TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3110
SUBJECT: GRB 050315, SMARTS optical afterglow decay
DATE:    05/03/17 21:04:47 GMT
FROM:    Bethany Cobb at Yale U  <cobb@astro.yale.edu>

B. E. Cobb and C. D. Bailyn (Yale), part of the larger SMARTS
consortium, report:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained additional optical/IR imaging of the reported optical afterglow
candidate (GCN 3100, Kelson & Berger) of GRB 050315
(GCN 3094, Parsons et al.) beginning ~35.6 hours post-burst.
Total summed exposure times amounted to 10 minutes R and I and
12.5 minutes in J.

The source identified as the afterglow candidate has dimmed
since our previous observations (GCN 3104, Cobb & Bailyn).
Preliminary comparison with Landolt Standards PG 1047+003 and RU 149
yields the following approximate afterglow magnitudes:

        R = 21.6
        I = 21.5

The counterpart was again not detected in the IR, with a
magnitude limit of J > 18.6.

Our R and I observations are summarized below:

        time post-burst:        R magnitude:    I magnitude:
        11.6 hours              20.9            20.3
        35.6 hours              21.6            21.5

We note that the I-band observations are close to the detection
limit, and have measurement errors of >0.2 magnitudes, so the
evidence for a color change should be treated with caution.

The R-band magnitudes have errors of ~0.2, so assuming
a power-law afterglow decay (afterglow flux proportional to
t^alpha), then this corresponds to an alpha of -0.57 +\- 0.25.