TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  239
SUBJECT: GRB 990123: Continued Radio Observations
DATE:    99/01/29 04:22:52 GMT
FROM:    Shri Kulkarni at Caltech  <srk@astro.caltech.edu>

S. R. Kulkarni (Caltech) and D. A. Frail (NRAO) report on behalf
of the Caltech-NRAO-CARA gamma-ray burst collaboration the following.

"We have continued monitoring the error box of GRB 990123
(GCN #202) with the Very Large Array (VLA). All our observations
were  conducted in the 8.46-GHz band. At the position of the
optical transient we obtain the following fluxes:


Here is a summary of all the X-band observations to date.

Jan 23:  <68 microJy  (GCN #200)
Jan 24:  260 microJy  (GCN #211)
Jan 26:  <78 microJy
Jan 27:  <50 microJy
Jan 28:  <50 microJy

Note: The upper limits are indicated by "<" and are 2-sigma.

To summarize, the radio afterglow has been detected only once, on
January 24 1999. The detection was very secure being 8-sigma. The
source was seen in both intermediate frequency (IF) bands and in both
senses of polarization (R and L). In contrast, the average of the flux
for the period January 26-28 is <32 microJy.

This high degree of variability could be due to interstellar scattering
and scintillation (ISS).  However, the factor of 10  variation in the
flux density requires rather extreme ISS.  In addition, we expect the
source to be gradually rising with time. Thus the absence of the source
on three successive days is quite puzzling.

Discarding the ISS hypothesis, we have two choices. First, the radio
emission detected on January 24 is some kind of precursor event to the
main afterglow. Typically, the radio afterglow in the 8.46-GHz band
rises to the peak flux in about 10-20 days. We have little data about
the behavior of radio afterglow within a few days after the burst.
Thus it is difficult to accept or reject this hypothesis.

The alternative hypothesis is that the GRB is lensed (GCN #216).  At
the present time, there is no firm evidence for lensing. However,  the
lensing idea is economical in energetics. Likewise, the lensing
hypothesis offers a simpler alternative to the curious phenomenon
discussed above.  As discussed by various people (S. Mao GCN#236, R.
Narayan, pers.  comm.) a robust expectation of strong lensing is that
we should first see the faint image "B1" (in the terminology of GCN
#236). In this framework, we identify the radio source of Jan 24th with
the radio afterglow of the B1 component.  If this interpretation is
correct then the radio afterglow of the brighter components B2 and B3
should become visible in in the next few weeks. We do not possess
sufficient astrometric accuracy to see if the reported (GCN #201)
optical afterglow (which is presumably due to B1+B2) and the radio
afterglow are offset on the sky.  Indeed, the excellent agreement (0.5
arcsec) between the positions of the optical and the radio afterglow
places a limit of a delay of 30 days between components B1 and B2+B3.
Clearly, radio monitoring will refute or confirm this hypothesis.

This report is citeable."