[vsnet-grb-info 7620] GRB 090418A: Swift/UVOT Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9175
SUBJECT: GRB 090418A: Swift/UVOT Observations
DATE:    09/04/20 19:47:16 GMT
FROM:    Stephen Holland at USRA/NASA/GSFC/SSC  <Stephen.T.Holland at nasa.gov>

S. T. Holland (CRESST/GSFC/USRA) and V. Mangano (INAF-IASFPA) report
on the behalf of the Swift UVOT team:

        The Swift/UVOT observed the field of GRB 090418A starting 84 s
after the BAT trigger (Mangano, et al. 2009, GCN Circ. 9149).
Settled exposures started at T+160 s.  The refined UVOT position for the
optical afterglow is

       RA (J2000) =  17:57:15.17  =  269.31321 (deg)
      Dec (J2000) = +33:24:21.1   =  +33.40585 (deg)

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence,
statistical + systematic).  Preliminary 3-sigma magnitudes and upper
limits are

Filter   T_start   T_stop   Exp(s)      Mag  Err
-------------------------------------------------------------
u (fc)       160      410      246    17.31 0.09

v            466      485       19    17.31 0.24
b            415      435       19    18.22 0.25
u            539      559       19    18.55 0.39
uvw1         515      835       19   >18.1        3-sigma UL
uvm2         490      510       19   >17.6        3-sigma UL
uvw2         441      461       19   >18.1        3-sigma UL

uvw1         515   11,831     1337    20.82 0.26
          114,115  131,895     2064   >22.0        3-sigma UL
uvm2         490   18,413     2109   >21.8        3-sigma UL
uvw2         441   16,709     1357   >21.8        3-sigma UL
-------------------------------------------------------------

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 flight system described in Poole
et al. (2008, MNRAS, 383, 627).

      The detection in the uvw1 filter, combined with the lack of a
detection in the uvm2 filter, is consistent with this source having a
redshift of z = 1.608 (Charnock, et al., 2009, GCN Circ. 9151).