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.