GCN Circular 2318

D. A. Smith (U. of Michigan) & R. Quimby (U. of Texas) report on behalf of the
ROTSE collaboration:

The ROTSE-IIIb instrument at McDonald Observatory, Texas, responded
automatically to HETE-2 alert 2777, and began taking images within 5 s of when
the alert was distributed.  The first exposure began 47 seconds after the burst
trigger time.  All objects in the SXC 2' (radius) error circle that appear in
more than four individual ROTSE images match to sources in the USNO 2.0
catalog.  Unfiltered limiting magnitudes, calibrated to USNO R-band, for the
first ten (5-s) images were around 16.5, for the second ten (20-s) images
around 17.5, and for the next 40 (60-s) images around 18.0.  We then co-added
sets of ten frames each to drive the limiting magnitudes down to 18.1, 18.7,
and 19.1, respectively.  We also co-added the first twenty frames as a check
against false positives in the first two co-added images.

Only two non-USNO sources within the error circle appear in more than the first
of the co-added, composite images.  We find no sources that appear in the five
later co-adds that are not in the first one.  Both of the non-USNO sources in
the first image are dimmer in the second and third images, and both vanish by
the fourth image.  Neither source appears to be in the DSS or the 2MASS J and K
archival images of this field, and neither source appears in images taken the
following night (to a limiting magnitude of 18.6) at 0.95 d after the burst.

One of the sources is too close to a USNO-cataloged star for SExtractor to
de-blend the two.  The star is at coordinates 21h 49m 24s.8, -27o 40' 10".16
(J2000.0), and its derived ROTSE magnitude (which would contain emission from
both sources) fades by 1.6 mag from 17.3+-0.1 to 18.9+-0.2 as the nearby source
(which is about 7.5" East and 3" South) disappears.  The following night, this
star was measured at 18.9+-0.2.
 
The second source was isolated, and we derive the following light curve:
R.A.            Dec.  (J2000.0)  Magnitude
21h 49m 24s.379 -27o 40' 35".04  17.9+-0.2  18.2+-0.2 18.2+-0.1  > 19.1   > 19.1   > 19.1
(Time from burst)                (47.3 s)   (191.9 s) (8.1 m)    (19.7 m) (31.3 m) (42.9 m)

We cannot at this time determine if either of these sources is related to the
HETE-2 event.  Images and finding charts can be found at
http://grad40.as.utexas.edu/~quimby/HETE2777