GCN Circular 221

GRB 990123 : Multiple and Distorted Images of the Host Galaxy?

E. L. Turner (Princeton Univ. Obsv.):

Djorgovski et al. (GCN Circ. 216
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) suggested that GRB 990123 might be
strongly lensed by an intervening galaxy based on the extreme energetics
implied by its high gamma-ray fluence (Feroci et al., IAUC 7095) and
the redshift lower bound of 1.61 (Kelson et al., IAUC 7096) and on the
relatively bright coincident galaxy found on the digital POSS-II images
by Odewahn et al. (GCN Circ. 201
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 and IAUC 7094).  Their prediction that
this galaxy's redshift would be much smaller than 1.6 and probably in
the range 0.2 to 0.3 was quickly confirmed by Hjorth et al. (GCN Circ. 
219
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) who in fact found one definite galaxian redshift system at z = 0.286
and a second possible one at z = 0.210, thus providing strong apparent
support for the gravitational lensing hypothesis in GRB 990123.

Hjorth et al. also note that additional images of this GRB may appear on
a time scale of days to months due to differential lensing time delays.

The light from the host galaxy of GRB 990123 will also be affected by any
strong gravitational lensing which has influenced the burst and should
show strongly distorted and probably multiple images if the magnification
factor is large.  Detection and characterization of such lensed host galaxy
images would provide strong additional evidence for the lensing hypothesis
but, more importantly and much more urgently, would also allow detailed
modeling of the lensing mass distribution and geometry and, thus, far more
precise prediction of where and when additional images of the burst are
expected to occur.  

These predictions would make it far more practical (and economical of
observing resources) to detect and intensively study additional images
of the burst as they appear.  Such data would not only be valuable for
probing the early stages of GRB physics but might also allow a sufficiently
precise determination of the time delay(s) to use for a measurement of
the cosmic distance scale.  Thus, deep and high resolution imaging in
search of the possibly distorted and multiple images of the GRB 990123
host galaxy is urgently indicated.

This report may be cited.