TITLE:   GCN CIRCULAR
NUMBER:  15325
SUBJECT: Fermi403206457: NOT optical observations
DATE:    13/10/13 05:21:49 GMT
FROM:    Dong Xu at DARK/NBI  <dong.dark@gmail.com>

D. Xu, D. Malesani (DARK/NBI), T. Kruehler (ESO), J.P.U. Fynbo
(DARK/NBI), P. Jakobsson (U. Iceland) and John Telting (NOT) report on
behalf of a larger collaboration:

We observed the location of the candidate optical afterglow
(iPTF13dsw; Kasliwal et al., GCN 15324) of the Fermi-GBM trigger
403206457
with the NOT telescope equipped with ALFOSC. We obtained two R-band
images with an exposure time of 300 s each starting at 01:32 UT
2013-10-13 in a good seeing of ~0.65".

The candidate afterglow is clearly detected in each image as a
point-like source, and we measure an R-band magnitude of R = 21.6 mag
assuming R = 18.36 mag for the nearby star at RA = 02:10:07.95, Dec =
-04:24:41.7 at a midpoint of 01:35 UT.

Accounting for the different calibration system with respect to the
magnitudes given in Kasliwal et al. (GCN 15324), the source has faded
by approximately 1.6 magnitudes with respect to the earlier P60 measurement.

Under the assumption that the transient decays as a single power-law,
this fading corresponds to a decay index of ~1.6, not untypical of GRB
afterglows at the given epoch.

We thank S. B. Cenko for private information about the photometric
calibration of the P60 measurement.