TITLE:   GCN CIRCULAR
NUMBER:  10913
SUBJECT: GRB 100213B: Detection of Fading Optical Afterglow
DATE:    10/06/30 23:03:00 GMT
FROM:    Jules Halpern at Columbia U.  <jules@astro.columbia.edu>

Y. Li, J. Weston, X. Fernandez, A. Brown, J. H. Yoon, A. P. S. Crotts,
and J. P. Halpern (Columbia U.) report on behalf of the graduate
Observational Astronomy class:

"We obtained R-band images of the field of Swift GRB 100213B (Vetere et al.,
GCN 10412) on two consecutive, photometric nights using the MDM 1.3m
telescope, for total exposure times of 30 and 40 minutes, respectively.
Within 3.8" of the enhanced Swift XRT afterglow position (of 90% confidence
radius 3.5", Evans, GCN 10423), we find a fading object at

      (J2000) R.A. = 08h 17m 07.94s, Decl. = +43d 26' 54.94"

with positional uncertainty ~0.5", and the following magnitudes:

    ----------------------------------------------------------
     Date(UT)  Mid-time(UT) t-t0(hr)  Exp(min)      R(mag)
    ----------------------------------------------------------
     Feb. 14      03:44        4.8       30      22.00+/-0.14
     Feb. 15      03:35       28.6       40      23.01+/-0.24
    ----------------------------------------------------------

Magnitudes are referenced to Landolt (1992) standard stars, and are not
corrected for Galactic extinction.

The position of the afterglow coincides with the R ~ 21.5 object detected by
Malesani et al. (GCN 10413) 19 minutes after the GRB, and whose redshift
z = 0.604 was measured by Cucchiara et al. (GCN 10422). We conclude that the
observed variability and positional coincidence with an emission-line object
establishes the optical afterglow and host galaxy redshift of GRB 100213B.
Further observations of the host galaxy are encouraged.

MDM images of the field are posted at

http://www.astro.columbia.edu/~jules/grb/100213b/

This message may be cited."