GCN Circular 10399
Subject
GRB 100205A: Keck limits on an underlying host galaxy
Date
2010-02-10T13:06:10Z (14 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
D. A. Perley, J. S. Bloom, S. B. Cenko, B. E. Cobb, A. N. Morgan, A. A.
Miller, and C. R. Klein (UC Berkeley) report:
We conducted a deep optical imaging integration on the field of GRB
100205A (Racusin et al., GCN 10361) using LRIS on the Keck I 10m
telescope starting at 09:48 UT on 2010 Feb 7. We integrated for a total
exposure time of 4000 seconds in g-band and 3770 seconds in R-band
simultaneously under clear skies but variable and generally poor seeing
conditions (1.5 arcsec average). The mid-point of the integration was
at 10:28 UT.
We detect no source at the position of the putative infrared afterglow
(Tanvir et al., GCN 10366). Calibrating relative to USNOB1.0, we
measure limiting magnitudes (3-sigma) of:
R > 26.7
g > 26.6
as measured at the afterglow location. A faint source is marginally
detected (<2 sigma) in R-band only at the northwestern edge of the XRT
error circle (Evans et al., GCN 10367), not consistent with the position
of the infrared afterglow.
We also stacked the R-band and g-band images together to create a
white-light composite image. There is no significant detection in this
image, with an approximate limiting magnitude of White > 27 mag.
All properties of this GRB are consistent with it being at high redshift:
* Lack of optical detection starting at early times (e.g. Malesani et
al., GCN 10362; Updike et al., GCN 10364; Cobb et al, GCN 10365)
* An apparently fading afterglow candidate detected only in H- and
K-bands (Cucchiara et al., GCN 10374; Im et al., GCN 10398)
* Limited X-ray absorption: excess N_H(z=0) = 7(+6/-5) x 10^20 cm^-2
(Starling and Racusin, GCN 10369), generally suggestive of higher
redshift (Grupe et al. 2007, AJ 133:2216G) and inconsistent with the
alternative hypothesis of a large extinction column at low redshift for
typical values of A_V/N_H.
* Lack of host galaxy to deep limits (Perley et al. 2009, AJ 138:1690)
As mentioned by Cucchiara et al., the implied redshift of this GRB if
the red H-K color is due to Lyman-alpha absorption would be z ~ 11-13.
However, significant dust absorption at intermediate redshift (e.g.,
A_V~3 mag at z~4) is still generally consistent with the available data,
and unfortunately the X-ray afterglow is too faint to impose meaningful
constraints on extinction in the K-band during the Gemini observation.
An image of the field is posted to:
http://lyra.berkeley.edu/~dperley/100205a/100205a_keck_gR.png
We encourage continued deep infrared follow-up of the field.
