TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  236
SUBJECT: On the Lensing Interpretation of GRB 990123
DATE:    99/01/28 10:49:17 GMT
FROM:    Shude Mao at MPI,Garching  <smao@ibm-2.MPA-Garching.MPG.DE>

Shude Mao (Max Planck Inst. for Astrophysics) reports:

   Djorgovski et al. (GCN Circ. 216) suggested that GRB 990123 might be
highly magnified (A>10) by an intervening galaxy based on a variety of
arguments. A likely scenario is that the bursting source is very close to
a fold caustic. As a result, we have a PAIR of highly magnified images
(we call B2 and B3) plus other additional image(s).

   If this scenario is correct, then from the lensing theory, three points
can be immediately made:

1. There must be a fainter burst that has arrived before GRB 990123. This
   first burst (we call B1) would be offset from GRB 990123 by about ~2".
   This expectation is valid no matter what the lens potential.
   Notice that, in this scenario, all the optical and radio afterglows
   we see now are the sum of the first-arriving burst B1 and GRB 990123.
   High resolution imaging in the radio and in the optical (with HST)
   may reveal the presence of B1.

   The time interval and intensity ratio between B1
   and GRB 990123 does depend on the model. A rough estimate
   of the time delay is a few days to a month, and the intensity
   ratio is around a few to a few tens. These predictions can be made
   more precise when the astrometries and velocity dispersion of the
   foreground galaxy are better known. The gamma-ray burst data archive
   should be searched to see whether there was such a burst. A fourth
   image (B4) much fainter than GRB 990123 may also appear after about a month.
 
2. The gamma-ray burst host galaxy, since it is extended, will be distorted
   into a ring or arcs if the center of the host galaxy does not have
   a significant offset from GRB 990123. Such features can be detected with
   HST imaging (see also Turner, GCN 221). This expectation is independent
   of the models.

3. The close pair of images should have roughly equal intensities.
   The time delay between these two images depends strongly
   on the magnification; a simple model shows that it can be between
   tens of seconds to a fraction of a day. This close pair therefore
   should have ALREADY left imprints on the after glow light curves.
   The close pair should have an image splitting of about 0.05" to a
   fraction of arcsecond; HST imaging will either provide a confirmation
   or rule out this scenario.

   If the time delay between the close pair can be as short as tens
   of seconds, this raises an intriguing possibility: is GRB 990123
   itself lensed? That is, GRB 990123 may be superposed by two bursts
   coming from the close pair. These two bursts have a time lag
   of about 15 seconds, producing the first and second peaks in
   the light curve. It will be very interesting to check whether 
   the gamma-ray spectrum and light curve of GRB 990123 are consistent
   with this superposition scenario. If this hypothesis is correct, then
   the close pair should have an imaging splitting of ~0.05" and
   each is magnified by a factor of about 100.

   More information (including figures) can be found at 
	http://www.mpa-garching.mpg.de/~smao/grb.html

   This report may be cited.


[GCN OP NOTE:  This Circular was received at 09:19 UT, but was delayed until
an entry in the distribution list was created.]