TITLE:   GCN CIRCULAR
NUMBER:  10750
SUBJECT: GRB 100513A: Lick observations and R-band dropout
DATE:    10/05/13 07:03:07 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

D. A. Perley, C. R. Klein, and A. N. Morgan (UC Berkeley) report:

We began observing the location of the afterglow of GRB 100513A 
(Baumgartner et al., GCN 10746) at 04:06 UT using the Nickel 40-inch 
telescope at Lick Observatory.  A total of fourteen 300-second images 
were acquired in the R-band under clear sky conditions.

A co-add of the first 9 images (midpoint 04:35 UT) reveals a faint point 
source at the position reported by Morgan et al (GCN 10747).  The source 
is near the detection limit of the co-add and is not detected in 
individual images.

Photometry of the object relative to three nearby SDSS stars (converted 
to Rc via the Lupton transformation equation) gives the following magnitude:

R = 21.39 +/- 0.25    (t_mid = 2.46 hours)

We note that this is significantly fainter than expected from the 
preliminary JHK magnitudes of Morgan et al (GCN 10749).  The IR spectral 
index is relatively flat (beta~0.5), suggesting little host extinction. 
  Assuming no extinction in addition to the Galactic value, a direct 
power-law extrapolation of the JHK data to R-band over-predicts the 
observed flux by >2 magnitudes, strongly suggesting an R-band dropout 
and likely redshift of 4.6 < z < 6.0.  We note also that the X-ray 
column is consistent with no absorption beyond Galactic (Baumgartner et 
al.), consistent with a high-redshift origin.

We encourage I/z/Y-band follow-up to better constrain the redshift and 
dust extinction.  Spectroscopic observations are planned.