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GCN Circular 238

Subject
GRB990123, Preliminary Lens Search Results
Date
1999-01-28T21:35:38Z (25 years ago)
From
Robert Nemiroff at Michigan Tech. <nemiroff@mtu.edu>
R. J. Nemiroff (Michigan Tech.), 
G. F. Marani (NRC/NASA), J. T. Bonnell (USRA/NASA),
J. P. Norris (NASA/GSFC), and C. A. Meegan (NASA/MSFC) report:

There is, as yet, no primary indication that GRB 990123 has undergone
any type of strong gravitational lensing.

Weak lensing:  Most sources at z>1.6 will be either gravitationally
amplified or (more likely) de-amplified by >5% by inhomogeneities in
the gravitational field between the observer and the source (see, for
example, Holz et al. astro-ph/9804271).

Strong Lensing:

Galaxy lensing: There is, as yet, no primary indication that GRB 990123
is one of multiple macro-images created by an intervening galaxy lens.
In comparison with QSOs, it is not unusual for single image QSOs to be
found within 3 arcseconds of a low mass galaxy (e.g. Claeskens & Surdej
1998, A&A 335, 69) or for a single image QSO to have absorption lines.

As GRB 990123 was the brightest event ever detected with a measured
cosmological redshift, its macrolensing probability is high relative to
other GRBs and afterglows, but still, quite possibly, low in absolute
terms.  The foreground galaxy near GRB 990123 is intriguing but not yet
defining.  The "probability" now all hinges on the unknown masses and
mass distributions internal to foreground galaxies.

No other BATSE triggered GRB has been found that is consistent with a
lensing interpretation.  Twenty-three BATSE GRBS have occurred in the
past three years within a 3-sigma error contour of GRB 990123.  The
closest two in light curve shape were judged to be BATSE triggers 6279
and 6698.  A preliminary visual inspection reveals none of them is a
close light-curve match to GRB 990123.

Millilensing: There is, as yet, no primary indication that GRB 990123
has undergone significant amplification by a compact mass on the
globular-cluster scale.  The two main peaks in the GRB light curve do
not appear to be co-added replicas of a single light curve seperated by
major peak are significantly different (> 5 sigma,  preliminary
analysis) in BATSE channels 1 and 4.  Similarly, the ratios in peak
flux between the first major peak and the last peak are also
significantly different (> 5 sigma, preliminary analysis).

Microlensing: There is, as yet, no primary indication that GRB 990123
has undergone significant light curve distortion due to microlensing by
compact masses on the stellar mass scale.  Microlensing at low optical
depths would create images separated in time by only microseconds
(Nemiroff 1994, ApJ 432, 478; Nemiroff 1998, ApJ 494, L173) to
milliseconds (Williams & Wijers 1997, MNRAS 286, L11), generally below
minimum time scale of BATSE GRB resolution (64-ms).

We encourage, however, continued attempts to recover secondary images
from any GRB or afterglow, including GRB 990123.
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