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GCN Circular 7140

Subject
GRB 070429B: Probable host galaxy and redshift
Date
2007-11-30T23:25:17Z (16 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
D. A. Perley, J. S. Bloom, M. Modjaz, D. Poznanski (UC Berkeley) and C. 
C. Thoene (DARK) report:

On the night of 2007-07-18 we re-observed the field of GRB 070429B (GCN 
6358, Markwardt et al.), likely to be a short-hard burst (T90 = 0.5 +- 
0.1 s, GCN 6365, Tueller et al.) under photometric conditions using the 
Keck I telescope + LRIS, in g and R filters simultaneously for a total 
integration of 930s(g) / 840s(R) under relatively poor seeing.  We 
further imaged the field using GMOS on Gemini-South on 2007-11-27 for 
1200s in r filter under excellent seeing.

The bright source reported by Cucchiara et al. (GCN 6368), designated 
object "A" by Antonelli et al. (GCN 6372) and likely the host galaxy of 
the GRB, is well-detected in R and r and weakly detected in g.  Using 
Landolt standard stars we measure an aperture magnitude for this object 
(in a 2.1" radius aperture) of

g = 24.79 +/- 0.14
R = 23.24 +/- 0.05

This is consistent within errors with the magnitudes reported by 
Cucchiara et al. and Antonelli et al. 4 and 5 hours after the burst, 
respectively.  Image subtraction of the new Gemini imaging versus the 
earlier epoch (GCN 6368) reveals no variability to a limiting magnitude 
of R > 24.5, ruling out an afterglow contribution in the first epoch 
(4.84 hours after the burst) at this level.

Object "B" is also detected in both filters, and also shows no evidence 
for variability.


On the night of 2007-10-09 we performed longslit spectroscopy covering 
both targets ("A" and "B") in two integrations of 1500s each, using Keck 
I + LRIS.

The trace of object "A" is faint and the spectrum is mostly featureless, 
but a faint line signature is observed centered at 7098 Angstroms, with 
a FWHM of 6 Angstroms.  The feature appears present in both exposures, 
though this site is severely affected by a cosmic ray in one exposure. 
We identify this feature as most likely being the [OII]3727 doublet. 
Other line identifications (H-alpha, H-beta, or [OIII]) are disfavored 
due to the absence of corroborating lines that would be expected over 
our spectral range (3500-8900 Angstroms) in those cases.

Association of this feature with [OII] indicates a redshift for this 
object of z=0.904.  Calibrating relative to R-band photometry, we 
estimate a preliminary line flux of 3e-17 erg/s/cm^2, corresponding to 
an unextincted star formation rate (Kewley et al. 2002) of 0.7 M_sun/yr, 
comparable to that observed in previous short burst hosts.  We note also 
the red color of this galaxy.

No obvious trace or line features are observed for object B.
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