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GCN Circular 216

GRB 990123: Possible Gravitationally Lensed Burst?
1999-01-25T01:57:11Z (25 years ago)
George Djorgovski at Caltech/Palomar <>
GRB 990123: Possible Gravitationally Lensed Burst?

S. G. Djorgovski, S. R. Kulkarni, J. S. Bloom, S. C. Odewahn, R. R. Gal 
(CIT), and D. A. Frail (NRAO) note on behalf of the Caltech-CARA-NRAO 

The lower limit to redshift of GRB 990123 (Kelson et al., IAUC, 7096), 
z =< 1.61, implies a luminosity distance D >= 3.73e28 cm, assuming a
standard Friedmann cosmology with H_0 = 65 km/s/Mpc, Omega_0 = 0.2, and
Lambda_0 = 0 (larger if Lambda_0 > 0).

Assuming the gamma-ray fluence reported by Feroci et al. (IAUC 7095), the
implied isotropic gamma-ray energy of this burst would be > 2.3e54 erg,
higher than most (or all) currently viable theoretical models can easily
accomodate.  The absolute magnitude of the optical counterpart at maximum 
light (Akerlof and McKay, GCN Circ. 205) would be M =< -36.5, i.e.,
> 2e16 L_sun, a remarkable number itself.

The apparent host galaxy of this burst, coincident with the optical
transient (Odewahn et al., GCN Circ. 201 and IAUC 7094) detected on DPOSS
(digital POSS-II) images calibrated with new CCD photometry in the field,
has a Gunn r magnitude 21.9 +- 0.4, i.e., R ~ 21.5 +- 0.5 (cf. Gal et al.,
GCN Circ. 213).  Its probable redshift is likely to be approximately 0.2 to
0.3, and (unless an active nucleus is present, for which there is no
spectroscopic evidence so far) almost certainly not at z = 1.61, the 
absorption line redshift measured by Kelson et al.

We therefore suggest that this may be the first documented case of a 
gravitationally lensed GRB, magnified by at least an order of magnitude by 
a foreground galaxy detected in DPOSS (possibly including some microlensing).  
This hypothesis naturally accounts for all of the data so far: the large 
fluence of the burst, its enormous implied gamma-ray luminosity, its 
remarkably high peak optical brightness and implied luminosity (and makes 
it consistent with previous non-detections of other bursts by the ROTSE and 
LOTIS experiments, with m_lim ~ 14), and the presence of an apparent 
foreground galaxy positionally coincident with the optical transient.

The conclusive test of this hypothesis will be a direct measurement of the 
galaxy's redshift, once the OT fades sufficiently.  If microlensing is
involved, a signature may be seen in the shape of the OT light curve.
Possible image splitting may be detectable, and high-resolution imaging
in radio, near-IR, and optical is urged.

In any case, the early detection by the ROTSE team (GCN Circ. 205), even if
corrected for a putative large gravitational lensing magnification factor,
suggests that routine early detections of GRBs in the visible light are
within reach, perhaps at ~ 15 mag level.

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