[vsnet-grb-info 7990] GRB090618 - Continued optical monitoring and light curve break

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9531
SUBJECT: GRB090618 - Continued optical monitoring and light curve  break
DATE:    09/06/19 08:56:19 GMT
FROM:    Zach Cano at ARI/John Moores Liverpool  <zec at astro.livjm.ac.uk>

Z. Cano (Liverpool JMU), C. Guidorzi (U. Ferrara), D. Bersier,
A. Melandri, I.A. Steele, R.J. Smith, C. Mundell (Liverpool JMU)
report, on behalf of a large collaboration:

Following our report of observations (GCN 9520) we continued monitoring
the field of GRB090618 (Swift trigger 355083) on the Liverpool Telescope
(La Palma) from 13.6 hours to 17.9 hours post burst in R and i bands.
Below is a summary of a few R-band images:

t_exp  t-T0          R-mag

300s   13.6 hours    18.9 ± 0.1
300s   15.6 hours    19.1 ± 0.1
300s   17.9 hours    19.3 ± 0.1

The calibration is still preliminary and determined against stars in
the USNO catalog.

We note a break in the R-band light curve, and find a decay index of
alpha~1.4 by 17.9 hours post burst, as compared to alpha~0.7 previously
reported (GCNs 9517, 9520).