[vsnet-grb-info 6980] GRB 081126: Confirmation of the Optical Afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8564
SUBJECT: GRB 081126: Confirmation of the Optical Afterglow 
DATE:    08/11/27 15:56:19 GMT
FROM:    Stephen Holland at USRA/NASA/GSFC/SSC  <Stephen.T.Holland at nasa.gov>

S. T. Holland (CRESST/GSFC/USRA), P. A. Evans (U Leicester),
F. E. Marshall (NASA/GSFC), P. Schady (MSSL), and R. Margutti (U
Bicocca & OAB) report on the behalf of the Swift UVOT team:

      We confirm the optical afterglow of GRB 081126 reported by
Sergeev & Pozanenko (2008, GCN Circ. 8558). A re-examination of the
UVOT data shows that there is a fading source at

      RA (J2000)  21:34:03.59  =  323.51496 (deg)
     Dec (J2000) +48:42:38.3   =  +48.71064 (deg)

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence,
statistical + systematic).  This is consistent with the XRT error
circle.  Preliminary photometry and 3-sigma upper limits for this
source are

Filter   T_start   T_stop   Exp(s)     Mag   Err
------------------------------------------------
  white        75      225     147     18.82 0.09
     u        289      420     127     18.17 0.13

     v       5161     5361     197     19.29 0.32
     b       4546     4746     197     20.20 0.28
     u       5778     5976     197    >20.4
   uvw1      5572     5771     197    >20.0
   uvm2      5366     5566     197    >19.6
   uvw2      4957     5156     197    >20.0
  white      4751     4951     197     20.33 0.21
------------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight corresponding to a
reddening of E_{B-V} = 0.78 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 in the u filter,
combined with the lack of detections in the UV filters, is consistent
with the afterglow having a redshift of approximately 2.4 < z < 3.8.
However, this result is uncertain due to the large Galactic reddening
along the line of sight to this source.

      The source identified in Holland & Margutti 2008, (GCN
Circ. 8559) as being possibly extended is consistent with the
catalogued position of the 2MASS point source 21340333+4842361.  The
separation between this 2MASS source and the afterglow is 3.2 arcsec.
Since the photometry of the 2MASS source and the afterglow are
consistent with both sources having at the same redshift it is
possible that the 2MASS source is the host galaxy.  Further
observations are encouraged to confirm or reject this hypothesis.

      Holland & Margutti 2008, (GCN Circ. 8559) misidentified the
optical afterglow as as the 2MASS point source 21340333+4842361.  We
apologize for any confusion caused by the misidentification in the
previous UVOT Circular on GRB 081126.