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GCN Circular 205

Subject
GRB990123, early optical counterpart detection
Date
1999-01-23T23:15:16Z (25 years ago)
From
Carl Akerlof at U.Michigan <akerlof@mich1.physics.lsa.umich.edu>
C. W. Akerlof and T. A. McKay (Univ. of Michigan) report on behalf of the ROTSE
collaboration (Michigan/LANL/LLNL):

We observed the error box of GRB 990123 provided by the BACODINE Burst Position
Notice dated 23-Jan-99 09:46:59 using the ROTSE-I telephoto camera array
located at Los Alamos, New Mexico. The first exposure began at 9:47:18.30,
22.18 seconds after the nominal burst trigger time. A rapidly fading object was
discovered at the coordinates, RA = 231.3754, DEC = 44.7666 (J2000) which is
within 1/3 of a pixel of the optical counterpart reported by Odewahn et al.
(GCN #201). The light curve for this object is relatively complex: the
luminosity increases by 3 magnitudes between the first and second exposures.
Estimated magnitudes for the first six exposures are given below:

        UTC     exposure     m_v

    9:47:18.3    5 secs.    11.82
    9:47:43.5    5 secs.     8.95
    9:48:08.8    5 secs.    10.08
    9:51:37.5   75 secs.    13.22
    9:54:22.8   75 secs.    14.00
    9:57:08.1   75 secs.    14.53

Note that the ROTSE-I detector system uses an unfiltered broadband CCD so that
magnitude estimates are based on comparisons to catalog values for nearby
stars. Sky patrol images of the same coordinates taken 133 minutes earlier
showed no evidence of the transient to a limit of at least two magnitudes
deeper. A more extensive analysis of this data will be available in the near
future.

The discovery images will be posted on the ROTSE Web page at:
   http://www.umich.edu/~rotse/gifs/grb990123/990123.gif

This message is quotable in publications.


[GCN OP NOTE:  This archived copy has had a typo fixed.  The original version
had the 9:48:08.8 timestamp for the 3rd exposure go out as 9:47:08.8.]
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