[vsnet-grb-info 3543] GRB060729: ROTSE-III Observations of the Slow Optical Decay

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5377
SUBJECT: GRB060729: ROTSE-III Observations of the Slow Optical Decay
DATE:    06/08/01 03:57:17 GMT
FROM:    Robert Quimby at U of Texas/ROTSE  <quimby at astro.as.utexas.edu>

R. Quimby (U Texas) and E. S. Rykoff (U Mich), report on behalf of the
ROTSE collaboration:

ROTSE-IIIa, located at the Siding Spring Observatory, Australia, responded
to Swift trigger 221755 (GRB 060729; Grupe et al., GCN 5365) with an
automatic sequence of 10 5-second, 10 20-second, and 34 60-second
integrations before twilight set in (Quimby et al., GCN 5366). Another 30
60-second images were taken on July 30, and 60 further 60-second frames
were recorded on July 31.

No optical source was detected in our first image, but seconds later we
clearly detect a 15.7 magnitude transient object at the location of the
UVOT source (GCN 5365; Immler, GCN 5367). The object faded over the next
100 seconds, and then brightened to about 16.5 magnitude around 400
seconds after the BAT trigger. After this, the optical transient slowly
began to fade, approximately following a t^-0.23 power-law at least until
the July 30 observations. On July 31, the optical transient was about 19.4
magnitude, significantly fainter than an extrapolation of the simple
power-law would predict. The 2.9 magnitude drop from t~400-seconds marks
one of the slowest 2 day average decline rates ever recorded by ROTSE-III.

We set the following representative detections and limits:

    tstart        tend    exp    mag   emag
-----------------------------------------------
     64.26        69.26     5  >16.60
     92.86        97.86     5   15.67  0.10
    187.06       219.96    15  >17.28
    448.57       604.68   120   16.56  0.05
   2269.84      2830.05   480   16.93  0.05
  72997.24     75133.01  1800   18.00  0.18
 171304.69    175581.44  3600   19.39  0.18

(times are seconds after the BAT trigger; magnitudes are unfiltered and
calibrated against the USNO-B1.0 R2)