TITLE:   GCN CIRCULAR
NUMBER:  10903
SUBJECT: GRB 090709A: Host galaxy detection
DATE:    10/06/29 04:44:26 GMT
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 observed the field of GRB 090709A (Morris et al., GCN 9625) using 
NIRI on Gemini-North on the night of 2010-06-23 between 11:54 and 14:46 
UT, under good conditions.  Our co-added stack contains a total of 146 
images of 60 seconds each (2.43 hours of imaging) in the K' filter.  The 
3-sigma limiting magnitude of the stack is K' ~ 23.7.  (Calibration of 
the field was established relative to several 2MASS stars in the image.)

Matching our image directly to early-time PAIRITEL imaging (Morgan et 
al., GCN 9635), we detect a faint, extended source slightly offset (0.6 
arcsec) from the position of the IR afterglow.  The orientation of the 
offset is along the apparent direction of extension of the source (E-W). 
   Aperture photometry within a 1.0 arcsec aperture centered on the 
object gives a magnitude of K' = 22.0 +/- 0.2 mag.  The chance 
probability for this alignment is approximately one percent (Bloom et 
al. 2002, AJ 123:1111, Djorgovski et al. 1995, ApJL 438:L13).

We therefore associate this object with the host galaxy of GRB 090709A. 
   The detection of a host galaxy confirms the extragalactic, 
cosmological nature of this event (de Luca et al. 2010, MNRAS 402:1870; 
Cenko et al. 2010, AJ 140:224).  We note that deep optical imaging of 
this field by the GTC (Castro-Tirado et al., GCN 9655) detected no 
object at this position to a limit of i' > 25.5, suggesting that the 
host galaxy may be internally dust-reddened.  The infrared afterglow 
also showed evidence of strong reddening (Cenko et al. 2010), and 
foreground extinction in this direction is low; E(B-V) = 0.09 mag. 
Additional imaging is encouraged to test this hypothesis.

An image of the field is posted to:
http://lyra.berkeley.edu/~dperley/090709a/090709a_niri.png

We are happy to thank Marie Lemoine-Busserolle, Matthew Dillman, and 
Andrew Stephens for their support during our classical NIRI run.