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GCN Circular 9601

Subject
GRB 090404: Deep optical imaging and possible host association
Date
2009-07-02T06:33:36Z (15 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
D. A. Perley, S. B. Cenko, and J. S. Bloom (UC Berkeley) report:

We imaged the field of dark gamma-ray burst GRB 090404 (Ziaeepour et 
al., GCN 9086) using the Keck I telescope (+LRIS) on the night of 
2009-06-25 (UT) for 1670 seconds in g-band and 1560 seconds in I-band 
under excellent (0.5 arcsecond) seeing and photometric conditions.

The object outside the XRT error circle mentioned by Malesani et al. 
(GCN 9093, GCN 9095) is clearly detected as an extended source, likely a 
moderate-redshift galaxy (hereafter "G1").  A second, slightly fainter 
extended source ("G2") is also detected 3.5 arcseconds to its northeast, 
and the two sources appear to be connected by faint emission, perhaps a 
tidal bridge.  Both sources are marginally detected in Sloan Digital Sky 
Survey archival images (see also GCN 9096).

Additional faint, extended emission is detected just west of the 
brighter galaxy G1.  The emission appears to be spread over several 
arcminutes in the g-band image, with an obvious "knot" located 1 
arcsecond northeast of the millimeter afterglow position reported by 
Catro-Tirado et al. (GCN 9100).  The extended region itself passes 
directly through the millimeter position.  A false-color image of the 
field, with the millimeter and enhanced XRT positions 
(http://www.swift.ac.uk/xrt_positions/00348428/image.php) superimposed, 
is posted to: http://lyra.berkeley.edu/~dperley/090404/090404_gI.png

We hypothesize that the extended emission may be a tidal tail extending 
from G1 as a result of interaction with its northeastern neighbor. 
Alternatively, the extended region may represent a very faint background 
host galaxy.  Aperture photometry at the location of the millimeter 
afterglow gives a magnitude of g = 27.3 +/- 0.2.

We encourage spectroscopy of the two bright galaxies G1 and G2 to 
determine their redshifts.
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