TITLE:   GCN CIRCULAR
NUMBER:  9183
SUBJECT: GRB 090418A: Swift/UVOT Detection of the Neighbouring Galaxy 
DATE:    09/04/21 21:52:49 GMT
FROM:    Stephen Holland at USRA/NASA/GSFC/SSC  <Stephen.T.Holland@nasa.gov>

S. T. Holland (CRESST/GSFC/USRA) reports on the behalf of the Swift
UVOT team:

      Swift/UVOT observed the POSS2 galaxy (Pavlenko, et al., 2009,
GCNC 9179) located approximately 4 arcsec northwest of the afterglow
of GRB 090418A (Mangano, et al.,2009, GCNC 9149).  We obtain the
following preliminary magnitudes and 1-sigma errors for this galaxy.

Filter  Exposure      Mag  Err  Sigma
-------------------------------------
v           6163     21.0  0.2    5.5
b           4361     22.2  0.5    2.3
u           6057     21.3  0.2    5.6
uvw1        6178     21.6  0.2    4.7
uvm2        2109     21.5  0.4    2.9
uvw2        1357     21.3  0.3    3.4
-------------------------------------

Exposure is the total exposure time in seconds.  The quoted magnitudes
have not been corrected for the expected Galactic extinction along
the line of sight corresponding to a reddening of E_{B-V} = 0.04 mag
(Schlegel, et al., 1998, ApJS, 500, 525).  All photometry is on the
UVOT photometric system described in Poole et al. (2008, MNRAS, 383,
627).

      The detection of the galaxy in the uvw2 filter suggests that the
galaxy has a redshift of less than approximately 1.3.  The redshift of
the afterglow is at least z = 1.608 (Chornock, et al., 2009, GCNC
9151).  Therefore, we conclude that this galaxy is unlikely to be the
host galaxy of GRB 090418A, in agreement with Pavlenko, et al. (2009,
GCNC 9179).