu^{-0.5}), suggesting both a relatively young
stellar population and that the galaxy has a redshift, z < 3.  XRF/GRB
011030 thus appears to be similar to GRBs in its probable redshift and
in its association with a faint, blue galaxy; this in turn suggests that 
XRFs are not a radically different class of object, but are very soft GRBs.
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